Entropy Increase


		As a dog pants, water evaporates spontaneously from its tongue, cooling the dog's bloodstream. Evaporating water is an endothermic process, and should not happen spontaneously if it was a process driven by the tendency to minimum energy.

Entropy Increase

The second of the natural "driving forces" behind change is an increase in entropy of the reacting system. Consider the following observations. If you have a gas at high pressure, and allow it to expand to a lower pressure, it will do so of its own accord spontaneously, but the temperature will drop.

When liquid water evaporates, it cools the surface that it is evaporating from. This of course is the reason we perspire, since sweating cools our bodies. However, look at the equation for the reaction:

H₂O (l) + energy H₂O (g)

h2opeincrease.gif (1237 bytes)
The evaporation of water is an endothermic process. The liquid water must gain energy as it changes to vapor.

How is this possible? The temperature is dropping, so the reaction is endothermic. It must have a positive

H. Potential energy must be increasing. Yet we've just indicated that a potential energy decrease should be spontaneous, not a potential energy increase. A spontaneous reaction with a potential energy increase is exactly like water flowing up hill of its own accord – and that just shouldn't happen. However there is absolutely no doubt that gases expand, or that water spontaneously evaporates. This kind of spontaneous endothermic change can take place, so there must be something else happening.

Also, heat always moves from a hot object to a cold one. In other words, if you put two objects side-by-side, one hot and one cold, the cold one always gets warmer and the warm one always gets colder. Never in the entire history of measurement has it ever gone the other way. Not once has the hot one ever gotten hotter, while the cold one got colder. Why?

In order to explain these kinds of phenomena, Sadi Carnot (or perhaps it was Rudolf Clausius – no one seems to know for sure) invented the concept of entropy. In any change, entropy is the part of the energy involved that is never available to do any useful work. Entropy (the symbol for entropy is S) is far from a simple concept, so most introductions to chemistry don't treat it very completely. We won't do any differently here.

Entropy is really a measurement of the number of ways in which energy can be scattered. The more dispersed or spread out the energy, the greater the entropy. It turns out that changes in entropy,

S, are the real reason behind all change. In any change, the entropy of the universe must increase. This simple statement is so fundamentally important that it is called the second law of thermodynamics.