Page 48 - Bruno Linder Elementary Physical Chemistry

August 18, 2010 11:36 9in x 6in b985-ch05 Elementary Physical Chemistry

Chapter 5

The Free Energy Functions

There is a drawback in working with entropy except for isolated systems.
If the system is not isolated, the system and surroundings have to be
combined, and at constant T and P,using Eq.(4.4) (∆S surr = −∆H sys /T )
yields

∆S tot =∆S sys +∆S surr =∆S sys − ∆H/T (5.1)
This requires that

∆S tot > 0 if process is spontaneous; (5.2a)
= 0 if process is reversible; (5.2b)
< 0ifprocessisimpossible. (5.2c)
Or, for short,

∆S tot ≥ 0 (5.2d)

There is a way to get around the problem of having to use an isolated
system or a system plus surrounding, rather than focusing on the system
only. This can be accomplished by using free energy functions,ofwhich
there are two kinds: the Helmholtz free energy and the Gibbs free energy (also
called Gibbs energy). In this course, only the Gibbs function will be used.

5.1. The Gibbs Free Energy

The Gibbs free energy is a property of the system only (surroundings not
included). The Gibbs energy is defined as

G = H − TS (5.3)

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