The
Gibbs free energy
is the Helmholtz free energy
plus the product of the system
volume and the external pressure. Changes in the Gibbs free energy at a constant pressure thus include work done against external pressure as a system undergoes volumetric changes. This proves
convenient for describing equilibria in gases and liquids at a constant
pressure (e.g., at one atmosphere), but is of little use in describing
machine-phase
chemical processes. Changes in the Gibbs free energy
caused by a change in the applied pressure (at constant volume) have no
direct physical significance. (See also enthalpy.)
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