A measure of a system's ability to do work. The Gibbs free energy (or Gibbs function), G, is defined by G = H – TS, where G is the energy liberated or absorbed in a reversible process at constant pressure and constant temperature (T), H is the enthalpy and S the entropy of the system. Changes in Gibbs free energy, ΔG, are useful in indicating the conditions under which a chemical reaction will occur. If ΔG is positive the reaction will only occur if energy is supplied to force it away from the equilibrium position (i.e. when ΔG = 0). If ΔG is negative the reaction will proceed spontaneously to equilibrium.
The Helmholtz free energy (or Helmholtz function), F, is defined by F = U – TS, where U is the internal energy. For a reversible isothermal process, ΔF represents the useful work available.