Surface Chemistry-Hsslive PDF
Surface Chemistry-Hsslive PDF
Surface Chemistry-Hsslive PDF
It is the branch of chemistry that deals with the study of nature of surfaces and the different processes taking
place at the surface. The important surface phenomena are adsorption, corrosion, electrode process, heterogeneous
catalysis, dissolution etc.
ADSORPTION
It is the process of concentration or accumulation of a substance on the surface of another substance. The
substance which is adsorbed is called adsorbate and the substance whose surface on which adsorption takes place is
called adsorbent. The commonly used adsorbents are charcoal, silica gel, alumina gel, clay, colloids, metals in finely
divided state etc.
Adsorption is a surface phenomenon. Some examples of adsorption are:
1. Powdered charcoal adsorbs gases like H2, O2, CO, Cl2, NH3, SO2 etc.
2. Silica gel adsorbs moisture
3. Animal charcoal adsorbs colouring material from sugar solutions
Desorption: The process of removal of an adsorbed substance from the surface of adsorbent is called desorption.
i.e. it is the reverse of adsorption.
Distinction between adsorption and absorption
In adsorption, the substance is concentrated only at the surface while in absorption, the substance is uniformly
distributed throughout the bulk of the solid. So adsorption is a surface phenomenon while absorption is a bulk
phenomenon.
Sorption: If adsorption and absorption occur simultaneously, the process is called sorption.
Mechanism of Adsorption
The surface particles of the adsorbent are not in the same environment as the particles inside the bulk (inner part).
Inside the adsorbent, all the forces are mutually balanced. But at the surface, there is always some unbalanced or
residual forces. These forces of the adsorbent are responsible for adsorption.
Heat of adsorption (Enthalpy of Adsorption)
Adsorption is an exothermic process. i.e. some heat is always evolved during adsorption. The amount of heat evolved
when 1 mole of an adsorbate is adsorbed on the surface of an adsorbent is called heat of adsorption.
Thermodynamic aspects of adsorption
Adsorption is an exothermic process. When a gas is adsorbed, the degree of freedom (randomness) of its
molecules decreases and hence the entropy decreases. i.e., ΔS becomes negative. Adsorption is thus accompanied by
decrease in enthalpy as well as decrease in entropy of the system. For a process to be spontaneous, ΔG must be
negative. On the basis of equation, ΔG = ΔH – TΔS, ΔG can be negative if ΔH >TΔS. As the adsorption proceeds, ΔH
becomes less and less negative ultimately ΔH becomes equal to TΔS and ΔG becomes zero. At this state equilibrium is
attained.
Types of adsorption
Depending on the force of attraction between adsorbent and adsorbate, adsorption is of two types – physical
adsorption or physisorption and chemical adsorption or chemisorption.
If the force of attraction between adsorbent and adsorbate is weak van der Waals force, it is called physical
adsorption or physisorption. For physisorption, the heat of adsorption is low and it is not specific since the van der
Waals forces are universal. That is any substance can form van der Waals force with any surface.
In chemisorptions, the force of attraction between adsorbent and adsorbate is chemical bond. It is also called
activated adsorption since it involves some activation energy. For chemisorption, the enthalpy of adsorption is high and
it takes place at high temperature. It is highly specific in nature and it will occur only if there is some possibility of
chemical bonding between adsorbent and adsorbate.
A physisorption at low temperature may pass into chemisorption at high temperature. For e.g. Hydrogen gas is
first adsorbed on nickel by van der Waals force. But at high temperature, the molecules of H2 dissociate to form H atoms
and they are adsorbed on the surface of Ni by chemical bond.
Both physisorption and chemisorption increases with increase in surface area of the adsorbent. Surface area can
be increased by powdering the adsorbent.
From the graph it is clear that x/m (extend of adsorption) increases with
pressure upto a certain pressure called saturation pressure (Ps) and
after that it becomes constant.
If we take logarithm of the above equation, we get x/m
log x/m = logk + 1/n logP
This equation is of the form y = mx+c, equation for a straight line. So if we
plot log x/m against log P, we get a straight line, which verifies Freundlich isotherm.
The value of 1/n in Freundlich isotherm ranges from 0 to 1
When 1/n = 0, x/m = a constant. i.e. the adsorption is independent of pressure. P Ps
When 1/n = 1, x/m = k.p, the adsorption varies linearly with pressure.
Freundlich adsorption isotherm failed to explain adsorption at very high pressures.
Adsorption from solution
Certain solid adsorbents can adsorb solute particles from solution. This is known as adsorption from solution. E.g.:
1. When a solution of acetic acid in water is shaken with charcoal, a part of the acid is adsorbed by the charcoal
and the concentration of the acid decreases in the solution.
