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Perplexed about entropy? You are not alone. Josiah Willard Gibbs (1839–1903) understood this confusion all too well, almost 150 years ago, “ . . . a method involving the notion of entropy, the very existence of which depends upon the second law of thermodynamics, will doubtless seem to many far-fetched, and may repel beginners as obscure and difficult of comprehension. This inconvenience is perhaps more than counter-balanced by the advantages of a method which makes the second law of thermodynamics so prominent, and gives it so clear and elementary an expression. . . . (1).” Gibbs profoundly altered our understanding of chemistry with his insights. At a time when it was mostly a philosophical concept, Gibbs went straight for application and made entropy relevant. Rapid advancements and heralded achievements in the chemical sciences ensued. Enthalpy (H) is a measure of the internal energy of a system, but this energy has an availability issue; some of that energy is useful, some is not. Enthalpy also provides no information about the spontaneity of energy exchange. Entropy (S) does indicate the probability of energy exchange (i.e., spontaneous, −∆S, or nonspontaneous, +∆S), but it is not useful energy and so it provides little information on the quantity of energy that is available to perform work. Energy that is available to perform useful work is known as Gibbs energy, symbolized as G. Gibbs energy has also been termed free energy. Yet energy is anything but “free” and so that term will not be used here
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