(also Gibbs function), one of the characteristic functions of a thermodynamic system, denoted by G and determined by enthalpy H, entropy S, and temperature T by the equality

(1)G = H - TS

The Gibbs free energy is a thermodynamic potential. In an isothermic equilibrium process occurring at constant pressure, the decrease in the Gibbs free energy of a given system equals the total work done by the system in this process minus the work against external pressure (that is, the Gibbs free energy equals the maximum useful work). It is usually expressed in kilojoules per mole or kilocalories per mole. With the aid of the Gibbs free energy and its derivatives, simple expressions can be found for other thermodynamic functions and properties of the system (including internal energy, enthalpy, and chemical potential) under conditions of constant temperature and pressure. Under these conditions, any thermodynamic process can occur without work expenditure externally only in the direction that corresponds to a decrease in G (dG < 0). The limit of its occurrence without work expenditure, that is the equilibrium condition, is the minimum value of G (dG = 0, d²G > 0). The Gibbs free energy is widely used in examining various thermodynamic processes occurring at constant temperature and pressure. It is used to determine the work of the reverse magnetization of magnets and the polarization of a dielectric under these conditions. Knowledge of the Gibbs free energy is important for a thermodynamic study of phase changes. The equilibrium constant K_a of a chemical reaction at some temperature T is determined by the standard change in the Gibbs free energy ΔG_T° according to the relation

(2) ΔG_T° = -RT ln K_a,T

Also widely used is the Gibbs free energy ΔG_for° of the formation of a chemical compound, equal to the change in the Gibbs free energy in the reaction of forming a given compound (or simple substance) from the standard state of the corresponding simple substances. For any chemical reaction, ΔG_for° equals the algebraic sum of the products ΔG_T° of the substances participating in the reaction and their coefficients in the reaction equation. For 298.15° K the ΔG_for° are already known for several thousand substances, which makes it possible to calculate the corresponding ΔG° and K_a for a large number of reactions.

Along with equation (1), the Gibbs free energy can also be determined by internal energy U, the Helmholtz free energy A, and the product of volume V and pressure p, from the equalities

(3)G = U - TS + pV

(4)G = A + pV

For a long time various authors called the characteristic function of the Gibbs free energy by various names—for example, free energy, free energy at constant pressure, thermodynamic potential, Gibbs thermodynamic potential, isobaric-isothermic potential, and free enthalpy. Different symbols were used to denote this function (Z, F, Φ). The term “Gibbs free energy” and the symbol G used here correspond with a decision of the 18th congress of the International Union of Pure and Applied Chemistry held in 1961.