Energy

Because energy takes many forms, only atoms and molecules. Temperature is a
some of which can be seen or felt, it is defined quantitative measure of “hot” or “cold.”
by its effect on matter. For example, When the atoms and molecules in an
microwave ovens produce energy to cook object are moving or vibrating quickly,
food, but we cannot see that energy. In they have a higher average kinetic energy
contrast, we can see the energy produced by a (KE), and we say that the object is “hot.”
light bulb when we switch on a lamp. In this When the atoms and molecules are
section, we describe the forms of energy and moving slowly, they have lower average
discuss the relationship between energy, KE, and we say that the object is “cold”
heat, and work.
(Figure 4.1). If no chemical reaction or
phase change (such as melting or
Energy can be defined as the capacity to vaporizing) occurs, increasing the amount
supply heat or do work. One type of work (w) of thermal energy in a sample of matter
is the process of causing matter to move will cause its temperature to increase.
against an opposing force. Like matter, And, if no chemical reaction or phase
energy comes in different types. One scheme change (such as condensation or freezing)
occurs, decreasing the amount of thermal
classifies energy into two types: potential
energy in a sample of matter will cause its
energy, the energy an object has because of
temperature to decrease.
its relative position, composition, or
condition, and kinetic energy, the energy that
an object possesses because of its motion. A
battery has potential energy because the
chemicals within it can produce electricity
that can do work.

Energy can be converted from one

form into another, but all of the energy
Figure 4.1 (a) The molecules in a
present before a change occurs always exists
sample of hot water move more rapidly
in some form after the change is completed.
than (b) those in a sample of cold water.
This observation is expressed in the law of
conservation of energy: during a chemical or Click on this interactive simulation (Links
physical change, energy can be neither to an external site.) to view the effects of
created nor destroyed, although it can be temperature on molecular motion.
changed in form. (This is also one version of
the first law of thermodynamics)
Heat
Thermal Energy,
Heat (q) is the transfer of
Temperature, and Heat thermal energy between two bodies
at different temperatures. Heat flow
Thermal energy is kinetic energy
associated with the random motion of (a redundant term, but one
commonly used) increases the same temperature and their
thermal energy of one body and molecules have the same average
decreases the thermal energy of the kinetic energy.
other.
Suppose we initially have a
high temperature (and high thermal Energy Units
energy) substance (H) and a low
temperature (and low thermal Matter undergoing chemical
energy) substance (L). The atoms reactions and physical changes can
and molecules in H have a higher release or absorb heat. A change
average KE than those in L. If we
that releases heat is called an
place substance H in contact with
substance L, the thermal energy will exothermic process. A reaction or
flow spontaneously from substance change that absorbs heat is an
H to substance L. The temperature endothermic process.
of substance H will decrease, as will
the average KE of its molecules; the Historically, energy was
temperature of substance L will measured in units of calories (cal).
increase, along with the average KE A calorie is the amount of energy
of its molecules. Heat flow will required to raise one gram of water
continue until the two substances by 1 degree centigrade. However,
are at the same temperature this quantity depends on the
(Figure 4.2 ).
atmospheric pressure and the
starting temperature of the water.
The ease of measurement of energy
changes in calories has meant that
the calorie is still frequently used.
Figure 4.2 (a) Substances H and L The Calorie (with a capital C), or
are initially at different
large calorie, commonly used in
temperatures, and their atoms have
different average kinetic energies. quantifying food energy content, is
(b) When they contact each other, a kilocalorie.
collisions between the molecules
The SI unit of heat, work, and
result in the transfer of kinetic
(thermal) energy from the hotter to energy is the joule. A joule (J) is
the cooler matter. (c) The two defined as the amount of energy
objects reach “thermal equilibrium” used when a force of 1 newton
when both substances are at the
moves an object 1 meter. It is heat required to raise the
named in honor of the English temperature of 1 gram of a
physicist James Prescott Joule. One substance by 1 degree Celsius (or 1
joule is equivalent to 1 kg m2/s2, kelvin):
which is also called 1 newton–
c= q / mΔT
meter. A kilojoule (kJ) is 1000
joules. To standardize its definition,
1 calorie has been set to equal
Specific heat capacity
4.184 joules.
depends only on the kind of
substance absorbing or releasing
heat. It is an intensive property—
Specific Heat the type, but not the amount, of the
substance is all that matters.
We now introduce two
concepts useful in describing heat The molar heat capacity, also
flow and temperature change. The an intensive property, is the heat
heat capacity (C) of a body of capacity per mole of a particular
matter is the quantity of heat (q) it substance and has units of J/mol °C.
absorbs or releases when it
experiences a temperature change
(ΔT) of 1 degree Celsius (or Energy Calculations
equivalently, 1 kelvin):
Sample Problem
C= q/ ΔT
Measuring Heat
Heat capacity is determined
by both the type and amount of A flask containing 8.0 × 102 g of
substance that absorbs or releases water is heated, and the
heat. It is therefore an extensive temperature of the water increases
property—its value is proportional from 21 °C to 85 °C. How much heat
to the amount of the substance. did the water absorb?

