Inthermodynamics, theGibbs free energy(IUPACrecommended name:Gibbs energyorGibbs function; also known asfree enthalpy to distinguish it fromHelmholtz free energy) is athermodynamic potentialthat can be used to calculate themaximumof reversibleworkthat may be performed by athermodynamic systemat a constanttemperatureandpressure(isothermal,isobaric). The Gibbs free energy (ΔGº = ΔHº-TΔSº) (JinSI units) is themaximumamount of non-expansion work that can be extracted from athermodynamically closed system(one that can exchange heat and work with its surroundings, but not matter); this maximum can be attained only in a completelyreversible process. When a system transforms reversibly from an initial state to a final state, the decrease in Gibbs free energy equals the work done by the system to its surroundings, minus the work of thepressureforces.

The Gibbs energy (also referred to asG) is also the thermodynamic potential that is minimized when a system reacheschemical equilibriumat constant pressure and temperature. Its derivative with respect to the reaction coordinate of the system vanishes at the equilibrium point. As such, a reduction inGis a necessary condition for the spontaneity of processes at constantpressureandtemperature.

Enthalpy. ... When a substance changes at constant pressure,enthalpytells how much heat and work was added or removed from the substance.Enthalpyis similar to energy, but not the same. When a substance grows or shrinks, energy is used up or released.