2. Animal charcoal adsorbs colouring materials from sugar solution. So it is used for the purification of sugar.
The important characteristics of adsorption from solution are:
(i) The extent of adsorption decreases with an increase in temperature.
(ii) The extent of adsorption increases with an increase of surface area of the adsorbent.
(iii) The extent of adsorption depends on the concentration of the solute and the nature of the adsorbent and the
adsorbate.
A catalyst is a substance that changes the rate of a chemical reaction without undergoing any permanent
chemical change by itself. The process of changing the rate of a chemical reaction by a catalyst is known as Catalysis.
Eg: MnO2 (Manganese dioxide) acts as a catalyst in the decomposition of KClO3 (Potassium chlorate)
2KClO3 → 2KCl + 3 O2
Promoters and poisons
Promoters are substances that enhance the activity of a catalyst while poisons decrease the activity of a catalyst.
For example, in Haber’s process for the manufacture of ammonia, molybdenum (Mo) acts as a promoter for the catalyst
iron.
N2 +3H2 Fe/Mo 2NH3
Types of Catalysis
Positive and Negative Catalyst
A catalyst that increases the rate of a chemical reaction is called Positive catalyst and that decreases the rate of
a chemical reaction is called negative catalyst (inhibitors).
E.g. In the Haber’s process for the manufacture of ammonia, Fe acts as a positive catalyst
N2 +3H2 Fe/Mo 2NH3
For decreasing the rate of dissociation of H2O2, Phosphoric acid is used as a negative catalyst.
Homogenous and Heterogeneous Catalysis
Homogeneous Catalysis
A catalytic process in which the reactants and the catalyst are in the same phase (i.e., liquid or gas), is said to be
homogeneous catalysis.
e.g.: (i) In the lead chamber process for the manufacture of Sulphuric acid, oxidation of sulphur dioxide into
sulphur trioxide is done in the presence of Nitric Oxide as catalyst
2SO2 (g) + O2 (g) NO (g) 2SO3 (g)
Here the reactants (sulphur dioxide and oxygen) and the catalyst (nitric oxide) are all in the same phase.
(ii)Acid catalysed hydrolysis of methyl acetate
CH3COOCH3(l) + H2O(l) HCI(l) CH3COOH(aq) + CH3OH(aq)
(iii) Hydrolysis of sugar is catalysed by H+ ions furnished by sulphuric acid.
C12H22O11(l) + H2O(l) H2SO4 C6H12O6(aq) + C6H12O6(aq)
Here the reactants are in gaseous state while the catalyst is in the solid state.
(ii) In Haber’s process for the manufacture of ammonia finely divided iron is used as catalyst.
N2(g) +3H2 (g) Fe(s) 2NH3
Here the reactants are in gaseous state while the catalyst is in the solid state.
(iii) Oxidation of ammonia into nitric oxide in the presence of platinum gauze in Ostwald’s process.
Here also the reactants are in gaseous state while the catalyst is in the solid state.
(iv) Hydrogenation of vegetable oils in the presence of finely divided nickel as catalyst.
Adsorption Theory of Heterogeneous Catalysis
This theory explains the mechanism of heterogeneous catalysis. According to this theory the catalytic activity
takes place on the surface of the catalyst. The mechanism involves five steps:
(i) Diffusion of reactants to the surface of the catalyst.
(ii) Adsorption of reactant molecules on the surface of the catalyst.
(iii) Occurrence of chemical reaction on the catalyst’s surface through formation of an intermediate.
(iv) Desorption of reaction products from the catalyst surface.
(v) Diffusion of reaction products away from the catalyst’s surface.
This theory explains why the catalyst remains unchanged in mass and chemical composition at the end of the reaction
and is effective even in small quantities. But it does not explain the action of
catalytic promoters and catalytic poisons.
Important features of solid catalysts
1. Activity
The activity is the ability of a catalyst to increase the rate of a chemical reaction. It depends upon the strength of
chemisorption.
e.g.: H2 combines with O2 to form H2O in presence of Platinum (Pt) catalyst
H2 + O 2 Pt H2O
In absence of Pt, the reaction does not take place.
2. Selectivity
It is the ability of a catalyst to direct a chemical reaction to a particular product.
e.g.: CO reacts with H2 to form different products based on the nature of the catalyst.
Enzyme Catalysis
Enzymes are complex nitrogenous organic compounds which are produced by living plants and animals. They
are actually protein molecules of high molecular mass. They are very effective catalysts and catalyse numerous reactions
taking place in plants and animals. So enzymes are also called biochemical catalysts and the phenomenon is known as
biochemical catalysis.
e.g.: (i) Inversion of cane sugar: The enzyme invertase converts cane sugar into glucose and fructose.
