Gibbs free energy
Gibbs free energy is an important function in chemical thermodynamics, defined by
G = H - TS
where H is the enthalpy, S the entropy, and T the thermodynamic temperature. Gibbs free energy is the energy liberated or absorbed in a reversible process at constant pressure and constant temperature. It is sometimes called Gibbs energy and, in older literature, simply "free energy."
Changes in Gibbs free energy, ΔG (= ΔH - TΔS), are useful in indicating the conditions under which a chemical reaction will occur. If ΔG is negative the reaction will proceed spontaneously to equilibrium. In equilibrium position ΔG = 0.