Engineering Chemistry
Engineering Chemistry
Engineering Chemistry
Polymers
Classification of Polymers
Module III
Polymerisation techniques
Bulk, Solution, Suspension and Emulsion
Concept of Tg - Factors affecting Tg
Crystallinity in polymers
Physical and Mechanical properties of polymers
Density, Tensile, Tear, Abrasion resistance, Resilience
Mechanism of lubrication
Thick film, Thin film, Extreme pressure
Classification of Lubricants
Solid, Liquid, Semisolid
Properties of Lubricants
Viscosity, Flash and Fire point,
Cloud and Pour point, Aniline point, Corrosion stability
Classification of fuels
Calorific Value of fuel
Cracking and Reforming
Petrol Knock and Octane number
Diesel knock and Cetane number
Bio-Diesel
Module IV
Theories of corrosion
Corrosion
Storage cells
Acid storage cells : Lead acid accumulator
Alkaline storage cells : Nickel cadmium cells
Coatings
Types of electrodes
Standard Hydrogen Electrode (SHE)
Calomel Electrode
Quinhydrone Electrode and Glass electrode
pH measurements using glass electrode
Water
Electrochemistry
Addition polymerization
Condensation polymerization
Coordination polymerization and Co-polymerisation
Theories of friction
Lubricants
Green chemistry
BioInorganic
chemistry
Definition
Classification based on the nature of M-C bond
18 electron rule
Metal carbonyls
Mononuclear and polynuclear carbonyls
(give examples of Fe, Co, Ni)
Fuels
Organo
Metallic
Compounds
Module I
Electrons in the complex may be counted by Donor Pair Method. This method considers ligands to donate
electron pairs to the metal. To determine the total electron count, we must take into account the charge on each ligand
and determine the formal oxidation state of the metal.
Cr(C0)6: A Cr atom has 6 electrons outside its noble gas core. Each CO is considered to act as a donor of 2
electrons. The total electron count is represented in Table 1. Cr(CO)6 is therefore considered an 18-electron complex. It is
thermally stable; for example, it can be sublimed without decomposition. Cr(CO)5, a 16-electron species, and Cr(CO)7, a
20-electron species, on the other hand, are much less stable and are known only as transient species.
Table 2
Table 1
(5-C5H5)Fe(CO)2Cl: Pentahapto-C5H5 is considered by this method as C5H5-, a donor of 3 electron pairs; it is a
6-electron donor. As in the first example, CO is counted as a 2-electron donor. Chloride is considered C1-, a donor of
2 electrons. This complex is formally an iron(II) complex. Iron(II) has 6 electrons beyond its noble gas core.
This electron count is represented in Table 2.
Metal Carbonyl (M-CO)
Carbon monoxide is the most common ligand in organometallic chemistry. It serves as the only ligand in binary carbonyls
such as Ni(CO)4, W(CO)6, and Fe2(CO)9 or, more commonly, in combination with other ligands, both organic and inorganic.
CO may bond to a single metal or it may serve as a bridge between two or more metals. In this section, we will consider the bonding
between metals and CO, the synthesis and some reactions of CO complexes, and examples of the various types of CO complexes.
Two features of the molecular orbitals of CO deserve attention. First, the highest energy occupied orbital (the HOMO) has
its largest lobe on carbon. It is through this orbital, occupied by an electron pair, that CO exerts its a-donor function, donating
electron density directly toward an appropriate metal orbital (such as an unfilled d or hybrid orbital). Carbon monoxide also has two
empty * orbitals (the lowest unoccupied, or LUMO); these also have larger lobes on carbon than on oxygen. A metal atom having
electrons in a d orbital of suitable symmetry can donate electron density to these * orbitals. These -donor and -acceptor
interactions are illustrated in Figure. The overall effect is synergistic. CO can donate electron density via a -orbital to a metal atom;
the greater the electron density on the metal, the more effectively it can return electron density to the * orbitals of CO. The net effect
can be strong bonding between the metal and CO; however, as will be described later, the strength of this bonding depends on several
factors, including the charge on the complex and the ligand environment of the metal.
The metal-carbon bond in metal carbonyls possess both s and p character. The MC bond is formed by the donation of
lone pair of electrons on the carbonyl carbon into a vacant orbital of the metal. The MC bond is formed by the donation of a pair
of electrons from a filled d orbital of metal into the vacant antibonding * orbital of carbon monoxide. The metal to ligand bonding
creates a synergic effect which strengthens the bond between CO and the metal
Bridging modes of CO
Although CO is most commonly found as a terminal ligand attached to a single metal atom, many cases are known in which
CO forms bridges between two or more metals. In cases in which CO bridges two metal atoms, both metals can contribute electron
density into * orbitals of CO to weaken the C - O bond. Ordinarily, terminal and bridging carbonyl ligands can be considered
2-electron donors, with the donated electrons shared by the metal atoms in the bridging cases. For example, in the complex the
bridging CO is a 2-electron donor overall, with a single electron donated to each metal.
Physical Properties: (1) Iron and nickel carbonyls are liquids at room temperature and pressure but all other
common carbonyls are solids (2) All the mononuclear carbonyls are volatile (3) Because they are non-polar,
all the mononuclear and many of the polynuclear carbonyls are soluble in hydrocarbon solvents (4) Most of the
mononuclear carbonyls are colorless or lightly coloured, but polynuclear carbonyls are coloured
Chemical Properties: [1] Most metal carbonyls can be reduced to metal carbonylates (anionic form)
[2] Some
me metal carbonyls disproportionate in the presence of a strongly basic ligand, producing the ligated cation and a
carbonylate anion [3] Most organometallic carbonyl compounds can be protonated at the metal centre
[4] Metal carbonyls are susceptible to oxidation by air; metal-metal
metal metal bonds undergo oxidative cleavage, but the rates of
oxidation vary (i) Co2(CO)8 reacts under ambient conditions, (ii) Fe(CO)5 and Ni(CO)4 are also easily oxidized
(their vapours forming explosive mixtures with air), (iii) But M(CO)6 (M = Cr, Mo, W) does not oxidize unless heated
[5] The C atom of CO is susceptible to attack by nucleophiles if it is attached to a metal atom that is electron
ele
poor
and the
he O atom of CO is susceptible to attack by electrophiles in electron-rich
electron
carbonyls
Synthesis of Polynuclear Carbonyls
[1] Thermal expulsion of CO from a metal carbonyl : The synthesis of [Co4(CO)12] by heating [Co2(CO)8]
[2] Diiron nonacarbonyl, Fe2(CO)9, is usually made by photolysis of Fe(CO)5
[3] Fe3(CO)12 is obtained by oxidation of [HFe(CO)4]- using MnO2
Fe2(CO)9
Bioinorganic Chemistry
The chemical elements essential to life forms can be divided into the following (i) Bulk elements: C, H, N, O, P, S
(ii) Macrominerals and ions: Na, K, Mg, Ca, Cl, PO43-, SO42- (iii) Trace elements: Fe, Zn, Cu (iv) Ultratrace elements
comprises of (a) Non-metals: F, I, Se, Si, As, B (b) Metals: Mn, Mo, Co, Cr, V, Ni, Cd, Sn, Pb, Li
+
Na and K
Mg2+
Ca2+
Si (silicate)
P (phosphate)
S and O
Se
FClI
Fe
Cu
Zn
V
Cr
Mn
Co
Ni
Mo
Porphyrins
One of the most important groups of compounds is the porphyrins, in which a metal ion is surrounded by the four
nitrogens of a porphine ring in a square-planar geometry and the axial sites are available for other ligands. Different side
chains, metal ions, and surrounding species result in very different reactions and roles for these compounds.
Oxygen transport
In the pulmonary alveoli, O2 is taken up by haemoglobin (Hb) and 1 L of blood can dissolve 200 ml of oxygen.
Simultaneously, hydrogencarbonate is converted to carbonic acid, which in turn is catalytically degraded into CO2 und
H2O (by the zinc enzyme carbonic anhydrase):
After transport of O2 by haemoglobin in the blood stream, the oxygen is transferred to tissue myoglobin (Mb). Mb has a
higher affinity to O2 than Hb.
Iron Porphyrins
Hemoglobin and Myoglobin: The best known iron porphyrin compounds are hemoglobin and myoglobin, oxygen
transfer and storage agents in the blood and muscle tissue, respectively. Each of us has nearly 1 kg of hemoglobin in our
body, picking up molecular oxygen in the lungs and delivering it to the rest of the body. Each hemoglobin molecule is
made up of four globin protein subunits, two and two . In each of these, the protein molecule partially encloses the
heme group, bonding to one of the axial positions through an imidazole nitrogen. The other axial position is vacant or has
water bound to it (the imidazole ring from histidine is too far from the iron atom to bond). When dissolved oxygen is
present, it can occupy this position, and subtle changes in the conformation of the proteins result. As one iron binds an
oxygen molecule, the molecular shape changes to make binding of additional oxygen molecules easier. The four irons
can each carry one O2, with generally increasing equilibrium constants: In hemoglobin, the Fe(I1) is about 70 pm out of
the plane of the porphyrin nitrogens in the direction of the imidazole nitrogen bonding to the axial position. When
oxygen bond to the sixth position, the iron becomes coplanar with the porphyrin, oxygen bonds at an angle of
approximately 130, also with considerable back bonding (as nearly that of Fe(II1) - O2-).
As soon as some oxygen has been bound to the molecule, all four irons are readily oxygenated. In a similar
fashion, initial removal of oxygen triggers the release of the remainder and the entire load of oxygen is delivered at the
required site. This effect is also favored by pH changes caused by increased CO2 concentration in the capillaries. As the
concentration of CO2 increases, formation of bicarbonate causes the pH to decrease and the increased acidity favors
release of O2 from the oxyhemoglobin, called the Bohr effect.
Myoglobin has only one heme group per molecule and serves as an oxygen storage molecule in the muscles. The
myoglobin molecule is similar to a single subunit of hemoglobin. Bonding between the iron and the oxygen molecule is
similar to that in hemoglobin, but the equilibrium is simpler because only one oxygen molecule is bound: When
hemoglobin releases oxygen to the muscle tissue, myoglobin picks it up and stores it until it is needed. The Bohr effect
and the cooperation of the four hemoglobin binding sites make the transfer more complete when the oxygen
concentration is low and the carbon dioxide concentration is high; the opposite conditions in the lungs promote the
transfer of oxygen to hemoglobin and the transfer of CO2 to the gas phase in the lungs. Myoglobin binds O2 more
strongly than the first O2 of hemoglobin.
Green Chemistry
Introduction
Green Chemistry is the use of chemistry techniques and methodologies that reduce or eliminate the use or
generation of feedstock, products, by-products, solvents, reagents, etc., that are hazardous to human health or the
environment. Green Chemistry is an approach to the synthesis, processing and use of chemicals that reduces risks to
humans and the environment. This approaches include new synthesis and processes as well as new tools for instructing
aspiring chemists how to do chemistry in a more environmentally benign (caring, kindly, gentle or compassionate)
manner.
In order to evaluate the greenness of a particular process attention must be paid in the first instance to issues
related to safety, health and protection of the environment, due to reactants (substrates and reagents), auxiliaries (mainly
solvents) and waste. While all elements of the lifecycle of a new chemical or process may not be environmentally benign,
it is however important to improve those stages where improvements can be made.
Definition
The term Green Chemistry is defined as -The invention, design and application of chemical products and
processes to reduce or to eliminate the use and generation of hazardous substances. Green Chemistry is defined as
environmentally benign chemical synthesis. Goal of Green Chemistry is to create better, safer chemicals while choosing
the safest, most efficient ways to synthesize them and to reduce wastes. Green chemistry is the sustainable practice of
chemical science and manufacturing within a framework of industrial ecology in a manner that is sustainable, safe, and
non-polluting, consuming minimum amounts of energy and material resources while producing virtually no wastes.
Principles of Green Chemistry
Green Chemistry aims to eliminate hazards right at the chemical design stage, then throughout the design,
production, use/reuse and disposal processes. Practitioners of Green Chemistry try hard to invent new chemical methods
that do not pollute and that minimize the consumption of energy and natural resources. In 1998, two US chemists, Dr.
Paul Anastas and Dr John Warner outlined Twelve Principles of Green Chemistry to demonstrate how chemical
production could respect human health and the environment while also being efficient and profitable.
1. It is better to prevent waste than to treat or clean up waste after it is formed: It is most appropriate to
carry out a synthesis by following a pathway so that formation of waste is minimum or absent. One type of waste product
common and often avoidable is the starting material or reagent that remains unreacted. The well known saying
Prevention is better than cure should be followed.
2. Synthetic methods should be designed to maximize the incorporation of all the materials used in
the process into the final product: A synthesis may generate significant amount of waste or by product, such a
synthesis, even though gives 100% yield, is not considered to be green synthesis. In order to find, if a particular reaction
is green, the concept of atom economy was developed by Berry Trost of Stanford University. This considers the amount
of stating materials incorporated into the desired final product. Thus by incorporation of greater amounts of the atoms
contained in the starting materials (reactants) in to the formed products, fewer waste by products are obtained.
3. Whenever practicable synthetic methodologies should be designed to use and generate a substance
that poses little or no toxicity to human health and the environment: Wherever practicable, synthetic
methodologies should be designed to use and generate substances that pose little or no toxicity to human health and the
environment. Redesigning existing transformations to incorporate less hazardous materials is at the heart of Green
Chemistry.
4. Chemical products should be designed to preserve efficiency of function while reducing toxicity:
The designing of safer chemical is now possible since the understanding of chemical toxicity. It is now fairly understood
that a correlation exist between chemical structure (presence of functional groups) and the existence of toxic effects.
The idea is to avoid the functionality related to the toxic effect.
5. The use auxiliary substances should be made unnecessary wherever possible and innocuous when
used: An auxiliary substance (e.g. solvents, separating agents) are used in the manufacture, processing at every step,
it is one that helps in manufacture of chemical, but does not become an integral part of the chemical. Major problem with
many solvents is their volatility that may damage human health and the environment. The problem of solvents has been
overcome by using such solvents which do not pollute the environment. Such solvents are known as green solvents.
Examples include liquid supercritical CO2, use near-critical water at higher temperatures where water behaves more like
organic solvent, ionic liquid water, or many new ionic liquids have been developed with a broad range of properties.
Even reactions have been conducted in solid state. Microwave technology can be used in some reactions to provide the
heat energy required to make the transformation go to completion .With microwave technology, reactions can take place
with less toxic reagents and in a shorter time, with fewer side reactions, all goals of Green Chemistry.
Microwave technology has also been used to create supercritical water that behaves more like an organic solvent and
could replace more toxic solvents in carrying out organic reactions. Another Green Chemistry approach is the use of a
catalyst which facilitates transformations without the catalyst being consumed in the reaction and without being
incorporated in the final product. Therefore, use of catalyst should be preferred whenever possible.
6. Energy requirements should be recognized for their environmental and economic impacts and
should be minimized: Energy generation, as we know has a major environmental effect. The requirement of energy can
be kept to a base minimum in certain cases by the use of a catalyst. It is now possible that the energy to a reaction can be
supplied by using microwaves, by sonication or photo chemically.
7. A raw material or feedstock should be renewable rather than depleting, whenever technically and
economically practicable: Non reversible or depleting sources can exhaust by their continual use. So these are not
regarded as sustainable from environmental point of view. The starting materials which are obtained agricultural or
biological processes are referred to as renewable starting materials. Substances like carbon dioxide (generated from
natural sources or synthetic routes like fermentation) and methane gas (obtained from natural sources such as marsh gas,
natural gas, etc) are available in reasonable amounts and so are considered as renewable starting material.
Methane, a constituent of biogas and natural gas can easily be converted into acetylene by partial combustion.
Acetylene is a potential source of number of chemicals such as ethyl alcohol, acetaldehyde, vinyl acetate, etc.
8.
Unnecessary
derivatization
(blocking
group,
protection,
deportation,
10. Chemical products should be designed so that at the end of their function they
do not persist in the environment and break down into innocuous degradation products:
It is extremely important that the products designed to be synthesized should be biodegradable. They should not be
persistent chemicals or persistent bio accumulators. It is now possible to place functional groups in a molecule that will
facilitate its biodegradation. Functional groups which are susceptible to hydrolysis, photolysis or other cleavage have
been used to ensure that products will be biodegradable. It is also important that degradation products do not possess any
toxicity and detrimental effects to the environment.
11.
in
Analytical
process
methodologies
monitoring
and
need
control
to
prior
be
to
further
the
developed
formation
to
of
allow
for
hazardous
real
time,
substances:
Methods and technologies should be developed so that the prevention or minimization of generation of hazardous waste
is achieved. It is necessary to have accurate and reliable reasons, monitors and other analytical methodologies to assess
the hazardous that may be present in the process stream. These can prevent any accidents which may occur in chemical
plants.
12. Substances and the form of a substance used in a chemical process should be chosen
so as to minimize the potential for chemical accidents, including releases, explosions and fires:
The occurrence of accidents in chemical industry must be avoided. It is well known that the incidents in Bhopal (India)
and Seveso (Italy) and many others have resulted in the loss of thousands of life. It is possible sometimes to increase
accidents potential inadvertently with a view to minimize the generation of waste in order to prevent pollution.
It has been found that in an attempt to recycle solvents from a process (for economic reasons) increases the potential for a
chemical accident or fire.
The principles of green chemistry and some examples of their applications to basic and applied
research are illustrated below:
Prevention of Waste: It is better to prevent waste than to treat or clean up waste after it is formed.
The ability of chemists to redesign chemical transformations to minimize the generation of hazardous waste is an
important first step in pollution prevention.
Maximize Atom Economy: Atom Economy is a concept that evaluates the efficiency of a chemical
transformation, and is calculated as a ratio of the total mass of atoms in the desired product to the total mass of atoms in
the reactants. Choosing transformations that incorporate most of the starting materials into the product are more efficient
and minimize waste. The examples are
[1] DielsAlder reaction is 100%. Atom Economy reaction as all the atoms of the reactants are incorporated in
the cycloadduct.
[2] Disinfection of water by chlorination. Chlorine oxidizes the pathogens there by killing them, but at the same
time forms harmful chlorinated compounds. A remedy is to use another oxidant, such as O3 or supercritical water
oxidation.
[3] Production of allyl alcohol CH2=CHCH2OH. Traditional route: Alkaline hydrolysis of allyl chloride, which
generates the product and hydrochloric acid as a by-product. CH2=CH-CH2-Cl + H2O CH2=CH-CH2-OH +HCl
Greener route, to avoid chlorine: Two-step using propylene (CH2=CHCH3), acetic acid (CH3COOH) and oxygen (O2).