The specific heat capacity (c) Solution

of a substance, commonly called its To answer this question, consider
“specific heat,” is the quantity of these factors:
the specific heat of the substance process is known as calorimetry.
being heated (in this case, water) Calorimetry is used to measure
amounts of heat transferred to or
the amount of substance being from a substance. To do so, the heat
heated (in this case, 8.0 × 102 g) is exchanged with a calibrated
object (calorimeter). The
the magnitude of the temperature temperature change measured by
change (in this case, from 21 °C to the calorimeter is used to derive the
85 °C). amount of heat transferred by the
process under study. The
The specific heat of water is measurement of heat transfer using
4.184 J/g °C, so to heat 1 g of water this approach requires the
by 1 °C requires 4.184 J. We note definition of a system (the
that since 4.184 J is required to heat substance or substances
undergoing the chemical or
1 g of water by 1 °C, we will need
physical change) and
800 times as much to heat 8.0 × 102 its surroundings (all other matter,
g of water by 1 °C. Finally, we including components of the
observe that since 4.184 J are measurement apparatus, that serve
required to heat 1 g of water by 1 to either provide heat to the system
°C, we will need 64 times as much or absorb heat from the system).
to heat it by 64 °C (that is, from 21 A calorimeter is a device
°C to 85 °C). used to measure the amount of heat
involved in a chemical or physical
Using the equation from the process. For example, when an
previous page, the value of q = exothermic reaction occurs in
210,000 J solution in a calorimeter, the heat
produced by the reaction is
Because the temperature increased, absorbed by the solution, which
the water absorbed heat and q is increases its temperature. When an
positive. endothermic reaction occurs, the
heat required is absorbed from the
thermal energy of the solution,
which decreases its temperature
Calorimetry (Figure 4.4). The temperature
change, along with the specific heat
One technique we can use to and mass of the solution, can then
measure the amount of heat
involved in a chemical or physical
be used to calculate the amount of
heat involved in either case.
Primary fuels or primary
energy sources are dense sources of
primary energy found as natural
resources. Primary fuels are fuels
that are found in nature and can be
extracted, captured, cleaned, or
graded without any sort of energy
conversion or transformation
Figure 4.3 In a calorimetric process. This means that all
determination, either (a) an processing and collecting of the fuel
exothermic process occurs and
is done before the fuel is converted
heat, q, is negative, indicating that
thermal energy is transferred from into heat or mechanical work.
the system to its surroundings, or These primary fuels tend to be non-
(b) an endothermic process occurs renewable, and some of the most
and heat, q, is positive, indicating known primary fuels are fossil
that thermal energy is transferred fuels. Energy harvested from
from the surroundings to the primary fuels tends to make up
system.
most of a country's total primary
Fuels energy supply.