C12H22O11(aq) + H2O(l) invertase C6H12O6(aq) + C6H12O6(aq)
Cane sugar Glucose Fructose
(ii) Conversion of glucose into ethyl alcohol: The enzyme zymase converts glucose into ethyl alcohol and carbon dioxide.
C6H12O6(aq) zymase 2C2H5OH(aq) + 2CO2(g)
Glucose Ethanol
(iii) Conversion of starch into maltose: The enzyme diastase converts starch into maltose.
2(C6H10O5)n(aq) + nH2O(l) Diastase n C12H22O11(aq)
Starch Maltose
(iv) Conversion of maltose into glucose: The enzyme maltase converts maltose into glucose.
C12H22O11(aq) + H2O(l) Maltase 2C6H12O6(aq)
Maltose Glucose
(v) Decomposition of urea into ammonia and carbon dioxide: The enzyme urease catalyses this decomposition.
NH2CONH2(aq) + H2O(l) Urease 2 NH3(g) + CO2(g)
Urea
Characteristics of enzyme catalysis
The important characteristics of enzyme catalysis are:
1. Enzyme catalysis is highly specific in nature. I.e., Each enzyme is specific for a given reaction or an enzyme that
catalyses a particular reaction does not catalyse another reaction.
2. Enzyme activity is highly efficient. i.e., one molecule of an enzyme may transform one million molecules of the
reactant per minute.
3. The rate of an enzyme reaction becomes maximum at a definite temperature, called the optimum temperature.
The optimum temperature range for enzymatic activity is 298-310K.
4. The rate of an enzyme-catalysed reaction is maximum at a particular pH called optimum pH, which is between
pH values 5-7.
5. The enzymatic activity is increased in the presence of certain substances, known as co-enzymes.
6. Enzymes activity is inhibited or poisoned by the presence of certain substances.
Mechanism of enzyme catalysis
There are a number of cavities present on the surface of colloidal particles of enzymes. These cavities have
characteristic shape and possess active groups such as -NH2, -COOH, -SH, -OH, etc. These are the active centers on the
surface of enzyme particles. The molecules of the reactant (substrate) fit into these cavities just like a key fits into a lock.
So an activated complex is formed, which then decomposes to yield the products. This theory is known as lock and key
theory. Thus, the enzyme-catalysed reactions may be considered to proceed in two steps.
Step 1: The enzyme combines with the substrate to form an activated complex.
E + S → ES*
Step 2: Decomposition of the activated complex to form product.
ES* → E + P
The schematic representation of the mechanism of enzyme catalysis is as follows:
The RCOO– ions are present on the surface with their COO– groups in water and the hydrocarbon chains (R) at
the surface. But at critical micelle concentration, the anions are pulled into the bulk of the solution and aggregate to
form a spherical shape. Thus a micelle is formed.
c) Peptization:
The process of conversion of a freshly prepared precipitate into a colloidal sol by shaking it with suitable
dispersion medium in the presence of small amount of electrolyte is called peptization. The electrolyte added is called
peptizing agent.
The charge on the sol particles is mainly due to preferential adsorption of ions from solution. When 2 or more
ions are present in the dispersion medium, preferential adsorption of the ion common to the colloidal particle takes
place.
e.g. when AgNO3 is added to KI, AgI is precipitated, which adsorbs iodide ions from the dispersion medium and
thus get a negative charge.
AgNO3 + KI → AgI + KNO3
But when KI is added to AgNO3, the precipitated AgI adsorbs Ag+ ions from the solution and thus get a positive
charge.
Due to the positive or negative charge in the sol particles, they attract the counter ions (opposite ions) from the
medium. Thus a double layer of opposite charges is formed. This is known as Helmholtz electrical double layer. The
layer in which the ions are directly adsorbed to the sol particles is termed as fixed layer. The second layer is mobile and
is termed as diffused layer.
Due to the opposite charges on the fixed and diffused layers, there arises a potential difference between these
layers. This potential difference between the fixed layer and the diffused layer of opposite charges is called the
electrokinetic potential or zeta potential.
The presence of similar charges on colloidal particles leads to repulsion between the particles and prevent them
from coagulation when they come closer. So the charge on the sol particles is mainly responsible for the stability of
colloidal solution.
5. Electrophoresis:
Since colloidal particles carry charge, they move under the influence of an electric field. This movement of colloidal
particles is called electrophoresis. The positively charged sol particles move towards cathode (cataphoresis) and the
negatively charged particles move towards the anode (anaphoresis).