Less Hazardous Chemical Syntheses: Synthetic methodologies should be designed to use and generate
substances that possess little or no toxicity to human health and environment. Some toxic chemicals are replaced by safer
ones for a green technology, when reagent choices exist for a particular transformation. This principle focuses on
choosing reagents that pose the least risk and generate only benign by-products.
Production of styrene (=benzene ring with CH=CH2 tail). Traditional route: Two-step method starting with
benzene, which is carcinogenic) and ethylene to form ethylbenzene, followed by dehydrogenation to obtain styrene.
Greener route: To avoid benzene, start with xylene (cheapest source of aromatics and environmentally
safer than benzene).
Phosgene, COCl2, is commonly used as a starting material for polycarbonate. Phosgene is a highly toxic
substance, and the by-products of many of its reactions are undesirable. A superior alternative might be
dimethyl carbonate
Polycarbonate Synthesis: Phosgene Process [phosgene is highly toxic, corrosive]
Designing Safer Chemicals for Accident Prevention: New products can be designed that are inherently safer for
the target application. Pharmaceutical products often consist of chiral molecules, and the difference between the two
forms can be a matter of life and death for example, racemic thalidomide when administered during pregnancy, leads to
horrible birth defects in many new borns. Evidence indicates that only one of the enantiomers has the curing effect while
the other isomer is the cause of severe defects. That is why it is vital to be able to produce the two chiral forms
separately. Catalysts that can catalyse important reactions that produce only one of the two mirror image forms are
developed. Design chemicals and their forms (solid, liquid, or gas) to minimize the chemical accidents including
explosions, fires and releases to the environment, e.g., manufacture of gold atom nano particles used diborane (highly
toxic and bursts into flame near room temperature) and cancer-causing benzene. Now, diborane has been replaced by
environmentally benign NaBH4 which also eliminates the use of benzene. Nanoscience and nanotechnology is another
important contribution to green chemistry. Nanotechnology provides huge savings in materials by development of
microscopic and submicroscopic electronic and mechanical devices.
Classical way of synthesis
Key factors in Green Synthesis, namely: [1] Minimization of Waste [2] Alternative Feedstocks [3] Benign Reagents/Synthetic Pathways
[4] Synthetic Transformations [5] Solvents/Reaction Conditions [6] Minimization of Hazardous Products/Design of Safer Chemicals [7]
Minimization of Energy Consumption [8] Prevention of Chemical Accidents
[1] Minimization of Waste
Substitution of classical organic syntheses employing
stoichiometric amounts of inorganic reagents with cleaner,
catalytic alternatives
The concept of atom economy has been proposed to join the
concept of reaction efficiency based on the yields
The second opening review deals with life-cycle assessment
(LCA), a useful tool for waste management
[2] Alternative Feedstocks
The substance itself poses a hazard in the form of either
toxicity, accident potential, possible ecosystem damage, or
another form
Hazardous reagent will be required in order to carry out the
requisite synthetic transformation, then this factor also needs
to be considered in the selection process
Renewable versus a depleting feedstock
For existing environmental problems
Utilization of waste biomass as a chemical feedstock
Use of renewable resources for the production of chemicals is
represented by biocatalytic processes
The utilization of carbon dioxide in manufacturing processes
[3] Benign Reagents/Synthetic Pathways
An analysis of the reagent itself (hazards associated with a
particular reagent), as well as an analysis of the synthetic
transformation associated with the use of that reagent
(i.e., to determine product selectivity, reaction efficiency,
separation needs, etc.).
For the generation of more or less waste than other reagents
Both high selectivity (more R into P) and high conversion
(higher yield) must be achieved in order for a synthetic
transformation to generate little or no waste
[4] Synthetic Transformations
Hazardous properties of all substances necessarily being
generated from the transformation, just as it is important to
evaluate the hazardous properties of all starting materials and
reagents that are added in a synthetic transformation
[5] Solvents/Reaction Conditions
The direct toxicity to humans is only one
aspect of the total hazards that solvents possess.
There are a number of environmental implications of the use
of large volumes of solvents
Alternative solvents includes supercritical fluids,
aqueous applications, ionic liquids, and solventless systems,
Polymers
There are several ways of classification of polymers based on some special considerations. The following are some of the
common classifications of polymers: [1] by Source [2] by Backbone of the chain [3] by Structure [4] by Composition
[5] by Mode of Polymerization [6] by Molecular force
Classification Based on Source: [1] Natural Polymers: These polymers are found in plants and animals. Examples are
proteins, cellulose, starch, resins and rubber. [2] Semi-synthetic Polymers: Cellulose derivatives as cellulose acetate (rayon) and
cellulose nitrate, etc. are the usual examples of this sub category. [3] Synthetic Polymers: A variety of synthetic polymers as
plastic (polythene), synthetic fibres (nylon 6,6) and synthetic rubbers (Buna - S) are examples of man-made polymers.
Classification Based on Backbone of the polymer chain: Organic and Inorganic Polymers: A polymer whose
backbone chain is essentially made of carbon atoms is termed as organic polymer. The atoms attached to the side valencies of the
backbone carbon atoms are, however, usually those of hydrogen, oxygen, nitrogen, etc. The majority of synthetic polymers are
organic. On the other hand, generally chain backbone contains no carbon atom is called inorganic polymers. Glass and silicone
rubber are examples of it.
Classification Based on Structure of Polymers: [1] Linear Polymers: These polymers consist of long and straight
chains. The examples are high density polythen, PVC, etc. Linear polymers are commonly relatively soft, often rubbery
substances, and often likely to soften (or melt) on heating and to dissolve in certain solvent. [2] Branched Polymers: These
polymers contain linear chains having some branches, e.g., low density polythene. [3] Cross-linked Polymers: These are usually
formed from bi-functional and tri-functional monomers and contain strong covalent bonds betweenvarious linear polymer chains,
e.g. vulcanized rubber, urea-formaldehyde resins, etc. Cross linked polymers are hard and do not melt, soften or dissolve in most
cases.
Linear Polymers
Branched Polymers
Cross-linked Polymers
Classification Based on Composition of Polymers: [1] Homopolymer: A polymer resulting from the polymerization of
a single monomer; a polymer consisting substantiallyof a single type of repeating unit. [2] Copolymer: When two different types
of monomers are joined in the same polymer chain, the polymer is called a copolymer.
Let's imagine now two monomers (A and B) made into a copolymer in many different ways. In an alternating copolymer,
the two monomers are arranged in an alternating fashion. In a random copolymer, the two monomers may following any order.
In a block copolymer, all of one type of monomers are grouped together, and all of the other are grouped together.
In graft copolymer, a block copolymer can be thought of as two homopolymers joined together at the ends: branched copolymers
with one kind of monomers in their main chain and another kind of monomers in their side chains.
Copolymerization: A heteropolymer or copolymer is a polymer derived from two (or more) monomeric species, as
opposed to a homopolymer where only one monomer is used. Copolymerization refers to methods used to chemically synthesize a
copolymer.
Commercially
relevant
copolymers
include
ABS
plastic,
SBR,
Nitrile
rubber,
styrene-acrylonitrile,
Addition Polymers: The addition polymers are formed by the repeated addition of monomer molecules possessing
double or triple bonds, e.g., the formation of polythene from ethene and polypropene from propene. However, the addition
polymers formed by the polymerisation of a single monomeric species are known as homopolymer, e.g., polythene.
The polymers made by addition polymerisation from two different monomers are termed as copolymers,
e.g.,
nylon
6,
is
formed
by
the
condensation
of
hexamethylene
diamine
with
adipic
acid.
It is also possible, with three functional groups (or two different monomers at least one of which is tri-functional), to have
long linkage sequences in two (or three) dimensions and such polymers are distinguished as cross linked polymers.
Classification Based on Molecular Forces: The mechanical properties of polymers are governed by intermolecular
forces, e.g., van der Waals forces and hydrogen bonds, present in the polymer. These forces also bind the polymer chains. Under
this category, the polymers are classified into the following groups on the basis of magnitude of intermolecular forces present in
them. They are (i) Elastomers (ii) Fibers (iii) Plastics [(a) Thermoplastic and (b) thermosetting plastic].
Elastomers: These are rubber like solids with elastic properties. In these elastomeric polymers, the polymer chains are
held together by the weakest intermolecular forces. These weak binding forces permit the polymer to be stretched.
A few crosslinks are introduced in between the chains, which help the polymer to retract to its original position after the force is
released as in vulcanised rubber. The examples are buna-S, buna-N, neoprene, etc.
Fibers: If drawn into long filament like material whose length is at least 100 times its diameter, polymers are said to have
been converted into fibre. Fibres are the thread forming solids which possess high tensile strength and high modulus. These
characteristics can be attributed to the strong intermolecular forces like hydrogen bonding. These strong forces also lead to close
packing of chains and thus impart crystalline nature. Examples are polyamides (nylon 6, 6), polyesters (terylene), etc.
Plastics: A polymer is shaped into hard and tough utility articles by the application of heat and pressure; it is used as a
plastic. Typical examples are polystyrene, PVC and polymethyl methacrylate. They are two types (a) thermoplastic and
(b) thermosetting plastic.
Thermoplastic Polymers: Some polymers soften on heating and can be converted into any shape that they can retain on
cooling. The process of heating, reshaping and retaining the same on cooling can be repeated several times. Such polymers, that
soften on heating and stiffen on cooling, are termed thermoplastics. These are the linear or slightly branched long chain
molecules capable of repeatedly softening on heating and hardening on cooling. These polymers possess intermolecular forces of
attraction intermediate between elastomers and fibres. Polyethylene, PVC, examples of thermoplastic polymers.
Thermosetting Polymers: Some polymers, on the other hand, undergo some chemical change on heating and convert
themselves into an infusible mass. They are like the yolk of egg, which on heating sets into a mass, and, once set, cannot be
reshaped. Such polymers, that become infusible and insoluble mass on heating, are called thermosetting polymers.
These polymers are cross linked or heavily branched molecules, which on heating undergo extensive cross linking in moulds and
again become infusible. These cannot be reused. Some common examples are bakelite, urea-formaldelyde resins, etc.
Types of Polymerization:
Polymerization: There are four types of polymerisation reactions; (a) Addition or chain growth
polymerisation (b) Condensation or step growth polymerisation and (c) Copolymerization
Addition Polymerisation: In this type of polymerisation, the molecules of the same monomer or different monomers add
together on a large scale to form a polymer. The monomers normally employed in this type of polymerization contain a carboncarbon double bond (unsaturated compounds, e.g., alkenes and their derivatives) that can participate in a chain reaction.
A chain reaction consists of three stages, Initiation, Propagation and Termination.
In the Initiation step an initiator molecule is thermally decomposed or allowed to undergo a chemical reaction to generate
an "active species." This "active species," which can be a free radical or a cation or an anion, then initiates the polymerization by
adding to the monomer's carbon-carbon double bond. The reaction occurs in such a manner that a new free radical or cation or
anion is generated. The initial monomer becomes the first repeat unit in the incipient polymer chain. In the Propagation step,
the newly generated "active species" adds to another monomer in the same manner as in the initiation step. This procedure is
repeated over and over again until the final step of the process, termination, occurs. In the Termination step, the growing chain
terminates through reaction with another growing chain, by reaction with another species in the polymerization mixture,
or by the spontaneous decomposition of the active site. Under certain conditions, anionic can be carried out without the
termination step to generate so-called "living" polymers.
The mechanism of addition polymerisation can be divided broadly into two main classes, free radical polymerization and
ionic polymerization, although there are some others. Ionic polymerisation was probably the earliest type to be noted, and is
divided into cationic and anionic polymerisations.
Free radical polymerization: A variety of alkenes or dienes and their derivatives are polymerised in the presence of a
free radical generating initiator (catalyst) like benzoyl peroxide, acetyl peroxide, tert-butyl peroxide, etc. A free radical may be
defined as an intermediate compound containing an odd number of electrons, but which do not carry an electric charge and are not
free ions. For example, the polymerization of ethene to polythene consists of heating or exposing to light a mixture of ethene with
a small amount of benzoyl peroxide initiator.
The first stage of the chain reaction is the initiation process; this process starts with the addition of phenyl free radical
formed by the peroxide to the ethene double bond thus generating a new and larger free radical. The second stage of the chain
reaction is the propagation process, the radical reacts with another molecule of ethene, and another bigger sized radical is formed.
The repetition of this sequence with new and bigger radicals carries the reaction forward and the step is chain propagating step.
The final stage of the chain reaction is the termination process; the product radical formed reacts with another radical to form the
polymerised product.
Initiation
Ionic
Propagation
Termination
Polymerisation: The addition polymerization that takes place due to ionic intermediate is called ionic
polymerization. Based on the nature of ions used for the initiation process ionic polymerization classified into two types; (a)
Cationic polymerization and (b) Anionic polymerization
Cationic polymerization depends on the use of cationic initiators which include reagents capable of providing positive
ions or H+ ions. Typical examples are aluminium chloride with water (AlCl3+H2O) or boron trifluoride with water (BF3+H2O).
They are effective with monomers containing electron releasing groups like methyl (-CH3) or phenyl (-C6H5) etc.
They include propylene (CH3CH=CH2) and the styrene (C6H5CH=CH2). (i) Chain Initiation: Decomposition of the initiator is
shown as BF3 + H2O H+ + BF3(OH). The proton (H+) adds to C C double bond of alkene to form stable carbocation.
(ii) Chain Propagation: Carbocation add to the C C double bond of another monomer molecule to from new carbocation.
(iii) Chain Termination: Reaction is terminated by combination of carbocation with negative ion (or) by loss of proton
Initiation
Propagation
Termination
Anionic polymerization depends on the use of anionic initiators which include reagents capable of providing negative
ions. Typical catalysts include sodium in liquid ammonia, alkali metal alkyls, Grignard reagents and triphenylmethyl sodium
[(C6H5)3C-Na]. They are effective with monomers containing electron withdrawing groups like nitrile (CN) or chloride (-Cl), etc.
They include acrylonitrile [CH2=C(CN)], vinyl chloride [CH2=C(Cl)], methyl methacrylate [CH2=C(CH3)COOCH3], etc.
(i) Chain Initiation: Potassium amide (K+NH2-) adds to C C double bond of alkene to form stable carbanion.
(ii) Chain Propagation: Carbanion adds to the C C double bond of another monomer molecule to from new carbanion.
(iii) Chain Termination: Reaction is terminated by combination of carbanion with cation.
Propagation
Termination
Figure. 1
Figure. 3
Figure. 2
Figure. 4
Figure. 5
Figure. 6
Condensation Polymerisation: This type of polymerisation generally involves a repetitive condensation reaction
(two molecules join together, resulting loss of small molecules) between two bi-functional monomers. These polycondensation
reactions may result in the loss of some simple molecules as water, alcohol, etc., and lead to the formation of high molecular mass
condensation polymers. In these reactions, the product of each step is again a bi-functional species and the sequence of
condensation goes on. Since, each step produces a distinct functionalised species and is independent of each other; this process is
also called as step growth polymerisation. The type of end polymer product resulting from a condensation polymerization is
dependent on the number of functional end groups of the monomer which can react.
Monomers with only one reactive group terminate a growing chain, and thus give end products with a lower molecular
weight. Linear polymers are created using monomers with two reactive end groups and monomers with more than two end groups
give three dimensional polymers which are cross linked. Polyester is created through ester linkages between monomers, which
involve the functional groups carboxyl and hydroxyl (an organic acid and an alcohol monomer). The formation of polyester like
terylene or dacron by the interaction of ethylene glycol and terephthalic acid is an example of this type of polymerisation.
Polyamide is created through amide linkages between monomers, which involve the functional groups carboxyl and amine
(an organic acid and an amine monomer). Nylon-6 is an example which can be manufactured by the condensation polymerisation
of hexamethylenediamine with adipic acid under high pressure and at high temperature.
This type of polymerization normally employs two difunctional monomers that are capable of undergoing typical organic
reactions. For example, a diacid can be allowed to react with a diol in the presence of an acid catalyst to afford polyester. In this
case, chain growth is initiated by the reaction of one of the diacid's carboxyl groups with one of the diol's hydroxyl groups. The
free carboxyl or hydroxyl group of the resulting dimer can then react with an appropriate functional group in another monomer or
dimer. This process is repeated throughout the polymerization mixture until all of the monomers are converted to low molecular
weight species, such as dimers, trimers, tetramers, etc. These molecules, which are called oligomers, can then further react with
each other through their free functional groups. Polymer chains that have moderate molecular weight can he built in this manner.
Copolymerization: It is a polymerisation reaction in which a mixture of more than one monomeric species is allowed to
polymerize and form a copolymer. The copolymer can be made not only by chain growth polymerisation but by step growth
polymerisation also. It contains multiple units of each monomer used in the same polymeric chain. For example, a mixture of
styrene and 1, 3 butadiene can form a copolymer called styrene butadiene rubber (SBR) and a mixture of acrylonitrile and
1, 3 butadiene can form a copolymer called nitrile rubber (NBR)
Technology of Polymerization
Monomers may be polymerised by the following methods (1) polymerization in homogeneous systems (2) polymerization
in heterogeneous systems
Polymerization in Homogeneous systems: The homogeneous polymerization techniques involve pure monomer
or homogeneous solutions of monomer and polymer in a solvent. These techniques can be divided into two methods:
(i) the bulk and (ii) the solution polymerizations.
Bulk polymerization: Bulk polymerization is the simplest technique and produces the highest-purity polymers.
Only monomer, a monomer-soluble initiator are used. This method helps easy polymer recovery and minimum contamination of
product. The viscosity of the mixture is low initially to allow ready mixing, heat transfer, and bubble elimination.
This method is used for the preparation of polyethene, polystyrene, etc.
Disadvantages: Reaction medium becomes increasingly viscous as reaction goes to higher conversion, making stirring,
heat removal and processing more difficult. It leads to uneven polymerization and loss of monomer. Free-radical polymerizations
are typically highly exothermic. An increase temperature will increase the polymerization rate; generate heat dissipation and a
tendency to develop of localized hot spots. Near the end of polymerization, the viscosity is very high and difficult to control the
rate as the heat is trapped inside. It leads to the autoacceleration process in which the propagation rate is very higher than that of
termination rate. This method is seldom used in commercial manufacture.
Solution polymerization: This method is used to solve the problems associated with the bulk polymerization because the
solvent is employed to lower the viscosity of the reaction, thus help in the heat transfer and reduce autoacceleration.
It requires the correct selection of the solvents. Both the initiator and monomer be soluble in each other and that the solvent are
suitable for boiling points, regarding the solvent-removal steps. It is often used to produce copolymers. This method is used for the
preparation of polyvinyl acetate, poly (acrylic acid), and polyacrylamide.