According to the law of

conservation of energy, energy can Most of the primary fuels used
never actually be “consumed”; it currently are non-renewable.
can only be changed from one form However, one major renewable
to another. What is consumed on a primary fuel is from biomass
huge scale, however, are resources sources. Other examples of primary
that can be readily converted to a fuel include coal, crude oil, bitumen,
form of energy that is useful for and natural gas.
doing work. energy that is not used
to perform work is either stored as
potential energy for future use or
transferred to the surroundings as
Nuclear Reactions as
heat. Source of Energy
 After the discovery of Many heavier elements with
radioactivity, the field of nuclear smaller binding energies per
chemistry was created and nucleon can decompose into more
developed rapidly during the early stable elements that have
twentieth century. Series of new intermediate mass numbers and
discoveries in the 1930s and 1940s, larger binding energies per nucleon.
along with World War II, combined This decomposition is called fission,
to usher in the Nuclear Age in the the breaking of a large nucleus into
mid-twentieth century. Scientists smaller pieces. The breaking is
learned how to create new rather random with the formation
substances, and certain isotopes of of a large number of different
certain elements were found to products. Fission usually does not
possess the capacity to produce occur naturally but is induced by
unprecedented amounts of energy, bombardment with neutrons. A
with the potential to cause tremendous amount of energy is
tremendous damage during war, as produced by the fission of heavy
well as produce enormous amounts elements.
of power for society’s needs during
peace.
Particles in Nuclear
The process of converting very
Reactions
light nuclei into heavier nuclei is Many entities can be involved
also accompanied by the conversion in nuclear reactions. The most
of mass into large amounts of common are protons, neutrons,
energy, a process called fusion. The alpha particles, beta particles,
principal source of energy in the positrons, and gamma rays, as
shown in Table 4.2. Protons and
sun is a net fusion reaction in which neutrons are the constituents of
four hydrogen nuclei fuse and atomic nuclei. Alpha particles also
produce one helium nucleus and represented by the symbol α are
two positrons. high-energy helium nuclei. Beta
particles, β, are high-energy
electrons, and gamma rays are
photons of
very high-energy electromagnetic indicates that there is a
radiation. Positrons are positively rearrangement during a nuclear
charged electrons (“anti- reaction, but of nucleons
electrons”). The subscripts and
(subatomic particles within the
superscripts are necessary for
balancing nuclear equations but are atoms’ nuclei) rather than atoms.
usually optional in other Nuclear reactions also follow
circumstances. conservation laws, and they are
balanced in two ways:

1. The sum of the mass numbers of

the reactants equals the sum of the
mass numbers of the products.

2. The sum of the charges of the

reactants equals the sum of the
charges of the products.
Figure 4.4 Some species If the atomic number and the mass
encountered in nuclear reactions,
number of all but one of the
this table summarizes the names,
symbols, representations, and particles in a nuclear reaction are
descriptions of the most common known, we can identify the particle
ones. by balancing the reaction.

Balancing Nuclear Redox Reactions

Reactions Electrochemistry deals with
A balanced chemical reaction chemical reactions that produce
equation reflects the fact that electricity and the changes
during a chemical reaction, bonds associated with the passage of
break and form, and atoms are electrical current through matter.
rearranged, but the total numbers The reactions involve electron
of atoms of each element are transfer, and so they are oxidation-
conserved and do not change. A reduction (or redox) reactions.
balanced nuclear reaction equation Many metals may be purified or
electroplated using electrochemical were ionic. The sum of oxidation
methods. Devices such as numbers for all atoms in a molecule
automobiles, smartphones, is equal to the charge on the
electronic tablets, watches, molecule.
pacemakers, and many others use
When it comes to redox
batteries for power. Batteries use
reactions, it is important to
chemical reactions that produce
understand what it means for a
electricity spontaneously and that
metal to be “oxidized” or “reduced”.
can be converted into useful work.
An easy way to do this is to
All electrochemical systems involve
remember the phrase “OIL RIG”.
the transfer of electrons in a
reacting system. In many systems,
the reactions occur in a region
OIL = Oxidization is Loss (of e-)
known as the cell, where the
transfer of electrons occurs at RIG = Reduction is Gain (of e-)
electrodes.

