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

Quality versus Quantity of Energy


· To see how the quality of energy changes as it is used.

One of the most important characteristics of energy is that it is conserved. Thus the total energy content of the universe will always be what it is now. If that is the case, why are we concerned about energy? For example, why should we worry about conserving our petroleum supply? Surprisingly, the “energy crisis” is not about the quantity of energy, but rather about the quality of energy. To understand this idea, consider an automobile trip from Chicago to Denver. Along the way you would put gasoline into the car to get to Denver. What happens to that energy? The energy stored in the bonds of the gasoline and of the oxygen that reacts with it is changed to thermal energy, which is spread along the highway to Denver. The total quantity of energy remains the same as before the trip but the energy concentrated in the gasoline becomes widely distributed in the environment:

Which energy is easier to use to do work: the concentrated energy in the gasoline or the thermal energy spread from Chicago to Denver? Of course, the energy concentrated in the gasoline is more convenient to use.

This example illustrates a very important general principle: when we utilize energy to do work, we degrade its usefulness. In other words, when we use energy the quality of that energy (its ease of use) is lowered.

In summary,

An illustration shows text: concentrated energy squeezed along its width and spread energy in oval contour. An arrow labeled “use the energy to do work” points from “concentrated energy” (squeezed along its width) to “spread energy” (oval contour).

You may have heard someone mention the “heat death” of the universe. Eventually (many eons from now), all energy will be spread evenly throughout the universe and everything will be at the same temperature. At this point it will no longer be possible to do any work. The universe will be “dead.”

We don’t have to worry about the heat death of the universe anytime soon, of course, but we do need to think about conserving “quality” energy supplies. The energy stored in petroleum molecules got there over millions of years through plants and simple animals absorbing energy from the sun and using this energy to construct molecules. As these organisms died and became buried, natural processes changed them into the petroleum deposits we now access for our supplies of gasoline and natural gas.

Petroleum is highly valuable because it furnishes a convenient, concentrated source of energy. Unfortunately, we are using this fuel at a much faster rate than natural processes can replace it, so we are looking for new sources of energy. The most logical energy source is the sun. Solar energy refers to using the sun’s energy directly to do productive work in our society. We will discuss energy supplies in the next section.