Advantages: (i) Solvent has low viscosity, reaction mixture can be stirred (ii) Solvent acts as a diluent and aids in
removal of heat of polymerization (iii) Solvent reduces viscosity, making processing easier (iv) Thermal control is easier than in
the bulk and (v) Cheap materials for the reactors (stainless steel or glass lined).
Disadvantages: (i) Reduce monomer concentration which results in decreasing the rate of the reaction and the degree of
polymerization (ii) Mobility is reduced and this can affect termination events, so the rate of reaction is increased
(iii) Solvent may terminate the growing polymer chain, leading to low molecular weight polymers (iv) Difficult to remove solvent
from final form, causing degradation of bulk properties (v) Clean up the product with a non solvent or evaporation of solvent.
Polymerization in heterogeneous systems: Polymerization occurs in disperse phase as large particles in water or
occasionally in another non-solvent (suspension polymerisation), or dispersed as fine particles. The last-named process is usually
known as emulsion polymerisation.
Suspension (Bead or Pearl) polymerisation: Monomer, initiator (must soluble in monomer) and polymer must be
insoluble in the suspension media such as water i.e., the reaction mixture is suspended as droplets in an inert medium.
Suspension polymerization consists of an aqueous system with monomer as a dispersed phase and results in polymer as a
dispersed solid phase. This method is used for the preparation of polystyrene, polyvinyl chloride, polyvinyl acetate, etc.
A reactor fitted with a mechanical agitator is charged with a water insoluble monomer and initiator. Droplets of monomer
(containing the initiator) are formed. As the polymerization proceeds, the viscosity of dispersed phase increases and they become
sticky. Aggregation of these sticky droplets is prevented by the addition of a dispersing agent (protective colloid,
e.g., water-soluble colloid such as gum acacia). Near the end of polymerization, the particles are hardened, are the bead or pearl
shaped polymers recovered by filtration, and followed by washing step.
Advantages: (i) Polymerisation to high conversion (ii) Low viscosity due to the suspension (iii) Easy heat removal due
to the high heat capacity of water (iv) Excellent heat transfer because of the presence of the solvent (v) Solvent cost and recovery
operation are cheap. Disadvantages: (i) Contamination by the presence of suspension and other additives low polymer purity
(ii) Must separate and purify polymer, or accept contaminated product.
Emulsion polymerisation: An emulsion polymerization consists of water (as the heat-transfer agent), monomer, initiator
(is soluble in water and insoluble in the monomer), a surfactant or emulsifier (such as sodium salt of long-chain fatty acid).
This method is used for the preparation of polyvinyl acetate, polychloroprene, butadiene/styrene/acrylonitrile copolymers, etc.
A typical recipe for emulsion polymerization consists of water, monomer, fatty acid soap (emulsifying agent), and water
soluble initiator. When a small amount of soap is added to water, the soap ionizes and the ions move around freely.
The soap anion consists of a long oil-soluble portion (R) terminated at one end by the water-soluble portion. So emulsifier
molecules arrange themselves into colloidal particles called micelles. In water containing a insoluble monomer molecule, the soap
anion molecules orient themselves at the watermonomer interfaces with the hydrophilic ends facing the water, while the
hydrophobic ends face the monomer phase. When the water-soluble initiator undergoes thermal decomposition to form the watersoluble radicals react with monomer dissolved in interior of the micelle. Emulsion polymerization takes place almost exclusively
in the micelles. As polymerization proceeds, the active micelles consume the monomers within the micelle. Monomer depletion
within the micelle is replenished first from the aqueous phase and subsequently from the monomer droplets. The active micelles
grow in size with polymer formation, to preserve their stability; these growing polymer particles absorb the soap of the parent
micelles. Advantages: (i) Overcomes many environmental problems: solvent is water (ii) If final desired product is polymer is
washed with water to remove the soap phase by coagulation.
Suspension
Emulsion
Physical properties of polymers: [1] Strength [2] Deformation (Plastic and Elastic) [3] Chemical Resistance and Solubility,
[4] Physical state of Polymers (Crystalline, Amorphous and Semicrystaline) and [5] Effect of Heat (Glass Transition Temperature)
[1] Strength: It depends upon the magnitude of force of attraction between polymeric chains. Two types: (a) Primary or
chemical bond and (b) Secondary or intermolecular forces (van der Wall force or hydrogen bonding). In cross-linked polymers,
all chains are interconnected by strong chemical covalent bond, resulting in a giant solid molecule, extending in three dimensions.
So they are strong and tough materials, since the movement of intermolecular chains are totally restricted. In linear or branched
polymers, the chains are held together by weak intermolecular force of attraction (secondary), strength increases with increase in
chain length or molecular weight. Polymers of lower chain length are soft and gummy, while higher chain polymers are hard and
strong. Strength of the polymer can be increased by increasing the intermolecular force by the introduction of groups like carboxyl,
hydroxyl, chlorine, fluorine, nitrile along the chain.
[2] Deformation: Deformation is the slipping of one chain over the other (on the application of heat or pressure or both)
or stretching and recoverance of original shape of the polymeric chains (after the removal of stress). Two types: Plastic
Deformation (Plasticity) and Elastic Deformation (Elasticity).
Plastic Deformation (Plasticity) is the slipping of one chain over the other on the application of heat or pressure or both.
It occurs only when the weak secondary intermolecular force is operating between the polymeric chains, when sufficient load is
applied permanent deformation occurs as slippage. The linear or branched polymers show the greatest degree of plastic
deformation. This type of material, in heated state, readily takes the shape of the mould, when it is injected into under pressure,
called thermoplastic. At high temperature, polymers deform easily due to the weakening of secondary intermolecular force between
chains so the chains can easily slip over each other. On cooling, the polymer becomes rigid in the moulded shape,
because plasticity decreases with fall of temperature. So plasticity of the polymer is reversible.
Elastic Deformation (Elasticity) is the stretching and recoverance of original shape of the polymeric chains after the
removal of stress. It arises from the fact that long polymeric chains having free rotating groups which assume peculiar
configuration of irregularly coiled and entangled snarls in unstressed condition, lead to amorphous state of polymer.
When such a polymer is stretched, the snarls begin to disentangle (like a spring) and straighten out, which in turn enhances the
attraction force between chains, thereby causing stiffening of polymer. When stress is released, the stretched snarls return to their
original arrangement.
[3] Chemical Resistance and Solubility: Chemical attack is internal, causing softening, swelling and loss of strength of
polymer. The chemical nature of monomeric units and their molecular arrangement determines the chemical resistance of the
polymer. Polymers having polar groups (-OH, COOH, or Cl) swollen or even dissolved in polar solvents whereas polymers
having non-polar groups (-CH3 or -C6H5) swollen or even dissolved in non polar solvents. Polymers of more aliphatic character are
more soluble in aliphatic solvents whereas polymers of more aromatic character are more soluble in aromatic solvents.
The tendency to swell or solubility of polymers decreases with the increase in chain length or molecular weight of polymer.
In crystalline polymers, denser close packing of polymeric chains makes the penetration of solvents or chemical reagents in the
polymeric material more difficult, so crystalline polymers exhibit more chemical resistance or lesser solubilty. Greater the degree
cross-linking in the polymer, lesser is its solubility and greater is its chemical resistance.
Physical state of Polymers: Relative arrangement of polymeric chains with respect to each other may result in an
amorphous or crystalline state of polymer. An amorphous state is characterized by a completely random, irregular, and
dissymmetrical arrangement of polymeric chains e.g., rubber, thermosetting. A crystalline state is characterized by a completely
regular, symmetrical and ordered arrangement of polymeric chains with uniaxial orientation e.g., fibers. A semi-crystalline state
consists of crystalline region called crystallites (ordered arrangement of polymeric chains) embedded in an amorphous matrix, e.g.,
thermoplastic.
[4] Crystallinity in Polymers: The degree to which chains of a polymer are arranged in orderly pattern with respect to each
other, is a measure of crystallinity. As solidification begins, the viscosity of the polymer rises, and the chains find more and more
difficulty in arranging their long chains in the regular pattern needed for crystal formation. Some parts of structure align during
cooling to form crystalline regions (places where the polymer chains are orderly arranged, they align along side with each other,
they lie in close proximity and are held together by strong intermolecular interactions) called crystallites, around crystallites get
amorphous regions (places where the polymer chains are randomly arranged, resulting in weak intermolecular interactions).
In fact polymers have regions of crystallinity, called crystallites, embedded in an amorphous matrix. The crystalline material shows
a high degree of order formed by folding and stacking of the polymer chains. An amorphous solid is formed when the chains have
little orientation throughout the bulk polymer; the chains are tangled and organized in disordered pattern. There are some polymers
that are completely amorphous, but most are combinations with the tangled and disordered regions (amorphous regions)
surrounding the crystalline areas called semicrystaline. In such polymers, the crystallites provide required hardness, toughness,
rigidity and heat resistance, whereas the amorphous matrix provides flexibility to a polymer
Crystallinity Vs Properties: Crystallization imparts a denser packing of chains, thereby increasing the intermolecular forces
of attraction, so higher and sharper softening point, greater rigidity and strength, and greater density of the crystalline polymer.
With increase in % of crystallinity - (a) Strength and stiffness of polymer increases but brittleness also increases (b) Solubility and
permeability decreases but chemical resistance increases (c) Density and melting point increases (d) Opacity of the polymer
increases. Short chains polymers are generally weaker in strength, although they are crystalline, only weak van der Waals forces
hold the lattice together; this allows the crystalline layers to slip past one another causing a break in the material. High chain length
(amorphous) polymers, however, have greater strength because the molecules become tangled between layers.
Factors affecting Crystallinity: [i] Polymer Morphology [ii] Chain length or Molecular weight [iii] Symmetry of the
repeating unit [iv] Chain branching [v] Cross-linking [vi] Rate of cooling
[i] Polymer Morphology: The polymer morphology is the size and shape of the monomers' substituent groups.
If the monomers are large and irregular, it is difficult for the polymer chains to arrange themselves in an ordered manner, resulting
in a more amorphous solid. Likewise, smaller monomers, and monomers that have a very regular structure (e.g. rod-like) will form
more crystalline polymers. Polymers having bulky side groups are attached at random to the main chain are typically amorphous.
[ii] Chain length or Molecular weight: The crystallization tendency of a polymer depends on the ease with which the chains
can be aligned in an orderly arrangement. In the crystallization process, it has been observed that relatively short chains organize
themselves into crystalline structures more readily than longer molecules. Polymers with a high chain length have difficulty
organizing into layers because they tend to become tangled - Large number of entanglement of polymeric chains which impose
restriction to the growth of crystallites. Linear long chain polymers do not form crystalline solids because their long chains prevent
efficient packing in a crystal lattice. Therefore, the chain length is an important factor in determining the Crystallinity of a polymer.
With increase in chain length or molecular weight of the polymer chain, % of crystallinity decreases.
[iii] Symmetry of the repeating unit: Symmetrical repeating unit structure facilitates the formation of crystallites.
Polymer with a low degree of symmetry does not crystallize easily, so they form amorphous structures, crystallization tendency
decreases by copolymerization, because it lowers structural symmetry. Random copolymers do not crystallize due to the
irregularity of the repeating unit. Geometrical regularity is also desired in a polymer for it to show crystallinity and only the
configurational regular forms of polymers crystallize.
[iv] Chain branching: In branched polymers, branching prevents chains from packing closely, so they are softer, too.
HDPE has almost perfectly linear structure and therefore it can be obtained in a highly crystalline state (80-85% crystallinity).
LDPE has a number of short chain and long chain branches, so it can only achieve 55% crystallinity.
[v] Cross-linking: A polymer with highly cross linked structure is devoid of crystallinity due to the presence of a dense
array of cross-links effectively eliminates crystallinity. But a few cross links may improve the crystallinity obtained on stretching
of a polymer, since orientation is increased by restricting the flow of polymer chains.
[vi] Rate of cooling: The cooling rate also influences the amount of Crystallinity. Slow cooling provides time for greater
amounts of crystallization to occur. Fast rates, on the other hand, such as rapid quenches, yield highly amorphous polymers.
Average cooling provides semi-crystalline polymers.
[5] Effect of Heat on Polymers: The thermal phase behavior of polymers differs markedly from that of common low
molecular weight compounds. Amorphous polymers do not possess any clear melting point, but crystalline polymers have clear
melting point. Highly crystalline polymers undergo first order transition from solid to liquid state (melt) at sharp melting point.
Amorphous and semicrystaline polymers undergo second order transition from solid to viscous fluid at a broader range of
temperature. Amorphous polymers do not melt but soften. At low temp, polymer behave as glassy materials where there chains
cannot move all, on heating, the polymers soften and becomes more flexible. In this state, polymer chains gain sufficient energy to
move slightly, this occurs at the glass transition temperature (Tg). Beyond Tg, amorphous and semi-crystalline polymers behave
differently. In amorphous or semicrystalline polymers, polymeric chains are disorderly arranged due to the entanglement of chains,
which prevent the close packing of polymeric chains, so fee volume increases, hence the internal movement of polymeric chains
becomes easy, so they have Tg. In semicrystalline polymers, amorphous region shows Tg and crystallites show Tm.
In crystalline polymers, polymeric chains are orderly arranged, which lead to dense close packing of polymeric chains, so fee
volume becomes zero, hence the internal movement of polymeric chains is prevented, so the crystalline polymers have no Tg. In
cross linked polymers, polymeric chains are connected through cross links or chemical bond, so the internal movement of
polymeric chains is prevented, so the they have infinite value of Tg.
The glass transition temperature (Tg) is the temperature at which the internal energy of the chains of the polymer increases
such as extends that the chains just starts leaving their lattice sites. Below glass transition temperature (Tg),
polymers are usually hard, brittle and glass-like in mechanical behavior. Above glass transition (Tg), polymers are usually more
soft, flexible and rubbery (elastic)-like in mechanical behavior. Tg is the temperature at which transformation of a polymer from a
rigid material to one that has rubber like characteristics and temperature has large effect on chain flexibility.
Factors affecting glass transition temperature: [1] Chain Flexibility [2] Geometric Factors [3] Inter-chain Attractive Forces
[4] Copolymerization [5] Chain Length [6] Cross-Linking and Branching [7] Crystallinity [8] Plasticization
[1] Chain Flexibility: Chain flexibility is determined by the ease with which rotation occurs about primary valence bonds.
Polymers with low hindrance to internal rotation have low Tg values. Long-chain aliphatic groups ether and ester linkages
enhance chain flexibility, while rigid groups like cyclic structures stiffen the backbone.
[2] Geometric Factors: Geometric factors, such as the symmetry of the backbone and the presence of double bonds on the
main chain, affect Tg. Polymers that have symmetrical structure have lower Tg than those with asymmetric structures. Additional
groups near the backbone for the symmetrical polymer would enhance steric hindrance and consequently raise Tg.
Double bonds in the cis form reduce the energy barrier for rotation of adjacent bonds, soften the chain, and hence reduce Tg.
[3] Inter-chain Attractive Forces: The presence of strong secondary attractive forces in a polymer chain, i.e., a high value
of cohesive energy density, will significantly increase Tg. The steric effects of the groups like CH3, Cl, and CN are similar,
but the polarity increases, consequently, Tg is increased. Secondary bonding forces are effective only over short molecular
distances. Therefore, any structural feature that tends to increase the distance between polymer chains decreases the cohesive
energy density and hence reduces Tg.
[4] Copolymerization: It is desirable to be able to control Tg, however, this is often impossible, polymer chemists have
circumvented this problem to some extent by copolymerization. A copolymer system may be characterized by the arrangement of
the different monomers (random, alternating, graft, or block). The increased disorder resulting from the random or alternating
distribution of monomers enhances the free volume and consequently reduces Tg.
[5] Chain Length: Since chain end segments are restricted only at one end, they have relatively higher mobility than the
internal segments, which are constrained at both ends. At a given temperature, therefore, chain ends provide a higher free volume
for molecular motion. As the number of chain ends increases (means short chain polymers), the available free volume increases,
and consequently there is a depression of Tg.
[6] Cross-Linking and Branching: By definition, cross-linking involves the formation intermolecular connections through
chemical bonds; this process necessarily results in reduction in chain mobility. Consequently, Tg increases. For lightly cross-linked
systems like vulcanized rubber, Tg shows a moderate increase over the uncross-linked polymer. For highly cross-linked systems
like phenolics and epoxy resins, the glass transition is virtually infinite. Like long and flexible side chains, branching increases the
separation between chains, enhances the free volume, and therefore decreases Tg.
[7] Crystallinity: In semicrystaline polymers, the crystallites may be regarded as physical cross-links that tend to reinforce
or stiffen the structure. Viewed this way, it is easy to visualize that Tg will increase with increasing degree of crystallinity.
[8] Plasticization: Plasticization is the process of inducing plasticity in a material. In polymers, this can be achieved by the
addition of low-molecular-weight organic compounds referred to as plasticizers which are usually nonpolymeric, organic liquids of
high boiling points and they are miscible with polymers. Addition of plasticizers to a polymer, even in very small quantities,
drastically reduces the Tg of the polymer. Plasticizers function through a solvating action by increasing intermolecular distance,
thereby decreasing intermolecular bonding forces.
Mechanical properties of polymers: For engineering applications of polymers, the designer using polymeric materials must
understand their mechanical behaviour with respect to the maximum permissible strains to avoid failure. A simple tensile stressstrain curve provides a good start towards understanding the mechanical behaviour of a particular polymer. This curve is usually
established by continuously measuring the force developed as the sample is elongated at constant rate of extension until it breaks.
Figure 1
Figure 2
From Figure 1, the initial slope provides a value for Young's modulus (or the modulus of elasticity) which is a measure of
stiffness. The curve also gives yield stress, strength and elongation at break, the area under the curve or work to break is a rough
indication of the toughness of the polymeric material. The stress at the knee in the curve (known as the yield point) is a measure of
the strength of the material and resistance to permanent deformation. The stress at the breaking point, commonly known as ultimate
strength, is a measure of the force required to fracture the material completely.
From Figure 2, Hard and Brittle polymers such as an amorphous polymer far below its Tg, usually has an initial
slope indicative of very high modulus, moderate strength, a low elongation at break, and a low area under the stress-strain
stress
curve,
e.g., polystyrene and phenol-formaldehyde
formaldehyde resins.
Hard and Strong polymers have high modulus of elasticity, high strength, and elongation at break of approximately
5 percent, e.g., thermoplastics like polyethene and polyvinyl chloride.