A redox reaction is one that

Voltaic Cells
entails changes in oxidation In redox reactions, electrons
number (or oxidation state) for one are transferred from one species to
or more of the elements involved. another. If the reaction is
The oxidation number of an spontaneous, energy is released,
which can then be used to do useful
element in a compound is
work. To harness this energy, the
essentially an assessment of how reaction must be split into two
the electronic environment of its separate half reactions: the
atoms is different in comparison to oxidation and reduction reactions.
atoms of the pure element. By this The reactions are put into two
description, the oxidation number different containers and a wire is
used to drive the electrons from
of an atom in an element is equal to
one side to the other. In doing so,
zero. For an atom in a compound, a Voltaic/ Galvanic Cell is created.
the oxidation number is equal to
the charge the atom would have in When a redox reaction takes
place, electrons are transferred
the compound if the compound
from one species to the other. If the
reaction is spontaneous, energy is
released, which can be used to do
work. Consider the reaction of a
solid copper (Cu(s)) in a silver
nitrate solution (AgNO3(s)). (Figure
4.5)

Figure 4.6 A Voltaic Cell.

A Voltaic Cell (also known as
a Galvanic Cell) is an
electrochemical cell that uses
spontaneous redox reactions to
generate electricity. It consists of
Figure 4.5 two separate half-cells. The salt
bridge is a vital component of any
This reaction releases energy.
voltaic cell. It is a tube filled with an
When the copper electrode solid is
electrolyte solution such as
placed directly into a silver nitrate
KNO3(s) or KCl(s). The purpose of the
solution, however, the energy is lost
salt bridge is to keep the solutions
as heat and cannot be used to do
electrically neutral and allow the
work. To harness this energy and
free flow of ions from one cell to
use it do useful work, we must split
another. Without the salt bridge,
the reaction into two separate half
positive and negative charges will
reactions: The oxidation and
build up around the electrodes
reduction reactions. A wire
causing the reaction to stop.
connects the two reactions and
allows electrons to flow from one
side to the other. In doing so, we
have created a Voltaic/ Galvanic Commercial Voltaic
Cell.
Cells
One of the main uses of
galvanic cells is the generation of
portable electrical energy. These Figure 4. 7 A variety of standard
cells are also popularly known sizes of primary cells. From left:
as batteries. The term battery is 4.5V multicell battery, D, C, AA,
generally used for two or more AAA, AAAA, A23, 9V multicell
Galvanic cells connected in series. battery, (top) LR44, (bottom) CR203
Thus, a battery is an arrangement 2
of electrochemical cells used as an
A common secondary cell is
energy source. The basis of an
the Lead-acid battery. This can be
electrochemical cell is an oxidation
commonly found as car batteries.
– reduction reaction.
They are used for their high voltage,
Types of commercial cells: There low costs, reliability, and long
are mainly two types of commercial lifetime. Lead-acid batteries are
cells, used in an automotive to start an
engine and operate the car's
(1) Primary cells: In these cells,
electrical accessories when the
the electrode reactions cannot be
engine is not running. The
reversed by an external electric
alternator, once the car is running,
energy source. In these cells,
recharges the battery.
reactions occur only once and after
use they become dead. Therefore,
they are not chargeable. Some
common examples are, dry cell,
mercury cell, Daniell cell and
alkaline dry cell
(2) Secondary cells: in these cells,
the reaction can be reversed by
running a current into the cell with
a battery charger to recharge it,
regenerating the chemical
reactants. Primary cells are made in
a range of standard sizes to power
small household appliances such as
flashlights and portable radios.