In
thermodynamics, the
Gibbs free energy (
IUPAC recommended name:
Gibbs energy or
Gibbs function; also known as
free enthalpy to distinguish it from
Helmholtz free energy) is a
thermodynamic potential that measures the maximum or reversible work that may be performed by a
thermodynamic 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 (
kJ in
SI units) 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
G 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.