Gibbs, Josiah Willard (1839–1903)
Josiah Gibbs was an American theoretical physicist and chemist best known for his pioneering work in chemical thermodynamics. While a professor at Yale, he devoted himself to establishing the basics of physical chemistry. In On the Equilibrium of Heterogeneous Substances (2 vols, 1876 and 1878) he stated Gibbs' phase rule (see phase equilibria). The quantity known as Gibbs free energy is named after him. Gibbs also made fundamental contributions to the subject of vector analysis.
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.