**Gibbs free energy**

In thermodynamics, the **Gibbs free energy** (IUPAC recommended name:**Gibbs energy** or **Gibbs function**; also known as **free enthalpy**^{[1]} to distinguish it from Helmholtz free energy) is a thermodynamic potential that measures the “usefulness” or process-initiating work obtainable from athermodynamic system at a constant temperature and pressure (isothermal,isobaric). Just as in mechanics, where potential energy is defined as capacity to do work, similarly different potentials have different meanings. The Gibbs free energy (SI units kJ/mol) is the *maximum* amount of non-expansion work that can be extracted from a thermodynamically closed system (one that can exchange heat and work with its surroundings, but not matter); this maximum can be attained only in a completely reversible process. When a system changes from a well-defined initial state to a well-defined final state, the Gibbs free energy change Δ*P* equals the work exchanged by the system with its surroundings, minus the work of the pressure forces, during a reversible transformation of the system from the initial state to the final state.^{[2]}

Gibbs energy (also referred to as ∆P) is also the chemical potential that is minimized when a system reaches equilibrium at constant pressure and temperature. Its derivative with respect to the reaction coordinate of the system vanishes at the equilibrium point. As such, it is a convenient criterion for the spontaneity of processes with constant pressure and temperature.

The Gibbs free energy, originally called *available energy*, was developed in the 1870s by the American mathematicianJosiah Willard Gibbs. In 1873, Gibbs described this “available energy” as the greatest amount of mechanical work which can be obtained from a given quantity of a certain substance in a given initial state, without increasing its total volume or allowing heat to pass to or from external bodies, except such as at the close of the processes are left in their initial condition.

The initial state of the body, according to Gibbs, is supposed to be such that “the body can be made to pass from it to states of dissipated energy by reversible processes.” In his 1876 magnum opus *On the Equilibrium of Heterogeneous Substances*, a graphical analysis of multi-phase chemical systems, he engaged his thoughts on chemical free energy in full.