Introductory Chemistry: A Foundation - Zumdahl S.S., DeCoste D.J. 2019

The Nature of Energy


Shaun Wilkinson/

A barn owl exerts a great deal of energy in order to fly.

Energy is at the center of our very existence as individuals and as a society. The food that we eat furnishes the energy to live, work, and play, just as the coal and oil consumed by manufacturing and transportation systems power our modern industrialized civilization.

Huge quantities of carbon-based fossil fuels have been available for the taking. This abundance of fuels has led to a world society with a huge appetite for energy, consuming millions of barrels of petroleum every day. We are now dangerously dependent on the dwindling supplies of oil, and this dependence is an important source of tension among nations in today’s world. In an incredibly short time we have moved from a period of ample and cheap supplies of petroleum to one of high prices and uncertain supplies. If our present standard of living is to be maintained, we must find alternatives to petroleum. To do this, we need to know the relationship between chemistry and energy, which we explore in this chapter.



Energy is a factor in all human activity.

The Nature of Energy


· To understand the general properties of energy.

Although energy is a familiar concept, it is difficult to define precisely. For our purposes we will define energy as the ability to do work or produce heat. We will define these terms below.

Energy can be classified as either potential or kinetic energy. Potential energy is energy due to position or composition. For example, water behind a dam has potential energy that can be converted to work when the water flows down through turbines, thereby creating electricity. Attractive and repulsive forces also lead to potential energy. The energy released when gasoline is burned results from differences in attractive forces between the nuclei and electrons in the reactants and products. The kinetic energy of an object is energy due to the motion of the object and depends on the mass of the object and its velocity : .

One of the most important characteristics of energy is that it is conserved. The law of conservation of energy states that energy can be converted from one form to another but can be neither created nor destroyed. That is, the energy of the universe is constant.

Although the energy of the universe is constant, it can be readily converted from one form to another. Consider the two balls in Fig. 10.1(a). Ball A, because of its initially higher position, has more potential energy than ball B. When ball A is released, it moves down the hill and strikes ball B. Eventually, the arrangement shown in Fig. 10.1(b) is achieved. What has happened in going from the initial to the final arrangement? The potential energy of A has decreased because its position was lowered. However, this energy cannot disappear. Where is the energy lost by A?

Figure 10.1.A set of two illustrations are shown. The first illustration shows two balls: one, labeled A, is held uphill, and the other, labeled B, is on the ground. Accompanying text reads: In the initial positions, ball A has a higher potential energy than ball B. The second illustration shows two balls; one, labeled A, is on the ground, and the other, labeled B, shows the ball at a slightly elevated level. Accompanying text reads: After A has rolled down the hill, the potential energy lost by A has been converted to random motions of the components of the hill (frictional heating) and to an increase in the potential energy of B.

Initially, the potential energy of A is changed to kinetic energy as the ball rolls down the hill. Part of this energy is transferred to B, causing it to be raised to a higher final position. Thus the potential energy of B has been increased, which means that work (force acting over a distance) has been performed on B. Because the final position of B is lower than the original position of A, however, some of the energy is still unaccounted for. Both balls in their final positions are at rest, so the missing energy cannot be attributed to their motions.

What has happened to the remaining energy? The answer lies in the interaction between the hill’s surface and the ball. As ball A rolls down the hill, some of its kinetic energy is transferred to the surface of the hill as heat. This transfer of energy is called frictional heating. The temperature of the hill increases very slightly as the ball rolls down. Thus the energy stored in A in its original position (potential energy) is distributed to B through work and to the surface of the hill by heat.

Imagine that we perform this same experiment several times, varying the surface of the hill from very smooth to very rough. In rolling to the bottom of the hill (see Fig. 10.1), A always loses the same amount of energy because its position always changes by exactly the same amount. The way that this energy transfer is divided between work and heat, however, depends on the specific conditions—the pathway. For example, the surface of the hill might be so rough that the energy of A is expended completely through frictional heating: A is moving so slowly when it hits B that it cannot move B to the next level. In this case, no work is done. Regardless of the condition of the hill’s surface, the total energy transferred will be constant, although the amounts of heat and work will differ. Energy change is independent of the pathway, whereas work and heat are both dependent on the pathway.

This brings us to a very important idea, the state function. A state function is a property of the system that changes independently of its pathway. Let’s consider a nonchemical example. Suppose you are traveling from Chicago to Denver. Which of the following are state functions?

· Distance traveled

· Change in elevation

Because the distance traveled depends on the route taken (that is, the pathway between Chicago and Denver), it is not a state function. On the other hand, the change in elevation depends only on the difference between Denver’s elevation ( ft) and Chicago’s elevation ( ft). The change in elevation is always ; it does not depend on the route taken between the two cities.

We can also learn about state functions from the example illustrated in Fig. 10.1. Because ball A always goes from its initial position on the hill to the bottom of the hill, its energy change is always the same, regardless of whether the hill is smooth or bumpy. This energy is a state function—a given change in energy is independent of the pathway of the process. In contrast, work and heat are not state functions. For a given change in the position of A, a smooth hill produces more work and less heat than a rough hill does. That is, for a given change in the position of A, the change in energy is always the same (state function) but the way the resulting energy is distributed as heat or work depends on the nature of the hill’s surface (heat and work are not state functions).

Critical Thinking

· What if energy were not conserved? How would this affect our lives?