Introductory Chemistry: A Foundation - Zumdahl S.S., DeCoste D.J. 2019
Energy as a Driving Force
· To understand energy as a driving force for natural processes.
A major goal of science is to understand why things happen as they do. In particular, we are interested in the driving forces of nature. Why do things occur in a particular direction? For example, consider a log that has burned in a fireplace, producing ashes and heat energy. If you are sitting in front of the fireplace, you would be very surprised to see the ashes begin to absorb heat from the air and reconstruct themselves into the log. It just doesn’t happen. That is, the process that always occurs is
The reverse of this process
Consider another example. A gas is trapped in one end of a vessel as shown below.
When the valve is opened, what always happens? The gas spreads evenly throughout the entire container.
You would be very surprised to see the following process occur spontaneously:
So, why does this process
occur spontaneously but the reverse process
In many years of analyzing these and many other processes, scientists have discovered two very important driving forces:
· Energy spread
· Matter spread
Energy spread means that in a given process, concentrated energy is dispersed widely. This distribution happens every time an exothermic process occurs. For example, when a Bunsen burner burns, the energy stored in the fuel (natural gas—mostly methane) is dispersed into the surrounding air:
The energy that flows into the surroundings through heat increases the thermal motions of the molecules in the surroundings. In other words, this process increases the random motions of the molecules in the surroundings. This always happens in every exothermic process.
Matter spread means exactly what it says: the molecules of a substance are spread out and occupy a larger volume.
After looking at thousands of processes, scientists have concluded that these two factors are the important driving forces that cause events to occur. That is, processes are favored if they involve energy spread and matter spread.
Do these driving forces ever occur in opposition? Yes, they do—in many, many processes.
For example, consider ordinary table salt dissolving in water.
This process occurs spontaneously. You observe it every time you add salt to water to cook potatoes or pasta. Surprisingly, dissolving salt in water is endothermic. This process seems to go in the wrong direction—it involves energy concentration, not energy spread. Why does the salt dissolve? Because of matter spread. The and that are closely packed in the solid become spread around randomly in a much larger volume in the resulting solution. Salt dissolves in water because the favorable matter spread overcomes an unfavorable energy change.
Entropy is a function we have invented to keep track of the natural tendency for the components of the universe to become disordered—entropy (designated by the letter ) is a measure of disorder or randomness. As randomness increases, increases. Which has lower entropy, solid water (ice) or gaseous water (steam)? Remember that ice contains closely packed, ordered molecules, and steam has widely dispersed, randomly moving molecules (Fig. 10.10). Thus ice has more order and a lower value of .
Comparing the entropies of ice and steam.
What do you suppose happens to the disorder of the universe as energy spread and matter spread occur during a process?
It seems clear that both energy spread and matter spread lead to greater entropy (greater disorder) in the universe. This idea leads to a very important conclusion that is summarized in the second law of thermodynamics :
The entropy of the universe is always increasing.
· What if the first law of thermodynamics was true, but the second law was not? How would the world be different?
A spontaneous process is one that occurs in nature without outside intervention—it happens “on its own.” The second law of thermodynamics helps us to understand why certain processes are spontaneous and others are not. It also helps us to understand the conditions necessary for a process to be spontaneous. For example, at atm ( atmosphere of pressure), ice will spontaneously melt above a temperature of but not below this temperature. A process is spontaneous only if the entropy of the universe increases as a result of the process. That is, all processes that occur in the universe lead to a net increase in the disorder of the universe. As the universe “runs,” it is always heading toward more disorder. We are plunging slowly but inevitably toward total randomness—the heat death of the universe. But don’t despair; it will not happen soon.