Hard and Tough polymers have high yield points, high modulus, high strengths and large elongations. e.g., polymer
fibers like cellulose acetate and nylons.
Soft and Tough polymers have low yield points, low modulus, moderate
moderate strength at break, and very high elongation
ranging from 20 to 100 percent, e.g., elastomers like Natural rubber, SBR, and NBR.
Important Mechanical Properties of Polymers: [1] Hardness [2] Toughness [3] Stiffness [4] Density
[5] Tensile Strength [6] Abrasion Resistance [7] Resilience [8] Wear and Tear
[1] Hardness: The ability of a polymer to resist scratching, abrasion, cutting, or penetration. It is measured by its ability
to absorb energy under impact loads.
oads. Hardness is associated with strength.
[2] Toughness: It is the amount of energy a polymer can absorb before actual fracture or failure takes place.
The ability of a polymer to withstand shock and vibrations. It is related
related to impact strength which is the resistance to breakage under
high velocity impact conditions, i.e., resistance to shock loading.
[3] Stiffness: The resistance of a polymer to elastic deformation, i.e., a polymer which suffers slight deformation under
load has a high degree of stiffness. Flexibility (has to do with bending)
bending is the opposite of stiffness.
[4] Density: Mass per unit volume (at define
definedd temperature) Relative Density is the mass of the polymer with the mass
of equal volume of a specific (reference) substance (water) Density is frequently measured as a quality control parameter.
Density of Polymer
[5] Tensile Strength: The strength of a polymer is its capacity to withstand destruction under the action of loads.
It determines the ability of a polymer to withstand stress without failure. Tensile strength or ultimate strength is the stress
stre
corresponding to the maximum load reached before rupturing the polymer.
[6] Abrasion Resistance: It is defined as the ability of a polymer to withstand mechanical action (such as rubbing,
scrapping, or erosion) that tends progressively to remove material from its surface. Abrasion is closely related to frictiona
frictional force,
load and true area of contact, an increase in any one of the three results in greater abrasion or wear. Abrasion process also creates
oxidation on the surface from the build up of localized high temperatures.
[7] Resilience: It is the capacity of a polymer to absorb energy elastically. Resilience gives capacity of the polymer to
bear shocks and vibrations. When a body is loaded, it changes its dimension, and on the removal of the load it regains its original
or
dimensions. In fact, the polymer behaves perfectly like a spring, so long as it remains loaded, it has stored energy in itself,
on removal of the load, the energy stored is given off exactly as in a spring when the load is removed.
[8] Wear and Tear: It occurs when a steady rate of increase in the use of polymers in bearing applications and
in situations where there is sliding contact e.g. gears, piston rings, seals, cams, etc. Wear and tear is characterized by fine
fi particles
of polymer being removed from the surface or the
the polymer becomes overheated to the extent where large troughs of melted
polymer are removed. The wear and tear of polymers is extremely complex subjects which depend markedly on the nature of the
application and the properties of the material. It is characterized
characterized by adhesion and deformation which results in frictional forces that
are not proportional to load but rather to speed. The mechanism of wear and tear is complex; the relative rates may change
depending on specific circumstance.
Lubricants
In all types of machines, the surfaces of moving or sliding or rolling parts rub against each other. Due to the
mutual rubbing of one part against another, a resistance is offered to their movement. This resistance is known as
friction. It causes a lot of wear and tear of surfaces of moving parts. Any substance introduced between two
moving/sliding surfaces with a view to reduce the friction (or frictional resistance) between them, is known as a
lubricants. The main purpose of a lubricant is to keep the moving/sliding surfaces apart, so that friction and consequent
destruction of material is minimized.
great
to
cause
deformation
of
the
peaks
to
create
weld
junctions
between
them.
(2) Mechanical Interlocking: When one surface moves over another, the peaks and valleys present on the surface
undergo interlocking; restrict the movement of one surface over the other. This accounts for static friction.
(3) Molecular Attraction: Atoms of one material are plucked out of the attractive range of their counterparts on the
mating surface, lead to the friction. (4) Electrostatic Attraction: When stick-slip phenomenon takes place between
rubbing metal surfaces, a net flow of electrons takes place producing clusters of charges of opposite polarity at the
interface. These charges are responsible for holding the surfaces together by electrostatic attraction.
Mechanism of Lubrication: The phenomenon of lubrication can be explained with the help of the following
mechanism; (a) Thick-Film lubrication (Fluid-Film or hydrodynamic lubrication) (b) Thin Film lubrication
(Boundary lubrication) and (c) Extreme Pressure lubrication
(a) Thick-Film lubrication: In this, moving/sliding surfaces are separated from each other by a thick film of
fluid (at least 1000 A thick), so that direct surface to surface contact and welding of welding of junctions rarely occurs.
The lubricant film covers/fills the irregularities of moving/sliding surfaces and forms a thick layer between them, so that
there is no direct contact between the material surfaces. This consequently reduces friction. The lubricant chosen should
have the minimum viscosity (to reduce the internal resistance between the particles of the lubricant) under working
conditions and at the same time, it should remain in place and separate the surfaces.
Hydrocarbon oils (mineral oils which are lower molecular weight hydrocarbons with about12 to 50 carbon
atoms) are considered to be satisfactory lubricants for thick-film lubrication. In order to maintain the viscosity of the oil
in all seasons of year, ordinary hydrocarbon lubricants are blended with selected long chain polymers.
(b) Thin Film lubrication: This type of lubrication is preferred where a continuous film of lubricant cannot
persist. In such cases, the clearance space between the moving/sliding surfaces is lubricated by such a material which can
get adsorbed on both the metallic surfaces by either physical or chemical forces. This adsorbed film helps to keep the
metal surfaces away from each other at least up to the height of the peaks present on the surface.
Vegetable and animal oils and their soaps can be used in this type of lubrication because they can get either
physically adsorbed or chemically react in to the metal surface to form a thin film of metallic soap which can act as
lubricant. Although these oils have good oiliness, they suffer from the disadvantage that they will break down at high
temperatures. On the other hand, mineral oils are thermally stable and the addition of vegetable/animal oils to mineral
oils, their oiliness can also be brought up. Graphite and molybdenum disulphide are also suitable for thin-film
lubrication.
(c) Extreme Pressure lubrication: When the moving/sliding surfaces are under very high pressure and speed, a
high local temperature is attained under such conditions, liquid lubricants fail to stick and may decompose and even
vaporize. To meet these extreme pressure conditions, special additives are added to minerals oils. These are called
extreme pressure additives. These additives form more durable films (capable of withstanding very high loads and high
temperatures) on metal surfaces. Important additives are organic compounds having active radicals or groups such as
chlorine (as in chlorinated esters), sulphur (as in sulphurized oils) or phosphorus (as in tricresyl phosphate).
These compounds react with metallic surfaces, at existing high temperatures, to form metallic chlorides, sulphides or
phosphides.
Classification of Lubricants: Lubricants are classified on the basis of their physical state, as follows;
(a) Liquid lubricants or Lubricating Oils, (b) Semi-solid lubricants or Greases and (c) Solid lubricants.
(a) Liquid lubricants or Lubricating oils: Lubricating oils also known as liquid lubricants. The characteristics
of good lubricating oils are: (1) high boiling point (2) low freezing point (3) adequate viscosity for proper functioning in
service (4) high resistance to oxidation and heat (5) non-corrosive properties and (6) stability to decomposition at the
operating temperatures. Lubricating oils are classified into three categories; (i) Animal and Vegetables oils,
(ii) Mineral or Petroleum oils and (iii) blended oils.
(i) Animal and Vegetables oils: Animal oils are extracted from the crude fat and vegetables oils such as cotton
seed oil and caster oils. These oils possess good oiliness and hence they can stick on metal surfaces effectively even
under elevated temperatures and heavy loads. But they suffer from the disadvantages that they are costly, undergo easy
oxidation to give gummy products and hydrolyze easily on contact with moist air or water. Hence they are only rarely
used these days for lubrication. But they are still used as blending agents in petroleum based lubricants to get improved
oiliness.
(ii) Mineral or Petroleum oils: These are basically lower molecular weight hydrocarbons with about 12 to 50
carbon atoms. As they are cheap, available in abundance and stable, hence they are widely used. But the oiliness of
mineral oils is less, so the addition of higher molecular weight compounds like oleic acid and stearic acid increases the
oiliness of mineral oil.
(iii) Blended oils: No single oil possesses all the properties required for a good lubricant and hence addition of
proper additives is essential to make them perform well. Such additives added lubricating oils are called blended oils.
Examples: The addition of higher molecular weight compounds like oleic acid, stearic acid, palmetic acid, etc or
vegetables oil like coconut oil, castor oil, etc increases the oiliness of mineral oil.
(b) Semi-solid Lubricants or Grease: A semi-solid lubricant obtained by combining lubricating oil with
thickening agents is termed as grease. Lubricating oil is the principal component and it can be either petroleum oil or a
synthetic hydrocarbon of low to high viscosity. The thickeners consist primarily of special soaps of Li, Na, Ca, Ba, Al,
etc. Non-soap thickeners include carbon black, silica gel, polyureas and other synthetic polymers, clays, etc.
Grease can support much heavier load at lower speed. Internal resistance of grease is much higher than that of lubricating
oils; therefore it is better to use oil instead of grease. Compared to lubricating oils, grease cannot effectively dissipate
heat from the bearings, so work at relatively lower temperature.
(c) Solid lubricants: They are preferred where (1) the operating conditions are such that a lubricating film
cannot be secured by the use of lubricating oils or grease (2) contamination (by the entry of dust particles) of lubricating
oils or grease is unacceptable (3) the operating temperature or load is too high, even for grease to remain in position and
(4) combustible lubricants must be avoided. They are used either in the dry powder form or with binders to make them
stick firmly to the metal surfaces while in use. They are available as dispersions in non-volatile carriers like soaps, fats,
waxes, etc and as soft metal films.
The most common solid lubricants are graphite, molybdenum disulphide, tungsten disulphide and zinc oxide.
They can withstand temperature upto 650 C and can be applied in continuously operating situations. They are also used
as additives to mineral oils and greases in order to increase the load carrying capacity of the lubricant.
Other solid lubricants in use are soapstone (talc) and mica.
Graphite: It is the most widely used of all the solid lubricants and can be used either in the powdered form or in
suspension. It is soapy to touch; non-inflammable and stable upto a temperature of 375 C. Graphite has a flat plate like
structure and the layers of graphite sheets are arranged one above the other and held together by weak van der Waals
forces. These parallel layers which can easily slide one over other make graphite an effective lubricant.
Also the layer of graphite has a tendency to absorb oil and to be wetted of it.
Molybdenum Disulphide: It has a sandwich-like structure with a layer of molybdenum atoms in between
two layers of sulphur atoms. Poor inter-laminar attraction helps these layers to slide over one another easily.
It is stable upto a temperature of 400 C.
Properties of Lubricants: (1) Viscosity (2) Flash Point and Fire Point (3) Cloud Point and Pour Point
(4) Aniline Point and (5) Corrosion Stability
(1) Viscosity: It is the property of liquid by virtue of which it offers resistance to its own flow (the resistance to
flow of liquid is known as viscosity). The unit of viscosity is poise. It is the most important single property of any
lubricating oil, because it is the main determinant of the operating characteristics of the lubricant. If the viscosity of the
oil is too low, a liquid oil film cannot be maintained between two moving/sliding surfaces. On the other hand,
if the viscosity of the oil is too high, excessive friction will result.
Effect of temperature on viscosity: Viscosity of liquids decreases with increasing temperature and, consequently,
the lubricating oil becomes thinner as the operating temperature increases. Hence, viscosity of good lubricating oil
should not change much with change in temperature, so that it can be used continuously, under varying conditions
of temperature.
The rate at which the viscosity of lubricating oil changes with temperature is measured by an
arbitrary scale, known as Viscosity Index (V. I). If the viscosity of lubricating oil falls rapidly as the temperature is
raised, it has a low viscosity index. On the other hand, if the viscosity of lubricating oil is only slightly affected on raising
the temperature, its viscosity index is high.
(2) Flash Point and Fire Point: Flash point is the lowest temperature at which the lubricant oil gives off enough
vapours that ignite for a moment, when a tiny flame is brought near it; while Fire point is the lowest temperature at which
the vapours of the lubricant oil burn continuously for at least five seconds, when a tiny flame is brought near it. In most
cases, the fire points are 5 C to 40 C higher than the flash points. The flash and fire do not have any bearing with
lubricating property of the oil, but these are important when oil is exposed to high temperature service.
A good lubricant should have flash point at least above the temperature at which it is to be used. This safeguard against
risk of fire during the use of lubricant.
(3) Cloud Point and Pour Point: When the lubricant oil is cooled slowly, the temperature at which it becomes
cloudy or hazy in appearance, is called its cloud point; while the temperature at which the lubricant oil cease to flow or
pour, is called its pour point. Cloud and pour points indicate the suitability of lubricant oil in cold conditions.
Lubricant oil used in a machine working at low temperatures should possess low pour point; otherwise solidification of
lubricant oil will cause jamming of machine. It has been found that presence of waxes in the lubricant oil raise
pour point.
(4) Aniline Point: Aniline point of the lubricant oil is defined as the minimum equilibrium solution temperature
for equal volumes of aniline and lubricant oil samples. It gives an indication of the possible deterioration of the lubricant
oil in contact with rubber sealing; packing, etc. Aromatic hydrocarbons have a tendency to dissolve natural rubber and
certain types of synthetic rubbers. Consequently, low aromatic content in the lubricant oil is desirable.
A higher aniline point means a higher percentage of paraffinic hydrocarbons and hence, a lower percentage of
aromatic hydrocarbons.
Aniline point is determined by mixing mechanically equal volumes of the lubricant oil samples and aniline in a
test tube. The mixture is heated, till homogenous solution is obtained. Then, the tube is allowed to cool at a controlled
rate. The temperature at which the two phases (the lubricant oil and aniline) separate out is recorded at the aniline point.
(5) Corrosion Stability: Corrosion stability of the lubricant oil is estimated by carrying out corrosion test.
A polished copper strip is placed in the lubricant oil for a specified time at a particular temperature. After the stipulated
time, the strip is taken out and examined for corrosion effects. If the copper strip has tarnished, it shows that the lubricant
oil contains any chemically active substances which cause the corrosion of the copper strip. A good lubricant oil should
not effect the copper strip. To retard corrosion effects of the lubricant oil, certain inhibitors are added to them.
Commonly used inhibitors are organic compounds containing P, As, Cr, Bi or Pb.
Essential requirements or characteristics of a good lubricant are as follows: [1] It should have a high
viscosity index [2] It should have flash and fire points higher than the operating temperature of the machine
[3] It should have high oiliness [4] The cloud and pour points of a good lubricant should always be lower than the
operating temperature of the machine [5] The volatility of the lubricating oil should be low [6] It should deposit least
amount of carbon during use [7] It should have higher aniline point [8] It should possess a higher resistance towards
oxidation and corrosion [9] It should have good detergent quality
Fuels
CLASSIFICATION OF FUEL
Fuels are classified as follows: [1] Primary fuels which occur in nature, e.g. coal, petroleum and natural gas
[2] Secondary fuels which are derived from the primary fuels, e.g. coke, gasoline and coal gas. Both primary and
secondary fuels may be further classified based upon their physical state as (a) solid fuels, (b) liquid fuels and
(c) gaseous fuels
Knocking
Knocking is a kind of explosion due to rapid pressure rise occurring in an IC engine. In a petrol engine, a mixture
of gasoline vapour and air (l: 17) is used as a fuel. The air and gasoline vapours are compressed and ignited by an electric
spark. The chemical reaction taking place is the oxidation of hydrocarbons. The products of the oxidation reaction drive
the piston down the cylinder. If the combustion proceeds in a regular way, there is no problem of knocking. But in certain
circumstances, the oxidation is sudden and the mixture detonates and produces an explosive sound called engine knock
which results in the loss of power. Knocking not only results in a decreased power output but can also cause mechanical
damage by overheating of the cylinder parts.
A good gasoline should resist knocking. It was recognized that chemical structures of the fuel hydrocarbons
largely determine their knocking tendency. The tendency to knock decreases in the following order: straight chain
paraffins > branched chain paraffins > cyclo-paraffins > olefins > aromatics.
Edgar introduced the octane number to express the knocking characteristics of a combustion engine fuel. It has
been found that n-heptane knocks very badly and hence its antiknock value is arbitrarily given as zero. On the other hand,
isooctane gives very little knocking and so its anti-knock value has been given as 100. Thus, the octane number is
defined as the percentage of isooctane in the n-heptane-isooctane blend which has the same knocking characteristics as
the gasoline sample, under the same set of conditions. Since isooctane has good anti-knock properties, it is clear that
greater the octane number, greater is the resistance to knocking.
Chemical structure and knocking: The knocking tendency decreases with increase in compactness of the
molecules, double bonds and cyclic structure. With normal paraffins, the antiknock properties decrease with the increase
in length of the hydrocarbon chain. Thus, the octane numbers of n-butane, n-pentane, n-hexane and n-heptane are 90, 60,
29 and 0 respectively. Branched chain paraffins have higher anti-knock properties than their normal isomers.
Olefins have higher anti-knock properties than the corresponding paraffins. Aromatic hydrocarbons such as benzene and
toluene have high octane numbers.
Leaded Petrol
The anti-knock properties of a gasoline can be improved by the addition of suitable additives. Tetraethyl lead
(TEL) is added to petrol and is called leaded petrol. This addition process is called doping. This addition was first
proposed by Thomas Midgley. TEL reduces the knocking tendency of hydrocarbons. Knocking is a free radical
mechanism leading to a chain reaction which results in an explosion. If the chains are terminated before their growth,
knocking will cease. TEL decomposes thermally to form ethyl free radicals which combine with the growing free radicals
of the knocking process and thus stop the chain growth. When this leaded petrol is used as a fuel, lead and lead oxide
vapours formed may contaminate the atmosphere. To avoid this, ethylene dibromide is added along with TEL.
This ethylene dibromide reacts with Pb and PbO to give PbBr2 which will escape into the atmosphere.
Improving the octane number of a fuel: The octane number of a fuel may be improved by the following:
[1] The addition of anti-knock compounds like TEL [2] Low octane petrol is blended with high octane compounds like
alcohol, e.g. straight run petrol is mixed with reformed petrol, benzol and alcohol [3] Reforming.
Diesel Oil
Diesel oil is a fraction obtained between 2S0-320C and is a mixture of C15H32 and C18H38 hydrocarbons,
Its calorific value is about 11000 kcal/kg. It is used as a diesel engine fuel.