If the movement of the sol particles is prevented by some suitable method, the particles of dispersion medium
itself move under the presence of electric field. This migration is termed as electro-osmosis.
6. Coagulation (precipitation or flocculation)
The process of settling of colloidal particles is called coagulation or precipitation of the sol. This can be done by
different ways:
i) By electrophoresis
ii) By mixing two oppositely charged sols
iii) By continuous dialysis
iv) By boiling
v) By the addition of electrolyte
When an electrolyte is added to the sol, the ions carrying opposite charge to that of the sol neutralize the charge
and causes precipitation. The ion of the electrolyte which causes the precipitation is called the coagulating ion or the
flocculating ion. A negatively charged ion causes the precipitation of positively charged sol and vice versa.
Generally, the greater the valency of the coagulating ion, the greater will be the coagulating power. This is known
as Hardy – Schulze rule.
Thus for the coagulation of a negative sol like As2S3, the flocculating power of the +ve ions is of the order:
Al3+ > Ba2+ > Na+
Similarly for a +ve sol like ferric hydroxide, the flocculating power of the counter ion is of the order:
[Fe(CN)6]4- > PO43- > SO42- > Cl-
Coagulating value: The minimum concentration of an electrolyte in millimoles per litre required for the coagulation of a
sol within 2 hours is called coagulating value. The smaller the coagulating value, the higher will be the coagulation
power.
Protection of colloids
Lyophilic sols are self stabilized, while lyophobic sols require some stabilizing agents. For this purpose, some
lyophilic sols are added to lyophobic sols. These lyophilic sols are called protective colloids.
Surface Chemistry-Anil-HSSLiVE Page 10
When a lyophilic sol is added to a lyophobic sol, the lyophilic particles form a layer around lyophobic particles
and thus protect them from electrolytes.
EMULSIONS
These are colloidal solutions in which a liquid is dispersed in another liquid. Generally one of the two liquids is
water. There are two types of emulsions: 1. Oil in water (O/W) type and 2. Water in oil (W/O) type
In oil in water type emulsion, oil is the dispersed phase and water is the dispersion medium.
E.g. milk. In milk, the liquid fat is dispensed in water
In water in oil type emulsion, water is the dispersed phase and oil is the dispersion medium.
E.g. butter and cream
An emulsion obtained by mixing oil with water or water with oil is not stable. In order to prepare a stable
emulsion, a third substance called emulsifying agent is added. The emulsifying agent forms an inter facial film between
dispersed phase and the dispersion medium.
The common emulsifying agents for O/W emulsions are proteins, gums, natural and synthetic soaps, etc., and
for W/O, heavy metal salts of fatty acids, long chain alcohols, lampblack, etc.
The two types of emulsions can be distinguished by dilution with dispersion medium.
The droplets in emulsions carry negative charge and they can be precipitated by electrolytes. They also show Brownian
movement and Tyndall effect.
Applications of Colloids
1. Formation of Delta:
Deltas are formed at the river mouth. This is because river water is a negatively charged colloid of sand particles.
When this water enters into sea, the positive ions present in sea water coagulate the colloidal solution of sand and
so the particles settle down. This will result in the formation of delta.
2. Electrical precipitation of smoke (Cottrell precipitation):
Smoke is a colloidal solution of carbon, arsenic compounds, dust particles etc. in air. The smoke before coming
out of the chimney is passed through a chamber (Cottrell precipitator) containing plates having a charge opposite to
that of smoke particles. Thus neutralization of charges occurs and the particles settle down and pure air flows out of
the chimney.
3. Purification of drinking water:
The water obtained from natural sources often contains suspended impurities. In order to coagulate these
impurities, alum is added to water. The positive ions present in alum neutralize the suspended impurities and hence
get purified.
4. Medicines:
Most of the medicines are colloidal in nature. This is because they have large surface area and are therefore
easily assimilated. For example, argyrol is a silver sol used as an eye lotion. Colloidal antimony is used in curing
kalaazar. Colloidal gold is used for intramuscular injection.
5. Tanning:
Animal hides are colloidal in nature. When a hide, which has positively charged particles, is soaked in tannin
(which contains negatively charged colloidal particles) mutual coagulation takes place. This results in the hardening
of leather. This process is termed as tanning.
6. Photographic plates and films:
Photographic plates or films are prepared by coating an emulsion of the light sensitive silver bromide in gelatin
over glass plates or celluloid films.
7. Rubber industry:
Rubber latex is a colloidal solution of rubber particles which are negatively charged. Rubber is obtained by
coagulation of the latex.
8. Food articles:
Milk, butter, halwa, ice creams, fruit juices, etc., are all colloids in nature.
9. Blood: