The Foundations of Chemistry

Some forms of energy are potential,
kinetic, electrical, nuclear, heat, and
light.

592 CHAPTER 15: Chemical Thermodynamics

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nergy is very important in every aspect of our daily lives. The food we eat supplies

the energy to sustain life with all of its activities and concerns. The availability of

relatively inexpensive energy is an important factor in our technological society.

This is seen in the costs of fuel, heating and cooling our homes and workplaces, and the

electricity to power our lights, appliances, and computers. It is also seen in the costs of

the goods and services we purchase, because a substantial part of the cost of production

is for energy in one form or another. We must understand the storage and use of energy

on a scientific basis to learn how to decrease our dependence on consumable oil and natural

gas as our main energy sources. Such understanding has profound ramifications, ranging

from our daily lifestyles to international relations.

The concept of energy is at the very heart of science. All physical and chemical processes

are accompanied by the transfer of energy. Because energy cannot be created or destroyed,

we must understand how to do the “accounting” of energy transfers from one body or

one substance to another or from one form of energy to another.

In thermodynamicswe study the energy changes that accompany physical and chem-

ical processes. Usually these energy changes involve heat— hence the “thermo-” part of

the term. In this chapter we study the two main aspects of thermodynamics. The first is

thermochemistry.This practical subject is concerned with how we observe, measure,and

predictenergy changes for both physical changes and chemical reactions. The second part

of the chapter addresses a more fundamental aspect of thermodynamics. There we will

learn to use energy changes to tell whether or not a given process can occur under spec-

ified conditions to give predominantly products (or reactants) and how to make a process

more (or less) favorable.

HEAT CHANGES AND THERMOCHEMISTRY

THE FIRST LAW OF THERMODYNAMICS

We can define energy as follows.

Energy is the capacity to do work or to transfer heat.

We classify energy into two general types: kinetic and potential. Kinetic energyis the

energy of motion. The kinetic energy of an object is equal to one half its mass, m,times

the square of its velocity, v.

Ekinetic^12 mv^2

The heavier a hammer is and the more rapidly it moves, the greater its kinetic energy and

the more work it can accomplish.

Potential energyis the energy that a system possesses by virtue of its position or

composition. The work that we do to lift an object is stored in the object as energy; we

describe this as potential energy. If we drop a hammer, its potential energy is converted

into kinetic energy as it falls, and it could do work on something it hits — for example,

drive a nail or break a piece of glass. Similarly, an electron in an atom has potential energy

because of the electrostatic force on it that is due to the positively charged nucleus and

the other electrons in that atom and surrounding atoms. Energy can take many other

forms: electrical energy, radiant energy (light), nuclear energy, and chemical energy. At

15-1

See the Saunders Interactive
General Chemistry CD-ROM,
Screen 6.4, Energy, and Screen 6.5,
Forms of Energy.

As matter falls from a higher to a
lower level, its gravitational potential
energy is converted into kinetic
energy. A hydroelectric power plant
converts the kinetic energy of falling
water into electrical (potential)
energy.