13. How To Measure Disorder   Previous PageNext Page

The difficulty with general laws is that they are so hard to prove but so easy to disprove.

If you are testing the proposition that "all Irishmen have red hair," then ten million red-haired Irishmen will not prove the law beyond challenge; they merely will make it more probable. But a single blond Irishman will wipe out the proposition completely. (All is not necessarily lost, however. If you look into the reasons for the yellow hair you may learn something about people.)

The analogy is not facetious. Any number of spontaneous, heat emitting reactions will not rigorously prove the statement "all spontaneous reactions tend to minimize energy," yet one lone, spontaneous but heat-absorbing process will scuttle it. If we look more closely at why some heat-absorbing reactions are spontaneous, we will discover a new fundamental principle about chemical reactions.

Exceptions to the principle that all spontaneous reactions emit heat are not hard to find. N205 is the oxide of nitrogen with its highest oxidation number, +5. The solid dissolves in water to form HNO3:


Crystalline N205 is unstable and will explode spontaneously:

The remarkable aspect is that when N205 decomposes it absorbs 26 kcal of heat per mole. Here is a spontaneous and rapid reaction that clearly goes to a state of higher energy.

  Page 4 of 45 HomeGlossary