Diesel knock: In a diesel engine, air is first drawn into the cylinder and compressed, this compression is
accompanied by a rise in temperature to about 500C. Near the completion of the compression stroke, oil is sprayed into
the heated air; Droplets of the oil in the atomized form get vaporized and ignited. This raises temperature as well as
pressure, the piston is pushed by the expanding gases and this constitutes a power stroke.
The combustion of a fuel in a diesel engine is not instantaneous and the interval between the start of fuel
injection and its ignition is called ignition delay and is an important quality of the diesel fuel. This delay is due to the
time taken for the vaporization of individual droplets and rising of the vapour to its ignition temperature.
Long ignition delays lead to accumulation of more vapours in the engine and when ignited an explosion results as the
combined effect of increased temperature and pressure. This is responsible for diesel knock. In order to avoid diesel
knock, the ignition delay period should be as short as possible. The cetane number decreases in the following order:
straight chain paraffins > cycloparaffins > olefins > brandied paraffins > aromatics
The diesel fuels are graded by means of cetane rating. Cetane, i.e. n-hexadecane [CH3(CH2)14CH3] having a very
short ignition delay is given the value of 100 in the rating scale. -methylnaphthalene having a longer ignition delay
represents zero of the scale, The percentage of cetane in the cetane--methylnaphthalene mixture which has the same
ignition delay as the fuel under test is the cetane number of the fuel. High cetane number fuels eliminate diesel knock.
The cetane number of a diesel fuel may be increased by the addition of ethyl nitrite, amyl nitrite, etc.
Biodiesel
There has been an increase in efforts to reduce the reliance on petroleum fuel for energy generation. Among the
alternative fuel, biodiesel has received much attention for diesel engines due to their advantages as the renewable,
domestically produced energy resources and they are environmentally friendly (biodegradable spills, reduction of
unburned hydrocarbon, CO and particulate matter). Biodiesel can be produced from animal or vegetable oils via base
catalyzed trans-esterification process. The term biodiesel refers to mixtures of alkyl esters of long chain fatty acids. The
reaction scheme for the production of biodiesel is as follows:
(1) Trans-esterification: Vegetable oils (Triacylglycerols are triesters) on reaction with methanol (renewable resource
and does not raise toxicity) in the presence of catalyst like NaOH or KOH, produce glycerol and methyl ester of fatty acid
(Biodiesel).
Vegetable Oil + Methanol + Catalyst Biodiesel + Glycerol
MgHCO
MgOH
+ 2CO
and CaHCO
CaCO + CO
+ H
O
Permanent Hardness is due to the presence of soluble salts of magnesium and calcium in the form of chlorides
and sulphates in water (CaCl2, CaSO4, MgCl2 and MgSO4). Permanent hardness is not removed by boiling. It is also
called non-carbonate hardness (NCH).
Total Hardness: Temporary hardness and permanent hardness constitute the total hardness which is also
expressed as the sum of the concentration of calcium and magnesium ions.
Total Hardness = Temporary Hardness + Permanent Hardness or = [Ca2+] + [Mg2+]
Determination of Total Hardness of Water by Complexometric Titration [EDTA Method]:
Eriochrome Black-T [EBT] is the indicator used in the determination of hardness by complexometric titration with
EDTA. Here, Eriochrome Black-T is a complex organic compound [sodium-1-(1-hydroxy 2-naphthylato)-6-nitro-2naphthol-4-sulphonate] and EDTA is a hexadentate ligand [disodium salt of ethylenediamine tetraacetic acid].
NaOOCH 2C
HO
OH
SO 3Na
CH2 COOH
N
H2
C
H2
C
HOOCH 2C
NO 2
CH2 COONa
EDTA
[Disodium salt of ethylenediamine tetraacetic
acid]
Principle: Estimation of hardness by EDTA method is based on the principle that EDTA forms metal complexes
with hardness producing metal ions in water. These complexes are stable when the pH is maintained between 8 and 10
and also the indicator becomes effective only at this pH range. In order to maintain the pH, buffer solution (NH4Cl and
NH4OH mixture) is added. The completion of the complexation reaction is indicated by Eriochrome Black-T indicator.
When this indicator is added to the sample water it forms indicator-metal complexes of wine red colour.
Now when this wine red-coloured solution is titrated against EDTA solution, EBT in the unstable complex is
replaced by EDTA to form a stable metal-EDTA complex and liberates the free Eriochrome Black-T. At this point, the
colour of the solution changes from wine red to original blue colour which showing the end point of the titration.
[Ca2+/ Mg2+ EBT] + EDTA
Wine red coloured
unstable complex
free EBT
Blue colour
The temporary hardness is removed by boiling and after the removal of precipitate by filtration; the permanent hardness
in the filtrate is determined by titration with EDTA as before.
Therefore, total hardness - permanent hardness - temporary hardness
Experimental procedure: A known volume of the sample of hard water (V ml) is treated with about 10 ml of a
buffer solution and 5 drops of Eriochrome Black-T indicator. This solution is then titrated against the standard EDTA
reagent (standardized such that 1 ml of the reagent corresponds to 1 mg of CaCO3). The end point is the colour change
from wine red to blue.
A known volume (V ml) of sample water is taken in a beaker and boiled for 15 minutes.
After cooling the mixture, it is filtered and thoroughly washed. The filtrate is collected and made up to a known volume
(V ml). This solution is titrated against EDTA as before. The volume of EDTA consumed is V2 ml. Then,
Problem: 0.5 g of CaCO3 was dissolved in dil. HCl and diluted to 500 ml. 50 ml of this solution required 48
ml of EDTA solution for titration. 50 ml of a hard water sample required 15 ml of the same EDTA solution for
titration. Calculate the total hardness of water.
500 ml of CaCl2 solution = 0.5 g of CaCO3
= 0.5 x 1000 mg of CaCO3
= 500 mg of CaCO3 hence 1 ml of CaCl2 solution = 1 mg of CaCO3
Standardization of EDTA
1 ml of CaCl2 solution = 1 mg of CaCO3
50 ml of CaCl2 solution = 50 mg of CaCO3
48 ml of EDTA solution = 50 mg of CaCO3
!.#
Alkalinity: The alkalinity of water is due to the presence of a wide variety of salts of weak acids such as
carbonates, bicarbonates, phosphates, etc., and also due to the presence of weak and strong bases (due to contamination
with industrial wastes). The major portion of alkalinity in natural water is caused by the presence of bicarbonates that are
formed when water containing free carbon dioxide percolates through soils containing calcium carbonate and magnesium
carbonate. CaCO3 + CO2 + H2O Ca(HCO3)2 The alkalinity of natural water may be taken as an indication of the
concentration of hydroxides, carbonates and bicarbonates. Caustic Embrittlement: It is the phenomenon during which
the boiler material becomes brittle due to the accumulation of caustic substances. It is a very dangerous form of stress
corrosion occurring in mild steel boiler metals exposed to alkaline solution at high temperatures, resulting in the failure
of the metal. Stressed parts like bends, joints and rivets are severely affected. Boiler water usually contains a small
proportion of Na2CO3. In high pressure boilers,
this breaks up to give NaOH and makes the boiler water more
Procedure: Take 100 ml of given water sample into a conical flask, and titrate slowly against N/50 standard
sodium thiosulphate solution (taken in the burette). When the colour of the solution is very light yellowish add about 2
ml of freshly prepared starch solution, so the colour of the solution turned into blue. Continue the titration till the
disappearance of blue colour of the solution and note down the volume of the titrant used. The titration is repeated until a
concordant volume is obtained.
1
= 0.02N
50
Volume of standard Na2S2O3 solution, V1
=
ml
Volume of given water sample, V2
=
.....ml
Normality of given water sample, N2 can be calculated from the normality formula,
i.e., N1 x V1 = N2
x V2
N! V!
=
Normality of given water sample, N2
V
=
..N
Amount of Dissolved Oxygen
=
N
Eq. wt of Oxygen = N
8 g/Lit
=
g/Lit
Amount of Dissolved Oxygen in ppm
=
. . 1000 mg/Lit
=
..ppm
Normality of standard Na2S2O3 solution, N1
Removal of Dissolved O2: Dissolved oxygen can be removed from water by chemical and mechanical means.
Sodium sulphite (Na2SO3), hydrazine (N2H4), etc. are some of the chemicals used for removing oxygen.
2Na2SO3 + O2 2Na2SO4 and N2H4 + O2 N2 + 2H2O Hydrazine is found to be an ideal compound for removing
dissolved oxygen since the products are water and inert N2 gas. It removes oxygen without increasing the concentration
of dissolved salts.
Impurities
in
[1] Dissolved impurities: The dissolved impurities are mainly the carbonates, bicarbonates, chlorides and sulphates of
calcium, magnesium, iron, sodium and potassium. The presence of these salts imparts hardness to water.
The
dissolved
impurities
also
include
dissolved
gases
like
oxygen
and
carbon
dioxide.
[2] Suspended impurities: The following are the types of suspended impurities: (a) Inorganic: Clay and sand (b) Organic:
Oil globules, vegetable and animal matter. The above suspended impurities impart turbidity, colour and odour to water.
[3] Colloidal impurities: They are finely-divided silica and clay, organic waste products, protein amino acids, etc.
[4] Microorganisms: They are algae, fungi and bacteria.
Potable Water (water for domestic supply): Municipalities have to supply potable water, i.e., it is a water of
sufficiently high quality that it can be consumed or used without risk of immediate or long term harm. The following are
characteristics of potable water: - [1] It should be sparking clear, soft, pleasant in taste, perfectly cool and odourless
[2] Its turbidity should not exceed 10 ppm [3] Its alkanity should not be high (pH = 8.0) [4] Its dissolved solids should
be less than 500 ppm [5] It should be free from objectionable minerals such as lead, arsenic, chromium and manganese
salts and also free from objectionable dissolved gases like hydrogen sulphide [6] It should be free from disease producing
micro-organisms.
Drinking water comes from two major sources: (a) Surface water such as lakes, rivers, and reservoirs,
but it requires both filtration and disinfection in order to use as drinking water. (b) Ground water, which is pumped from
wells, it is considered to be the purest source of water. Rivers, lakes and wells are the most common sources of water
used by municipalities. The actual treatment methods depend directly on the impurities present. For removing various
types of impurities the following treatment processes are employed.
Sterilization: The complete removal of harmful bacteria is known as sterilization. The following sterilizers
are generally used for sterilizing water.
[1] Sterilization by chlorine or bleaching powder: Chlorine is the most common sterilizing agent in water
treatment. Chlorine may be added in the form of bleaching powder or directly as a gas or in the form of concentrated
solution in water.
When bleaching powder is added to water, HOCI which acts as a powerful germicide is produced. It is believed
that HOCI reacts with bacteria and inactivate the enzymes present in the cells of bacteria. These enzymes are responsible
for the metabolic activities of microorganisms. Since these enzymes are inactivated, microorganisms become dead.
CaOCl2 + H2O Ca(OH)2 + Cl2; Cl2 + H2O HCI + HOCI; HOCI + Bacteria Bacteria are killed
[2] Sterilization by ultraviolet radiations: Ultraviolet radiations emanating from electric mercury vapour lamp is capable
of sterilizing water. This process is particularly useful for sterilizing swimming pool water. This process is highly
expensive.
[3] Sterilization by ozone: Ozone is a powerful disinfectant and is readily absorbed by water. Ozone is highly unstable
and decomposes to give nascent oxygen which is capable of destroying the bacteria. O3 O2 + [O]. This process is
relatively expensive.
Break point chlorination
In break point chlorination a sufficient amount of chlorine is added to oxidize (a) organic matter,
(b) reducing substances (Fe2+, H2S, etc.) and (c) free ammonia in raw water leaving behind mainly free chlorine which
destroys pathogenic bacteria. When chlorine is added to water, initially it reacts with ammonia and there will formation
of chloramines (See equations). Thus the amount of combined residual chlorine (chloramines) increases with increasing
dosage. Then the oxidation of chloramines and other impurities start and there is a fall in combined chlorine content.
Thus combined residual chlorine decreases to a minimum at which oxidation of chloramines and other organic
compounds complete. This minimum is the breaking point of chlorine (Fig)
The reason for such behaviour is due to the fact that some organic compounds which defy oxidation at lower
chlorine concentration get oxidized when the break point chlorine concentration is reached. Since, it is these organic
compounds which are generally responsible for bad taste and odour in water, it is clear that break point chlorination
eliminates bad taste and odour. Further chlorination increases the free residual chlorine (CI2, HOCI, OCI-). Hence, it use
chlorine as a good disinfectant, the chlorine dosage has to be given more than the break point.
Electrochemistry is the study of production of electricity from energy released during spontaneous chemical reactions
and the use of electrical energy to bring about non-spontaneous chemical transformations. Batteries and fuel cells convert
chemical energy into electrical energy and are used on a large scale in various instruments and devices. The reactions carried out
electrochemically can be energy efficient and less polluting. Therefore, study of electrochemistry is important for creating new
technologies that are eco-friendly.
Electrochemical cells: A device for producing an electrical current from a chemical reaction (redox reaction) is called an
electrochemical cell. How a Redox reaction can produce an electrical current? When a bar of zinc is dipped in a solution of copper
sulphate, copper metal is deposited on the bar. The net reaction is Zn + Cu2+ Zn2+ + Cu. This is a redox reaction and the two
half-reactions are: Zn Zn2+ + 2e and Cu2+ + 2e Cu. In this change, Zn is oxidized to give Zn2+ ions and Cu2+ ions are
reduced to Cu atoms. The electrons released in the first half-reaction are used up by the second half-reaction.
Both the half reactions occur on the zinc bar itself and there is no net charge.
Now, let the two half-reactions occur in separate compartments which are connected by a wire. The electrons produced in
the left compartment flow through the wire to the other compartment. However the current will flow for an instant and then stop.
The current stops flowing because of the charge build up in the two compartments. The electrons leave the left compartment and it
would become positively charged. The right compartment receives electrons and becomes negatively charged. Both these factors
oppose the flow of electrons (electrical current) which eventually stops.
This problem can be solved very simply. The solutions in the two compartments may be connected, say, by a salt bridge.
The salt bridge is a U-tube filled with an electrolyte such as NaCl, KCl, or K2SO4. It provides a passage to ions from one
compartment to the other compartment without extensive mixing of the two solutions. With this ion flow, the circuit is complete
and electrons pass freely through the wire to keep the net charge zero in the two compartments.
Cell diagram or Representation of a Cell: A cell diagram is an abbreviated symbolic depiction of an electrochemical
cell. For this purpose, we will consider that a cell consists of two half-cells. Each half-cell is again made of a metal electrode
contact with metal ions in solution. (1) A single vertical line (|) represents a phase boundary between metal electrode and ion
solution (electrolyte). Thus the two half-cells in a voltaic cell are indicated in Figure 1. It may be noted that the metal electrode in
anode half-cell is on the left, while in cathode half-cell it is on the right of the metal ion. (2) A double vertical line (||) represents
the salt bridge, porous partition or any other means of permitting ion flow while preventing the electrolyte from mixing.
(3) Anode half-cell is written on the left and cathode half-cell on the right. (4) In the complete cell diagram, the two half-cells are
separated by a double vertical line (salt bridge) in between. (5) The symbol for an inert electrode, like the platinum electrode is
often enclosed in a bracket. (6) The value of emf of a cell is written on the right of the cell diagram. Thus a zinc-copper cell has
emf 1.1V and is represented as
Voltaic Cells: A Voltaic cell, also known as a galvanic cell is one in which electrical current is generated by a
spontaneous redox reaction. Here the spontaneous reaction of zinc metal with an aqueous solution of copper sulphate is used.
Zn + Cu2+ Zn2+ + Cu. A bar of zinc metal (anode) is placed in zinc sulphate solution in the left container. A bar of copper
(cathode) is immersed in copper sulphate solution in the right container. The zinc and copper electrodes are joined by a copper
wire. A salt bridge containing potassium sulphate solution interconnects the solutions in the anode compartment and the cathode
compartment. The oxidation half-reaction occurs in the anode compartment. Zn Zn2+ + 2eThe reduction half-reaction takes
place in the cathode compartment. Cu2+ + 2e Cu
When the cell is set up, electrons flow from zinc electrode through the wire to the copper cathode. As a result,
zinc dissolves in the anode solution to form Zn2+ ions. The Cu2+ ions in the cathode half-cell pick up electrons and are converted to
Cu atoms on the cathode. At the same time, SO42 ions from the cathode half-cell migrate to the anode half-cell through the salt
bridge. Likewise, Zn2+ ions from the anode half-cell move into the cathode half-cell. This flow of ions from one half-cell to the
other completes the electrical circuit which ensures continuous supply of current. The cell will operate till either the zinc metal or
copper ion is completely used up.
Daniel Cell: It is a typical voltaic cell. It was named after the British chemist John Daniel. It is a simple zinc copper cell
like the one described above. In this cell the salt-bridge has been replaced by a porous pot. Daniel cell resembles the above voltaic
cell in all details except that Zn2+ ions and SO42 ions flow to the cathode and the anode respectively through the porous pot
instead of through the salt-bridge. Inspite of this difference, the cell diagram remains the same.
Voltaic cell
Daniel Cell
(a) Half-reactions:
Zn(s) Zn2+ (aq) + 2e and
Cu2+ (aq) + 2e Cu(s)
(b) Cell reaction by adding up the
half-reactions:
Zn(s) + Cu2+ (aq) Zn2+ (aq) + Cu(s)
Cell reaction
Helmholtz double layer: The transition region between two phases consists of a region of charge unbalance known as
the electric double layer [Helmholtz double layer]. As its name implies, this consists of an inner monomolecular layer of adsorbed
water molecules and ions, and an outer diffuse region that compensates for any local charge unbalance that gradually merges into
the completely random arrangement of the bulk solution. In the case of a metal immersed in pure water, the electron fluid within
the metal causes the polar water molecules to adsorb to the surface and orient themselves so as to create two thin planes of
positive and negative charge. If the water contains dissolved ions, some of the larger (and more polarizable) anions will loosely
bond (chemisorb) to the metal, creating a negative inner layer which is compensated by an excess of cations in the outer layer.
Electrochemistry is the study of reactions in which charged particles (ions or electrons) cross the interface between two phases of
matter, typically a metallic phase (the electrode) and a conductive solution, or electrolyte. A process of this kind can always be
represented as a chemical reaction and is known generally as an electrode process. Electrode processes take place within the
double layer and produce a slight unbalance in the electric charges of the electrode and the solution.
