1.10.1: Gibbs Energy
The Gibbs energy is an extensive state variable defined by the following equation.
\[\mathrm{G}=\mathrm{U}+\mathrm{p} \, \mathrm{V}-\mathrm{T} \, \mathrm{S} \nonumber \]
Instead of Gibbs energy the terms Gibbs free energy and Gibbs function are often used. Physicists prefer the term Gibbs function [1]. The term ‘free energy’ is not encouraged. Everyday experience tells us that no energy is ‘free’.
Footnote
[1] Nevertheless the French term ‘enthalpie libre’ ( i.e. free enthalpy) for \(\mathrm{G}\) has merit. Enthalpy is defined by \(\mathrm{H} = \mathrm{ U} + \mathrm{p V}\). Then \(\mathrm{G} = \mathrm{ H} – \mathrm{ TS}\). The product \(\mathrm{TS}\) is the linked energy in a system from which no work can be produced. Hence the available or ‘free’ part of the enthalpy is the Gibbs energy.