Electrode Potential: At each electrode-electrolyte interface there is a tendency of metal ions from the solution to deposit
on the metal electrode trying to make it positively charged. At the same time, metal atoms of the electrode have a tendency to go
into the solution as ions and leave behind the electrons at the electrode trying to make it negatively charged. At equilibrium, there
is a separation of charges and depending on the tendencies of the two opposing reactions, the electrode may be positively or
negatively charged with respect to the solution. A potential difference develops between the electrode and the electrolyte which is
called electrode potential. When the concentrations of all the species involved in a half-cell is unity then the electrode potential is
known as standard electrode potential. In a galvanic cell, the half-cell in which oxidation takes place is called anode and it has a
negative potential with respect to the solution. The other half-cell in which reduction takes place is called cathode and it has a
positive potential with respect to the solution. Thus, there exists a potential difference between the two electrodes and as soon as
the switch is in the on position the electrons flow from negative electrode to positive electrode.
Single electrode potential: An electrochemical cell consists of two half-cells. With an open-circuit, the metal electrode
in each half-cell transfers its ions into solution. Thus an individual electrode develops a potential with respect to the solution.
The potential of a single electrode in a half-cell is called the Single electrode potential. Thus in a Daniel cell in which the
electrodes are not connected externally, the anode Zn/Zn2+ develops a negative charge and the cathode Cu/Cu2+, a positive charge.
The amount of the charge produced on individual electrode determines its single electrode potential. The single electrode potential
of a half-cell depends on: (a) concentration of ions in solution; (b) tendency to form ions; and (c) temperature.
Cell potential or emf: In a Zn-Cu voltaic cell, electrons are released at the anode and it becomes negatively charged.
The negative electrode pushes electrons through the external circuit by electrical repulsions. The copper electrode gets positive
charge due to the discharge of Cu2+ ions on it. Thus electrons from the outer circuit are attracted into this electrode. The flow of
current through the circuit is determined by the push, of electrons at the anode and attraction of electrons at the cathode. These
two forces constitute the driving force or electrical pressure that sends electrons through the circuit. This driving force is called
the electromotive force (abbreviated emf) or cell potential. The emf of cell potential is measured in units of volts (V) and is also
referred to as cell voltage.
Calculating the emf of a cell: The potential difference between the two electrodes of a galvanic cell is called the cell
potential and is measured in volts. The cell potential is the difference between the electrode potentials (reduction potentials) of the
cathode and anode. It is called the cell electromotive force (emf) of the cell when no current is drawn through the cell. It is now an
accepted convention thatwe keep the anode on the left and the cathode on the right while representing the galvanic cell.
A galvanic cell is generally represented by putting a vertical line between metal and electrolyte solution and putting a double
vertical line between the two electrolytes connected by a salt bridge. Under this convention the emf of the cell is positive and is
given by the potential of the half-cell on the right hand side minus the potential of the half-cell on the left hand side. The emf of a
cell can be calculated from the half-cell potentials of the two cells (anode and cathode) by using the following formula,
E = E
E
= E
E where ER and EL are the reduction potentials of the right-hand and left-hand electrodes
respectively. It may be noted that absolute values of these reduction potentials cannot be determined. These are found by
connecting the half-cell with a standard hydrogen electrode whose reduction potential has been arbitrarily fixed as zero.
Measurement of emf of a cell [Poggendorfs compensation method]: The emf of an unknown cell can be measured
with the help of a potentiometer. It consists of a wire AB which is about one meter long. The two ends of this wire are connected
to a working battery W. A standard cell C1 (i.e., a cell of known emf) is connected to the end A. At the other end, the cell C1 is
connected to a galvanometer through a key K1. The galvanometer is then joined to a sliding contact that moves on the wire AB.
The cell C2 whose emf is to be measured is similarly connected to the key K2, the galvanometer and then the sliding contact.
By using the key K1, the cell C1 is put into the circuit and the contact is moved to and fro along AB. When no current flows
through the galvanometer, the point of contact X1 is recorded. Then by using the key K2, the cell C2 is put into the circuit and the
procedure is repeated to find the corresponding point X2. The emf of the cell C2 is calculated by using the following equation:
Standard emf of a cell [E]: The emf generated by an electrochemical cell is given by the symbol E. It can be measured
with the help of a potentiometer. The value of emf varies with the concentration of the reactants and products in the cell solutions
and the temperature of the cell. When the emf of a cell is determined under standard conditions, it is called the standard emf.
The standard conditions are (a) 1 M solutions of reactants and products; and (b) temperature of 25C. Thus standard emf may be
defined as: the emf of a cell with 1 M solutions of reactants and products in solution measured at 25C. Standard emf of a cell is
represented by the symbol E. With gases 1atm pressure is a standard condition instead of concentration. For a simple Zn-Cu
voltaic cell, the standard emf, E, is 1.10 V. This means that the emf of the cell operated with [Cu2+] and [Zn2+] both at 1 M and
25C is 1.10 V.
Determination of emf of a half-cell: By a single electrode potential, we also mean the emf of an isolated half-cell or its
half-reaction. The emf of a cell that is made of two half-cells can be determined by connecting them to a voltmeter. However,
there is no way of measuring the emf of a single half-cell directly. A convenient procedure to do so is to combine the given
half-cell with another standard half-cell. The standard hydrogen half-cell or Standard Hydrogen Electrode (SHE) is selected for
coupling with the unknown half-cell. It consists of a platinum electrode immersed in a 1 M solution of H+ ions maintained at
25C. Hydrogen gas at one atmosphere enters the glass hood and bubbles over the platinum electrode. The hydrogen gas at the
platinum electrode passes into solution, forming H+ ions and electrons. H2 2H+ + 2e. The emf of the standard hydrogen
electrode is arbitrarily assigned the value of zero volts. So, SHE can be used as a standard for other electrodes. The half-cell,
whose potential is desired, is combined with the hydrogen electrode and the emf of the complete cell determined with a voltmeter.
The emf of the cell is E
= E
E
For example, it is desired to determine the emf of the zinc electrode, Zn | Zn2+.
It is connected with the SHE as shown in Figure. The complete electrochemical cell may be represented as:
The emf of the cell is E
= E
E = E
E/
= 0 0.76 = 0.76 V. Similarly,
the emf of the copper electrode, Cu2+ | Cu can be determined by pairing it with the SHE when the electrochemical cell as:
The emf of this cell has been determined to be 0.34 V which is the emf of the copper half-cell.
E = E
E = E !/ ! E
= 0.34 0 = 0.34 V
SHE
When it is placed on the right-hand side of the Zinc electrode, the hydrogen electrode reaction is 2H+ + 2e H2.
The electrons flow to the SHE and it acts as the cathode. When the SHE is placed on the left hand side, the electrode reaction is
H2 2H+ + 2e. The electrons flow to the copper electrode and the hydrogen electrode as the anode. Evidently, the SHE can act
both as anode and cathode and, therefore can be used to determine the emf of any other half-cell electrode. IUPAC convention
places the SHE on the left-hand side: The electrons flow from left-to-right and the given half-cell electrode gains electrons
(reduction). The observed emf of the combined electrochemical cell is then the emf of the half-cell on the right-hand. Such emf
values of half-cells, or half reactions, are known as the Standard reduction potentials or Standard potentials. However, if the SHE
be placed on the right hand side of the given half-cell, the potential so obtained is called as the Standard oxidation potential.
Electrochemical Series: The standard electrode potentials (reduction) of a number of electrodes are given in Table.
These values are said to be on the hydrogen scale since in these determinations, the potential of the standard hydrogen electrode
used as the reference electrode has been taken as zero. When the various electrodes are arranged in the order of their increasing
values of standard reduction potentials on the hydrogen scale, then this arrangement is called electrochemical series.
Applications of Electrochemical Series or Electrode Potentials: The following are the applications of electrochemical
= E
E
series: [1]The standard emf of a cell can be calculated if the standard electrode potential values are known. E
[2] The relative tendencies of metals to go into solution can be noted by the help of electrochemical series. Metals on the top are
more easily ionized into solution. [3] The metal with negative reduction potential will displace hydrogen from an acid solution:
Zn + H2SO4 ZnSO4 + H2 (EZn0 = -0.76 volt).The metal with positive reduction potential will not displace hydrogen from an acid
solution. Ag + H2SO4 no reaction EAg0 = +0.80 volt. [4] Metals which lie higher in the series can displace from the solution
those elements which lie below them in the series. Thus, zinc can displace copper from the solution. [5] The anodic or more active
metals in the series are more prone to corrosion. The cathodic or more noble metals are less prone to corrosion.
Using Electrochemical Series or Electrode Potentials: In Table the standard reduction potentials (E) are arranged in
the order of increasing potentials. The relative position of electrodes (M/M+) in the table can be used to predict the reducing or
oxidising ability of an electrode. The electrodes that are relatively positive indicate that reduction reaction involving addition of
electrons, M+ + e M is possible. In case of relatively negative potential involving loss of electrons, M M+ + e is indicated.
It also follows that the system with higher electrode potential will be reduced by the system with lower electrode potential.
[1] Predicting the Oxidising or Reducing Ability: (i) The more positive the value of E, the better the oxidising ability (the greater
the tendency to be reduced) of the ion or compound, on moving upward in the Table. (ii) The more negative the value of E the
better the reducing ability of the ions, elements or compounds on moving downward in the Table. (iii) Under standard conditions,
any substance in this Table will spontaneously oxidise any other substance lower than it in the Table. [2] Predicting cell emf:
The standard emf, E, of a cell is the standard reduction potential of right-hand electrode (cathode) minus the standard reduction
potential of the left-hand electrode (anode). E
= E
E
= Cathode Potential Anode Potential. Let us predict the
E
= E
E
= 0.80 2 0.7633 = 1.563V. [3] Predicting Feasibility of
Reaction: The feasibility of a redox reaction can be predicted with the help of the electrochemical series. The net emf of the
reaction, Ecell can be calculated from the eqn
[4] Predicting whether a metal will displace another metal
from its salt solution or not: The metals lying higher up in the series are strong oxidising agents and their ions are readily reduced
to the metal itself. In general we can say that a metal lower down the electrochemical series can precipitate the one higher up in
the series. [5] Predicting whether a metal will displace hydrogen from a dilute acid solution: Any metal lying below hydrogen is a
stronger reducing agent than hydrogen and will convert H+ to H2. This explains why Zn lying below hydrogen reacts with dilute
H2SO4 to liberate H2, while Cu lying above hydrogen does not react.
The Nernst Equation: We know experimentally that the potential of a single electrode or half-cell varies with the
concentration of ions in the cell. In 1889 Walter Nernst derived a mathematical relationship which enable us to calculate the
half-cell potential, E, from the standard electrode potential, E, and the temperature of the cell. This relation known as the Nernst
equation can be stated as E = E
6.77
8
9
temperature, n = number of electrons transferred in the half-reaction, F = Faraday of electricity and K = equilibrium constant for
the half-cell reaction as in equilibrium law
Derivation: Consider a reversible cell, e.g. Daniel cell. The overall cell reaction occurring in the cell is represented by the
equation Zn + Cu6? Zn6? + Cu. In general, for a reversible cell the equation is A + B C + D. The electrical energy of a
reversible cell can be measured by the free energy decrease (-G) of the reaction taking place in the cell. In the cell, if the reaction
involves the transfer of n number of electrons, then n Faradays of electricity will flow. If E is the emf of the cell, then the total
electrical energy produced in the cell is-G = nFE where -G = decrease in free energy. At standard conditions, -G0 = nFE0
where -Go = standard free energy change.
Standard free energy change is the change in free energy when the concentration of reactants and products are unity.
0
E is the standard emf of the cell in which the reactants and products are kept at 1 molar concentration at 25C, G and G0 are
related as G = G0 + RT ln K where K is the equilibrium constant of the reaction defined as the ratio of the concentration of the
products to the concentration of the reactants.
Divide the above equation At 25C, T = 298 K, R = 8.314 11K/mol, F = 96500 coulombs.
by nF
Therefore,
This is known as the Nernst equation. This equation may be used to calculate the emf of cells when concentration of reactants
and products of the cell reactions are known.
Applications of the Nernst Equation: [1] Calculation of Half-cell potential: For an oxidation half-cell reaction when the
metal electrode M gives Mn+ ion, M Mn+ + ne-. The Nernst equation takes the form E = E
CDE F .
6.77
8
log
CDF
9
The concentration of solid metal [M] is equal to zero. Therefore, the Nernst equation can be written as
E = E
6.77
8
logCM? F.
9
Substituting the values of R, F and T at 25C, the quantity 2.303 RT/F comes to be 0.0591.
.HIJ
which oxidation occurs. In case it is a reduction reaction, the sign of E will have to be reversed. [2] Calculation of Cell potential:
The Nernst equation is applicable to cell potentials as well. Thus, E = E
.HIJ
of the redox cell reaction. [3] Calculation of Equilibrium constant for the cell reaction: The Nernst equation for a cell is
E = E
0=
.HIJ
log K. At equilibrium, the cell reaction is balanced and the potential is zero. The Nernst equation becomes
.HIJ
E log K , hence log K
P
MNOO
= .HIJ
Concentration Cells: A concentration cell is an electrochemical cell which produces electrical energy by the transfer of
material from a system of higher concentration to a system of lower concentration. The difference in concentration may be due to
the difference in concentration of the electrodes or electrolytes. Based on this, concentration cells are divided into two categories;
(i) Electrode Concentration Cells and (ii) Electrolyte Concentration Cells
Electrode Concentration Cells: In electrode concentration cells, two electrodes of the same metal with different
concentrations are dipped in the same solution of the electrolyte, e.g. two hydrogen electrodes at unequal gas pressures immersed
in the same solution of hydrogen ions constitute electrode concentration cells, Pt; H2(p1)solution of H+ ionH2(p2); Pt
where p1>p2
R.H.S: 2H+ + 2e H2 (p2) and L.H.S: H2 (p1) 2H+ + 2e so the overall reaction: H2 (p1) H2 (p2) and E = E
.HIJ
Q
log Q
R
Electrolyte Concentration Cells: Cell potentials depend on concentration of the electrolyte. Thus a cell can be
constructed by pairing two half-cells in which identical electrodes are dipping in solution of different concentrations of the same
electrolyte. Such a cell called Electrolyte concentration cell. It may be described as: a cell in which emf arises as a result of
different concentrations of the same electrolyte in the component half-cells.
The electrode dipped in the solution of lower concentration is an anode and the electrode dipped in solution of higher
concentration is a cathode. The overall reaction involves the transfer of material from higher concentration to lower concentration.
The two solutions are connected together by a salt bridge. A typical concentration cell is shown in Figure. It consists of two silver
electrodes, one immersed in 0.1 M silver nitrate solution and the other in 1 M solution of the same electrolyte. The two solutions
are in contact through a membrane (or a salt bridge). When the electrodes are connected by a wire, it is found experimentally that
electrons flow from the electrode in more dilute (0.1M) solution to that in the more concentrated (1 M) solution.
Explanation: The concentration of Ag+ ions in the left compartment is lower (0.1M) and in the right compartment it is
higher (1M). There is a natural tendency to equalize the concentration of Ag+ ions in the two compartments. This can be done if
the electrons are transferred from the left compartment to the right compartment. This electron transfer will produce Ag+ ions in
the right compartment by the half-cell reactions: Ag Ag+ + e- (Left compartment) and Ag+ + e- Ag (Right compartment)
8
T
ln
.
9
U T V
Example is Standard Hydrogen Electrode (SHE). It consists of a platinum electrode immersed in a 1 M solution of H+ ions
maintained at 25C. Hydrogen gas at one atmosphere enters the glass hood and bubbles over the platinum electrode.
[2] Metal-metal insoluble salt electrode {M (s)/MX (s)/X- (aq)}: It consists of a pure metal [M(s)] coated by a layer
of its sparingly soluble salt [MX (s)] and kept immersed in a solution containing a common anion [X- (aq)]. It is represented as
M(s) /MX (s)/X- (aq). The electrode reaction can be written as MX (s) + ne- M (s) + X- (aq). Electrode potential is
X
ED/DW/WX = ED/DW/W
8
9
ln
Y . ZX
YZ
Example is Calomel electrode: The standard calomel electrode, SCE, consists of a wide glass-tube with a narrow
side-tube. It is set up as illustrated in Fig. 10. A platinum wire is dipping into liquid mercury covered with solid mercurous
chloride (Hg2Cl2, calomel). The tube is filled with a 1 M solution of KCl (or saturated KCl solution). The side-tube containing
KCl solution provides the salt bridge which connects the electrode to any other electrode. The calomel electrode is represented as
The half-cell reaction is Hg2Cl2 + 2e- 2Hg + 2Cl[3] Quinhydrone Electrode: It is a widely used secondary standard electrode. It involves the redox reaction between
quinine (Q) and hydroquinone (QH2), represented in Figure 1. The hydroquinone half-cell consists of a platinum strip immersed in
a saturated solution of quinhydrone at a definite H+ ion concentration (buffered solution). Quinhydrone is a molecular compound
which gives equimolar amounts of quinone and hydroquinone in solution represented in Figure 2. The potential developed is
measured against a hydrogen electrode or calomel electrode. The emf with respect to a standard hydrogen electrode is 0.2875 V.
[4] Glass electrode: It consists of a glass tube having a thin-walled bulb at the lower end. The bulb contains a 1M HCl
solution. Sealed into the glass-tube is a silver wire coated with silver chloride at its lower end. The lower end of this silver wire
dips into the hydrochloric acid, forming silver-silver chloride electrode. The glass electrode may be represented as
When placed in a solution, the potential of the glass electrode depends on the H+ ion
concentration of the solution. The potential develops across the glass membrane as a result of a concentration difference of H+
ions on the two sides of the membrane. This happens much in the same way as the emf of a concentration cell develops.
A commonly used secondary standard electrode is the so-called glass electrode. Its emf is determined by coupling with a standard
calomel electrode (SCE). The glass electrode provides one of the easiest methods for measuring the pH of a given solution.
Calomel electrode
Gas Electrode
Quinhydrone Electrode
Glass Electrode
Determination of pH of a solution using glass electrode: A half-cell is set up with the test solution as electrolyte.
The emf of the cell depends on the concentration of H+ ions or pH of the solution. The emf of the half-cell is determined by
coupling it with another standard half-cell and measuring the emf of the complete cell. The commonly used standard electrode is
the glass electrode. A glass electrode is immersed in the solution of unknown pH. It is coupled with a standard calomel electrode
(SCE) as shown in Figure. The emf of the complete cell can be determined experimentally.
Glass electrode
Calomel electrode
Calculations: The potential of the glass electrode, EG, at 25C is given by the above equation. The value of the potential of
calomel electrode is known while Ecell can be found experimentally. Therefore, we can find pH of a given solution if EG is known.
It can be determined by using a solution of known pH in the cell and measuring Ecell. This value of EG is constant for a particular
glass electrode and can be used for any subsequent determinations of pH of unknown solutions with the help of equation.
The potential of the cell, Ecell, cannot be measured using ordinary potentiometer or voltmeter as the resistance of the glass
membrane is very high and the current small. Therefore, an electronic voltmeter is required which reads pH directly.
Cathode
During the discharge process the consumption of sulphuric acid is replaced by an equivalent quantity of water and the
sulphuric acid concentration decreases. On charging the reverse reaction takes place. During the reverse reaction water is
consumed and sulphuric acid is regenerated. Hence the original strength of acid is restored. Since both of these changes are
associated with variations in the specific gravity of the acid, the extent of charge or discharge of the cell at any time can be
determined by testing the specific gravity of the acid.
Uses: The following are the uses of lead-acid storage cells: [1] Lead-acid storage cells are used in automobiles, hospitals,
telephone exchanges, etc. [2] As it is rechargeable it is used in UPS (uninterrupted power supply) a power system which maintains
current flow without even a momentary break in the event of current failure.
Nickel-Cadmium Battery (Ni-Cd Battery) or Alkaline Storage Battery: A nickel-cadmium battery is a type of alkaline
storage battery. This battery consists of a cadmium anode, nickel oxyhydroxide cathode and an alkaline electrolyte (potassium
hydroxide). During discharge cadmium metal oxidizes to cadmium hydroxide at the anode:
By accepting the electrons, nickel oxyhydroxide [NiO(OH)] is reduced to nickel hydroxide [Ni(OH)2]
at the cathode:
The overall cell reaction is:
The emf of the cell is 1.3 volt. This is a rechargeable battery. When the discharged battery is connected to an external voltage
source, the cell reaction is reversed.
Advantages: Nickel--cadmium cells are characterized by long life, relatively high rates of discharge and charge and
ability to operate at low temperatures. Uses: Ni-Cd cells are used in calculators, electrical shavers, etc. The good low temperature
performance has lead to wide use of nickel-cadmium batteries in aircraft and space satellite power systems.
Fuel cells: A fuel cell is an electrochemical cell in which the chemical energy of the fuel-oxidant system is directly
converted into electrical energy. It is an energy conversion device or electricity generator. A fuel cell operates like a galvanic cell
with the exception that the reactants are supplied from outside. It is an example for a primary cell. This is capable of supplying
current as long as it is provided with the supply of reactants.
Hydrogen-Oxygen Fuel Cell: A fundamental and important example of a fuel cell is the hydrogen/oxygen cell. Like an
electrochemical cell, the fuel cell is also having two electrodes and an electrolyte. The two electrodes are made up of porous
graphite admixed with nickel powder. The electrolyte used is potassium hydroxide solution maintained at 200C and
20-40 atmospheres. Hydrogen and oxygen gases are bubbled through the anode and cathode compartments respectively.
Hydrogen is oxidized at the anode whereas the oxygen gets reduced at the cathode (Figure 3).
The cell reaction is the same as combustion of hydrogen in air or oxygen. Generally a large number of these cells are
stacked together in series to make a battery called fuel cell battery or fuel battery. In the fuel cells, gaseous fuels used are
hydrogen, alkanes and co. Among the liquid fuels methanol, ethanol, etc. are very important. Oxygen, air, hydrogen peroxide, etc.
are some of the oxidants used.
Advantages of fuel cells: The following are the advantages of fuel cells: [1] The energy conversion efficiency is very
high (75-83%). [2] They are used as power sources in spacecrafts. [3] The product of a hydrogen-oxygen fuel cell is pure water
which can be used for drinking purpose. [4] Noise and thermal pollution are very low. [5] The maintenance cost is very low.
[6] It saves fossil fuels. Limitations: The following are the limitations of fuel cells: [1] The cost of power from a fuel cell is high
as result of the cost of electrodes and pure hydrogen gas. [2] As the fuels used are gases, they have to be stored in big tanks under
high pressures.
Solar Cells
Solar Cells or Photovoltaic Cells: Photovoltaic cells convert solar energy directly into electrical energy.
Description of solar cells as follows; the conventional solar cells made up of p-type doped semiconductor (i.e., silicon doped with
boron) and n-type doped semiconductor (i.e., silicon doped with phosphorus). The surface layer is made up of
n-type semiconductor and it is extremely thin (~ 0.2 m) so that sun light can penetrate through it. The bottom layer is made up of
p-type semiconductor. The electrodes made of Ti-Ag solder are attached to both the sides to provide electrical contact.
The electrode on the top surface is in the form of a metal grid with fingers which permit sun light to pass through.
On the back side, the electrode completely covers the surface. An anti-reflection coating of silicon oxide having a thickness of
0.1 m is also put on the top surface.
Working: When p-type and n-type semiconductors are placed together, electrons from n-type side diffuse across
p-n junction to combine with holes present in p-type semiconductor side. As a result positive ions (on n-type side) and negative
ions (on p-type side) are created near the junction to certain thickness. The separation of charges produces an electric field across
p-n junction which is about 0.6-0.7 volt. This potential at p-n junction prevents the charges moving across it further.
When light strikes on the p-n junction, electron-hole pair is produced. As the potential barrier resists the flow of charge carriers
across it, they flow through the conductor/load connected externally to produce electric current. Since the emf of a single solar cell
is about 0.6 V, they are arranged into larger groupings called arrays in solar panels.
Applications: The following are the application of solar cells: [1] Solar cells are used to provide power supply for space
satellites. [2] They are used for the distillation of water to get pure drinking water. [3] Solar cells provide thermal energy for solar
cookers, solar furnaces, etc. [4] They provide electricity for street lighting in remote areas, and to run water pumps and radios in
desert areas. [5] Solar cells provide electric power to light houses.
Corrosion
Introduction: Metals and alloys are used as construction and fabrication materials in engineering. If the metal or alloy
structure is not properly maintained, they deteriorate slowly by the action of atmospheric gases, moisture and other chemicals.
This phenomenon of metals and alloys to undergo destruction by the act of environment is known as corrosion.
Corrosion is defined as the gradual eating away or deterioration of a metal by chemical or electrochemical reactions with its
environment. Due to corrosion the useful properties of a metal such as malleability, ductility and electrical conductivity are lost.
The most familiar example of corrosion is rusting of iron when exposed to atmospheric conditions. During this, a layer of reddish
scale and powder of oxide (Fe3O4) is formed and the iron becomes weak. Another example is the formation of green film or basic
carbonate [CuCO3 + Cu(OH)2] on the surface of copper when exposed to moist air containing CO2.
Cause of Corrosion: In nature, most metals are found in a chemically combined state known as an ore.
All the metals except gold, platinum and silver exist in nature in the form of their oxides, carbonates, sulphides, sulphates, etc.
These combined forms of the metals represent their thermodynamically stable state (low energy state). The metals are extracted
from these ores after supplying a large amount of energy. Metals in the uncombined condition have a higher energy and are in an
unstable state. It is their natural tendency to go back to the low energy state, i.e., combined state by recombining with the elements
present in the environment. This is the main reason for corrosion.
Effects of corrosion: The following are the effects of corrosion: [1] Lost production during a shut down [2] Replacement
of corroded equipment [3] High maintenance costs such as repainting [4] Loss of efficiency [5] Contamination of the product.
Theories of Corrosion: [1] Direct chemical attack or Chemical or Dry corrosion [2] Electrochemical theory or
Wet corrosion [3] Differential aeration or Concentration cell corrosion
Theories of Corrosion
[1] Direct Chemical Attack or Chemical or Dry Corrosion: Whenever corrosion takes place by direct chemical attack
by gases like' oxygen, nitrogen and halogens, a solid film of the corrosion product is formed on the surface of the metal which
protects the metal from further corrosion. If a soluble or volatile corrosion product is formed, then the metal is exposed to further
attack. For example, chlorine and iodine attack silver generating a protective film of silver halide on the surface.
On the other hand, stannic chloride formed on tin is volatile and so corrosion is not prevented.
Oxidation corrosion is brought about by direct action of oxygen at low or high temperatures on metals in the absence of
moisture. Alkali metals (Li, Na, K, etc.) and alkaline earth metals (Mg, Ca, Sn, etc.) are readily oxidized at low temperatures.
At high temperatures, almost all metals except Ag, Au and Pt are oxidized. Alkali and alkaline earth metals on oxidation produce
oxide deposits of smaller volume. This results in the formation of a porous layer through which oxygen can diffuse to bring about
further attack of the metal. On the other hand, aluminium, tungsten and molybdenum form oxide layers of greater volume than the
metal from which they were produced. These non-porous, continuous and coherent oxide films prevent the diffusion of oxygen
and hence the rate of further attack decreases with increase in the thickness of the oxide film.
The protective or non-protective nature of the oxide film is determined by a rule known as the Pilling-Bedworth rule.
The ratio of the volume of the oxide formed to the volume of the metal consumed is called the Pilling-Bedworth rule.
According to it, if the volume of the oxide layer is greater than the volume of the metal, the oxide layer is protective and nonporous. On the other hand, if the volume of the oxide layer formed is less than the volume of the metal, the oxide layer is nonprotective and porous.
[2] Electrochemical Theory or Wet Corrosion
According to the electrochemical theory, the corrosion of a metal in aqueous solution may be a two-step process, one
involving oxidation and another reduction. It is known that two metals having different electrode potentials form a galvanic cell
when they are immersed in a conducting solution. The emf of the cell is given by the difference between the electrode potentials.
When the electrodes are joined by a wire, electrons flow from the anode to the cathode. The oxidation reaction occurs at the
anode, i.e. at the anode the metal atoms lose their electrons to the environment and pass into the solution in the form of positive
ions. Fe Fe2+ + 2e-. Thus, there is a tendency at the anode to destroy the metal by dissolving it as ions. Hence corrosion always
occurs at anodic areas. The electrons released at the anode are conducted to the cathode and are responsible for various cathodic
reactions such as electroplating (deposition of metals), hydrogen evolution and oxygen absorption: (i) Electroplating: The metal
ions at the cathode collect the electrons and deposit on the cathode surface. Cu2+ + 2e- Cu (ii) Liberation of hydrogen: In an acid
solution, (in the absence of oxygen) hydrogen ions accept electrons and hydrogen gas is formed. 2H+ + 2e- H2.
In a neutral or alkaline medium, (in the absence of oxygen) hydrogen gas is liberated with the formation of OH- ions.
2H2O + 2e- H2 + 2OH- (iii) Oxygen absorption: In the presence of dissolved oxygen and in an acid medium, oxygen absorption
reaction takes place. 4H+ + O2 + 4e- 2H2O
In the presence of dissolved oxygen and in a neutral or weakly alkaline medium, OH- ions are formed.
2H2O + O2 + 4e- 4OH- Thus it is clear that the essential requirements of electrochemical corrosion are as follows:
(a) Formation of anodic and cathodic areas. (b) Electrical contact between the cathodic and anodic parts to enable the conduction
of electrons. (c) An electrolyte through which the ions can diffuse or migrate. This is usually provided by moisture.
Galvanic corrosion: Galvanic corrosion is a type of electrochemical corrosion in which two different types of metals in
contact are jointly exposed to corrosive atmosphere. Here the metal with more negative electrode potential will become the anode
and goes into solution or corrode, e.g. (a) zinc and copper metals and (b) steel pipe connected to copper plumbing. This corrosion
can be minimized by (a) avoiding galvanic couples and (b) providing an insulating material between the two metals.
Galvanic Series: the galvanic series is used to provide sufficient information in predicting the corrosion behaviour in a particular
set of environmental conditions. Oxidation potential measurements of various metals and alloys have been made using the
standard calomel electrode as the reference electrode and immersing the metals and alloys in sea water. These are arranged in
decreasing order of activity and this series is known as the galvanic series. The galvanic series gives more practical information
on the relative corrosion tendencies of the metals and alloys. The speed and severity of corrosion depends upon the difference in
potential between the anodic and cathodic metals in contact.
In a similar way, iron corrodes under drops of water or salt solution. Areas covered by droplets, having less access of
oxygen become anodic with respect to the other areas which are freely exposed to air. Differential aeration corrosion occurs when
one part of metal is exposed to a different air concentration from the other part. This causes a difference in potential between
differently aerated areas. It is experimentally found that poor oxygenated parts are anodic. Consequently, differential aeration of a
metal causes a flow of current.
Stress corrosion cracking: Corrosion of metals is also influenced by some physical differences like internal stresses in the
metals. Such differences result during manufacture, fabrication and heat treatment. Metal components are subjected to unevenly
distributed stresses during their manufacturing process. The electrode potential thus varies from one point to another.
Areas under great stress act as the anode while areas not under stress act as the cathode. Various treatments of metals and alloys
such as cold working or quenching, bending and pressing introduce uneven stress and lead to stress corrosion. Corrosion takes
place so as to minimize the stress. Most of the time it ends in breaking of the components into pieces. Corrosion of head and point
portions of a nail indicates that they have been acting as anode to the middle portion. Actually the head and the point portions
were put under stress during their manufacture. In the case of iron-wire hammered at the middle, corrosion takes place at the
hammered part and results in breaking of the wire into two pieces. Caustic embrittlement takes place in stressed parts such as
bends, joints and rivets in boilers.
.
The diagram shows clearly the zones of corrosion, immunity and passivity. In the diagram x is a point where pH is 7 and
the electrode potential is -0.4 V. It is present in the corrosion zone. This shows that iron rusts in water under those conditions.
This is noticed to be true in actual practice also. From figure, it is seen that the rate of corrosion can be altered by shifting the
point x into immunity or passivity regions. The iron would be immune to corrosion if the potential is changed to about -0.8 V and
this can be achieved by applying external current. On the other hand, the corrosion rate of iron can also be reduced by moving into
the passivity region by applying positive potential. The diagram clearly indicates that the corrosion rate can also be reduced by
increasing the pH of the solution by the addition of alkali without disturbing the potential.
[4] Nature of the electrolyte: The nature of the electrolyte also influences the rate of corrosion. If the electrolyte consists
of silicate ions, they form insoluble silicates and prevent further corrosion. On the other hand, if chloride ions are present, they
destroy the protective film and the surface is exposed for further corrosion. If the conductance of electrolyte is more, the corrosion
current is easily conducted and hence the rate of corrosion is increased.
[5] Concentration of oxygen and formation of oxygen concentration cells: The rate of corrosion increases with
increasing supply of oxygen. The region where oxygen concentration is lesser becomes anodic and suffers corrosion.
Corrosion often takes place under metal washers where oxygen cannot diffuse readily. Similarly, buried pipelines and cables
passing from one type of soil to another suffer corrosion due to differential aeration, e.g. lead pipeline passing through clay and
then through sand. Lead pipeline passing through clay get corroded because it is less aerated than sand.
Corrosion Control: Corrosion can be controlled by the following ways: [1] By selection of the material: Selection of
the right type of the material is the main factor for corrosion control. Thus, noble metals are used for surgical instruments and
ornaments as they are most immune to corrosion. [2] By using pure metals: Pure metals have higher corrosion resistance.
Even minute amount of impurities may lead to severe corrosion, e.g. 0.02% iron in aluminium decreases its corrosion resistance.
[3] By alloying: Both corrosion resistance and strength of many metals can be improved by alloying, e.g. stainless steels
containing chromium produce a coherent oxide film which protects the steel from further attack. [4] By annealing: Heat treatment
like annealing helps to reduce internal stresses and reduces corrosion. [5] By eliminating galvanic action: If two metals have to
be in contact, they should be so selected that their oxidation potentials are as near as possible. Further, the area of the cathode
metal should be smaller than that of the anode, e.g. copper nuts and bolts on large steel plate. The corrosion can also be reduced
by inserting an insulating material between the two metals. [6] By cathodic protection: The principle involved in cathodic
protection is to force the metal behave like a cathode. Since there will not be any anodic area on the metal, corrosion does not
occur. There are two types of cathodic protection. (a) Sacrificial anodic protection. (b) Impressed current cathodic protection.
(a) Sacrificial anodic protection: In this technique, a more active metal is connected to the metal structure to be protected
so that all the corrosion is concentrated at the more active metal and thus saving the metal structure from corrosion. This method is
used for the protection of sea going vessels such as ships and boats. Sheets of zinc or magnesium are hung around the hull of the
ship. Zinc and magnesium being anodic to iron get corroded. Since they are sacrificed in the process of saving iron (anode) they
are called sacrificial anodes. The corroded sacrificial anode is replaced by a fresh one, when consumed completely.
Important applications of sacrificial anodic protection are as follows: [i] Protection from soil corrosion of underground cables and
pipelines (Fig. a). [ii] Magnesium sheets are inserted into domestic water boilers to prevent the formation of rust water (Fig. b).
(b) Impressed current cathodic protection: In this method, an impressed current is applied in an opposite direction to
nullify the corrosion current and converting the corroding metal from anode to cathode. This can be accomplished by applying
sufficient amount of direct current from a battery to an anode buried in the soil and connected to the corroding metal structure
which is to be protected. The anode is in a backfill (composed of gypsum) so as to increase the electrical contact with the soil.
Since in this method current from an external source is impressed on the system, this is called impressed current method.
[8] By modifying the environment: The corrosion rate can be reduced by modifying the environment. The environment
can be modified by the following: (a) Deaeration: The presence of increased amounts of oxygen is harmful since it increases the
corrosion rate. Deaeration aims at the removal of dissolved oxygen. Dissolved oxygen can be removed by deaeration or by adding
some chemical substances like Na2SO3 (b) Dehumidification: In this method, moisture from air is removed by lowering the
relative humidity of surrounding air. This can be achieved by adding silica gel which can adsorb moisture preferentially on its
surface. (c) Inhibitors: In this method, some chemical substances known as inhibitors are added to the corrosive environment in
small quantities. These inhibitors substantially reduce the rate of corrosion.
(i) Anodic inhibitors: Anodic inhibitors include alkalis, molybdates, phosphates. chromates, etc. When these inhibitors are
added, they react with the ions of the anode and produce insoluble precipitates. The so formed precipitate is adsorbed on the anode
metal forming a protective film thereby reducing corrosion. (ii) Cathodic inhibitors: In an electrochemical corrosion, the cathodic
reactions are of two types depending on the environment. In acidic solution, the cathodic reaction is 2H+ + 2e- H2.
In acidic solution; the corrosion can be controlled by slowing down the diffusion of H+ ions through the cathode. This can be done
by adding organic inhibitor like amines and pyridine. They adsorb over the cathodic metal surface and act a protective layer.
In neutral solution, the cathodic reaction is H2O + 1/2O2 + 2e- 2OH- The formation of OH- ions is only due to the presence of
oxygen. By elimination the oxygen from the medium, the corrosion rate can be reduced. Oxygen can be removed by adding some
reducing agents (Na2SO3) or by deaeration. (iii) Vapour phase inhibitors: Vapour phase inhibitors (VPIs) are organic inhibitor
which readily sublime and form a protective layer on the metal surface, e.g., dicyclohexyl ammonium nitrite. They are used in the
protection of machinery, sophisticated equipment, etc. which are sent by ships. The condensed inhibitor can be easily wiped off
from the metal surface.
[9] By Passivation: Passivation is a phenomenon of converting an active surface of a metal into passive i.e. more
corrosion resistant by forming a thin, non-porous and highly protective film over the surface. When the metals like aluminium and
tin are exposed to the atmosphere or to the oxidizing environment, their surfaces rapidly get converted into oxides. The nonporous nature of these oxide layers prevents further corrosion. In other words, the metals are passivated. Similarly, metals which
are susceptible to corrosion are made passive by alloying with one or more metals which are passive or resist corrosion. For
example, iron is rendered passive by alloying it with any of the transition metals such as chromium, nickel and molybdenum.
[10] By applying protective coating: The surface of engineering material can be protected from corrosion by covering it
by a protective coating. This coating may be of organic or inorganic material.
Protective Coatings
Introduction: Protective coatings are used to protect the metals from corrosion. The main types of protective coatings are
classified as follows.
The protective coatings must be chemically inert to the environment and also sufficiently thick. Besides protection from
corrosive conditions, such coatings can also give decorative appearance to the base metals. To be more effective, these coatings
should adhere well to the surface.
Pretreatment of the surface or preparation of materials for coating: The outermost surface of the base metal
(which is to be protected) usually contains impurities like rust, scale and grease. These substances, if present at the time of
coating, will give porous and discontinuous coatings. In order to get a uniform, smooth and coherent protective coating, these
substances are removed by the following methods. [1] Degreasing: Oil and grease may be removed by cleaning with organic
solvents such as chloroform, toluene and acetone. Immersion in hot alkaline solutions is the most commonly
used cleaning technique. For example, sodium carbonate and sodium hydroxide are used for this purpose.
[2] Removal of Oxide Scales or Descaling: Removal of the oxide scales and corrosion products (rust) from the surface is called
descaling. In this process, the base metal is dipped inside the acid solution at higher temperatures. The acid penetrates through
cracks and pores of the scales and then their dissolution takes place. Acids like sulphuric acid, hydrochloric acid and nitric acid are
used under dilute conditions. [3] Mechanical Cleaning: Oxide scales, rust and corrosion products are also removed by abrasion
such as grinding, wire brushing and polishing. [4] Electrochemical Method: The electrochemical method is used to remove oxide
scales which are not removed by other methods. The base metal is made either anode or cathode with an electrolyte (acid or base).
At the anode the oxide scale is dissolved in the electrolyte and leaves the base metal, whereas at the cathode the metal oxides are
reduced to metal.
Metallic Coatings:
Coatings: Surface coatings made up of metals are known as metallic coatings. These coatings separate the
base metal from the corrosive environment and also function as an effective barrier for the protection of base metals.
Metallic coatings are mostly applied on iron and steel because they are cheap and commonly used. Metallic coatings are usually
imparted by the following methods.
Hot Dipping: In the process of hot dipping, the metal to be coated is dipped in the molten bath of the coating metal and
the thickness of the coating is adjusted by squeezing out the excess of the coating metal with rollers. Such hot dip coatings are
generally non-uniform. The common examples of hot dip coatings are galvanizing and tinning.
[1] Galvanizing: The process of coating a layer of zinc on steel is called galvanizing. The steel article is first pickled with
dilute sulphuric acid to remove traces of rust, dust, etc, at 60-90C for about 15-20 minutes. Then this metal is dipped in a molten
zinc bath maintained at 430C. The surface of the bath is covered with ammonium chloride flux to prevent oxide formation on the
surface of molten zinc. The coated base metal is then passed through rollers to correct the thickness of the film.
It is used to protect roofing sheets, wires, pipes, tanks, nails, screws, etc.
[2] Tinning: The coating of tin on iron is called tin plating or tinning. In tinning, the base is first pickled with dilute
sulphuric acid to remove surface impurities. Then it is passed through molten tin covered with zinc chloride flux. The tin coated
article is passed through a series of rollers immersed in a palm oil bath to remove the excess tin. Tin-coated utensils are used for
storing foodstuffs, oils, etc. Galvanizing is preferred to tinning because tin is cathodic to iron, whereas zinc is anodic to iron.
So, if the protective layer of the tin coating has any cracks, iron will corrode. If the protective layer of the zinc coating has any
cracks, iron being cathodic does not get corroded. The corrosion products fill up the cracks, thus preventing corrosion.
Cementation: In cementation, the base metal is heated with the coating metal in the form of fine powder in order to
promote the diffusion of the coating metal into the base metal. The coatings obtained are of uniform thickness. The base metal is
generally steel and the coating metals used is zinc, chromium and aluminium. When the coating metal is zinc, the process is called
sherardizing. When the coating metal is chromium, the process is called chromizing. When the coating metal is aluminium,
the process is called calorizing.
[1] Sherardizing: Cementation with zinc powder is called sherardizing. The base metal is heated with zinc dust in a metal
drum maintained at a temperature of 350-370C. The drum is closed tightly and rotated with constant heating for two to three
hours. During this process zinc gets diffused into iron forming an alloy of Fe-Zn on the surface. Sherardized coatings are used for
protecting small steel parts such as nuts and bolts against atmospheric corrosion.
[2] Chromizing: The base metal is heated with a powdered mixture of 55 per cent chromium and 45 per cent alumina at
a temperature of 1300-1400C for about 3-4 hours in a closed drum. The purpose of using alumina is to prevent the coalescing of
chromium particles. The outermost surface of the base metal is converted into a chrome alloy which protects the metal against
corrosion. This method is used to protect gas turbine blades. [3] Calorizing: Here the base metal is heated with a powdered
mixture of aluminium and alumina in a drum at a temperature of 840-930C for 4-6 hours.
Electroplating or Electrodeposition: Electroplating is a process in which metals are deposited or plated on base metals
from solutions containing metallic ions by means of electrolysis. The objectives of electroplating are as follows:
(1) To obtain improved resistance to corrosion and chemical attack (2) To get better appearance (3) To get increased hardness
(4) To change the surface properties of metals and non-metals. In the electroplating process, the freshly cleaned base metal which
is to receive the coat is made the cathode in a suitable electrolyte bath containing (a) a solution of the salt of the metal 10 be
electrodeposited, (b) buffer solution to control the pH, and (c) additional reagents to enhance conductivity and to aid the formation
of smooth, dense and coherent coating. The concentration of the salt solution is maintained by the addition of the metal salt at
regular intervals or by the use of continuously dissolving anode of the metal. The plating is usually done at a high current density.
The nature of the deposit depends upon the current density, pH and the concentration of the bath.
Organic Coatings: Organic coatings are inert organic barriers applied to the surface of base metals for corrosion
resistance and decoration. Paints, varnish, lacquers and enamels are the main organic coatings. Paints: Paint is a viscous
suspension of finely divided solid pigment in a fluid medium which on drying yields an impermeable film having considerable
hiding power. When paint is applied to a metal surface, the thinner evaporates, while the drying oil slowly oxidizes
forming a dry pigmented film. Requirements of a good paint: A good paint should essentially have the following:
[1] A good paint should form a good impervious and uniform film on the metal surface. [2] It should have a high hiding
(covering) power. [3] The film should not crack on drying. [4] A good paint should adhere well to the surface.
[5] It should spread on the metal surface easily. [6] It should give a glossy film. [7] It should be corrosion resistant. [8] A good
paint should give a stable and decent colour on the metal surface.
Constituents of paint and their functions: The important constituents of paint are as follows: (1) Pigments (2) Vehicle or
drying oils or medium (3) Thinners (4) Driers (5) Fillers or extenders (6) Plasticizers (7) Anti-skinning agents
(1) Pigments: A pigment is a solid and colour-producing substance in the paint. (2) Vehicle or drying oils or medium:
The liquid portion of the paint in which the pigment is dispersed is called a medium or vehicle. This is the film forming constituent
of the paint. Vehicles are high molecular weight fatty acids present in animal and vegetable oils, e.g. linseed oil, dehydrated castor
oil, soyabean oil and fish oil. (3) Thinners: Thinners are added to paints to reduce the consistency or viscosity of the paints so
that they can be easily applied to the metal surface. Thinners are volatile in nature and evaporate easily after application of the
paint, e.g. turpentine and petroleum spirit (4) Driers: Driers are used to accelerate or catalyze the drying of the oil film by
oxidation, polymerization and condensation, e.g. naphthenates, borates and tungstales of lead, cobalt and manganese.
(5) Fillers or extenders: Fillers are used to reduce the cost and increase the durability of the paint, e.g. talc, china clay, calcium
sulphate and calcium carbonate. (6) Plasticizers: Plasticizers are chemicals added to paints to give elasticity to the film and to
prevent cracking of the film, e.g. triphenyl phosphate and tricresyl phosphate. (7) Anti-skinning agents: They are chemicals
added to the paint to prevent skinning of the paint, e.g. polyhydroxy phenols.
Failure of paint: Paint may fail due to several reasons: (1) Chalking: Chalking is the gradual powdering of paint. It is
caused by continuous destructive oxidation of the oil after the original drying of paint. (2) Erosion: It is very quick chalking. (3)
Flaking: Flaking is caused due to poor adherence of a paint film to the surface because of the presence of grease on the surface.
(4) Checking: It is a very fine type of surface cracking. (5) Alligatoring: The centre portion of the film remains attached to the
surface, whereas the portion around the centre peels off.
The failure of paint can be prevented by the following ways: (1) Carefully preparing the surface before application of
paint. (2) Applying a suitable primer coat. (3) Applying the paint evenly. (4) Allowing each paint coat to dry sufficiently before
the next coat is applied.
Questions
Marks
What is the importance of bulk and trace metal ions in biological systems
What is green chemistry? How is it important? What are the goals of green chemistry and its limitations?
Write briefly about (a) Safer solvents; (b) Energy efficiency by design; (c) Atom economy
Explain about (i) Use of renewable raw materials and (ii) Shorter synthetic methods
Explain the different steps used for the prevention of chemical accidents
Write notes on any two of the synthetic methods used in green chemistry
10
Explain 18-electron rule with any two examples. Give their importance
What are pi-acceptor ligands? Discuss in details the nature of bonding involved in metal carbonyls
What are metal carbonyls? Discuss mononuclear and polynuclear carbonyls with examples
7
Module II - Lubricants
No
Questions
Marks
Draw and explain the structure of graphite and molybdenum disulphide. How are they used as lubricants?
10
How do viscosity and viscosity index influence the selection of lubricants for a particular purpose?
11
Write short notes on: (i) Solid lubricants (ii) Flash and Fire point (iii) Aniline point (iv) Cloud and Pour point
10
Module II - Fuels
No
Questions
Marks
Define calorific value of a fuel. How HCV and LCV are related?
Distinguish between Gross (or higher) and Net (or lower) calorific value of fuel
What is cracking and what for it used? What are the types of cracking?
10
How do you explain knocking in a diesel engine? How can it be controlled? What is cetane number?
10
Module II - Polymer
No
Questions
Marks
Differentiate between thermoplastic and thermosetting plastic. Give two examples of each type
10
10
11
What is glass transition temperature? Discuss the factors affecting Tg. What is its significance?
12
Module
Module IV - Water
No
Questions
Marks
What is the hardness of water? How is it expressed? Describe the different types of hardness
Calculate the temporary and permanent hardness of a sample of water containing Mg(HCO3)2 7.3 mg/L,
Ca(HCO3)2 16.2 mg/L, MgCl2 9.5 mg/L and CaSO4 13.6 mg/L. Atomic weight of Ca and Mg is 40 and 24 resp
Calculate the temporary and permanent hardness of a sample of water containing Mg(HCO3)2 75 mg/L,
CaCl2 278 mg/L and MgSO4 142 mg/L.
4
5
Distinguish between Hard water and Soft water. What is break point chlorination?
Why buffer is added during titration of hard water against EDTA solution?
Write the structure of EDTA and EBT
How is hardness of water determined experimentally by EDTA titration method?
Write the necessary calculations
0.30 g of CaCO3 was dissolved in HCl and the solution made on to one litre with distilled water. 100 ml of this solution
required 30 ml of EDTA solution on titration. 100 ml of hard water sample required 55 ml of EDTA solution on titration.
After boiling 100 ml of this water, cooling, filtering and then titration required 10 ml of EDTA solution.
Calculate the temporary and permanent hardness of water
20 ml of std hard water (containing 15 g CaCO3/litre) required 25 ml of EDTA solution for end point 100 ml of water
sample required 18 ml of EDTA solution while the same water after boiling required 12 ml EDTA solution.
Calculate total hardness of water
20 ml of standard hard water (containing 20g of CaCO3 per litre) requires 30 ml of EDTA solution for end point. 100 ml of
water sample requires 20 ml of EDTA solution. Calculate carbonate and non-carbonate hardness of water.
Why do we add buffer solution in this titration
10
5
5
7
5
8
11
12
A water sample contains Ca(HCO3)2 324 ppm, Mg(HCO3)2 73 ppm, MgCl2 190 ppm, MgSO4 60 ppm and
CaSO4 136 ppm. Determine the quantity of lime soda needed to soften 50,000 litres of water
13
With a neat diagram, discuss the demineralization of water using ion exchange method
14
What is potable water? What are the steps taken to obtain pure drinking water
15
Enumerate the various stages involved in the purification of water for domestic use
16
17
18
19
20
21
Questions
Marks
Describe the Standard Hydrogen Electrode and its use in the determination of single electrode potential
Outline the Poggendorfs compensation method for the measurement of emf of a cell
What is an electrochemical cell? How EMF is measured by using hydrogen electrode and glass electrode?
10
10
11
12
13
14
15
16
17
What is concentration cell? Explain its working with example. Write the electrode and cell reactions
18
19
The copper rods are placed in copper sulphate solution of concentration of 0.1 M and 0.01 M respectively
to form a cell. Give the cell representation and calculate its emf at 298 K
20
21
22
23
24
25
Derive the expression for the determination of pH of a solution using glass electrode. Mention its advantages
26
The emf of a glass electrode in a solution of an acid of unknown strength is 0.29V at 298K as measured
against standard calomel electrode. Calculate the pH of an acid solution (EG0 is 1 V)
27
28
What is alkaline battery? Describe the construction and working of Ni-Cd battery with relevant reactions
taking place during discharge. Mention the advantages and applications
29
30
31
Describe the construction and functioning of lead acid accumulators and Nickel-Cadmium cells
10
32
Write a short note on fuel cell. How is it different from commercial cells? Mention the advantage of fuel cell
33
Define fuel cell. Explain the construction and working of H2-O2 fuel cell. Write the cell reactions involved
34
35
Discuss the construction, functioning and applications of Fuel cells and Solar cells
10
Module IV Corrosion
No
Questions
Marks
01
02
Explain the mechanism of dry corrosion. Explain the role of oxide film in dry corrosion and classify them
03
04
05
Corrosion products can act as a protective layer. Explain this term with the help of pilling Bedworth rule
06
07
08
09
10
11
12
Explain the rusting of iron with the help of electrochemical theory of corrosion
10
13
Discuss the mechanism of rusting of iron in (a) acid medium (b) alkaline medium
10
14
15
10
16
17
18
19
20
21
22
10
23
24
We can use Al in the place of Zn for cathodic protection of rusting of Fe. Comment the statement
25
What is cathodic and anodic protection for controlling corrosion? Discuss their merits and demerits
26
Give details of corrosion protection through sacrificial anodic method and impressed current method
27
28
29
30
31
What are the necessities of protective coatings? Explain the mechanism and applications of anodized oxide coating
10
32
33
34
10
35
36
37
38
Write notes on: (a) Galvanizing (b) Sheradising (c) Anodisation of aluminium
39
40
41
42
43
46
What are paints? What are the characteristics? Mention the essential ingredients of paint.
What are their functions? Give examples
Theory
8 x 5 marks = 40 marks
There shall be minimum of two and maximum of three questions from each module with total ten questions.
Two questions from each module with choice to answer one question
4 x 15 marks = 60 marks
Minimum for Pass is 75 in 150 (minimum 40 in 100 from External and 35 in 50 from Internal)
Part A
Answer any eight questions
Each question carries 5 marks
[1] Discuss the 18-electron rule with two examples
[2] What is the importance of bulk and trace metal ions in biological systems
[3] Discuss about green synthesis
[4] Write about the concept of Tg and crystallinity in polymers
[5] Write a note on the term solid lubricants
[6] What is cracking and what for it used? What are the types of cracking?
[7] What is electrochemical series? What are its applications?
[8] How pH is measured by using glass electrode?
[9] Describe the various factors influencing the corrosion
[10] Among BOD and COD, which is greater? Why?
Part B
Answer one full question from each module
II
III
IV
Module I
(A) (i) What are organometallic compounds? Explain the classification of organometallic compounds with examples {8 marks}
(ii) Explain the twelve principles of green chemistry {7 marks}
Or
(B) (i) Give elementary idea about hemoglobin and myoglobin{7 marks}
(ii) What are metal carbonyls? Discuss the nature of bonding involved in it and write examples for mononuclear and polynuclear
carbonyls {8 marks}
Module II
(A) (i) Explain free radical, cationic and anionic mechanism of polymerization {8 marks}
(ii) Outline the mechanism of lubrication {7 marks}
Or
(B) (i) How does knocking occur in I.C engines? How can it be prevented? {8 marks}
(ii) Briefly explain the techniques of polymerization {7 marks}
Module III
(A) (i) Derive the Nernst equation for electrode potential {7 marks}
(ii) Describe the construction and functioning of lead acid accumulators and NickelCadmium cells {8 marks}
Or
(B) (i) How is EMF of an electrochemical cell determined through Poggendorfs compensation method? {7 marks}
(ii) Write a descriptive account on Fuel cells and Solar cells {8 marks}
Module IV
(A) (i) Give details of corrosion protection through sacrificial anodic method and impressed current method {8 marks}
(ii) Enumerate the various stages involved in the purification of water for domestic use {7 marks}
Or
(B) (i) How is hardness of water determined experimentally by EDTA titration method? Write the structure of EDTA,
EBT and necessary calculations {8 marks}
(ii) What is meant by differential aeration corrosion? Illustrate with suitable examples {7 marks}