In order to account for changes in both entropy and enthalpy J.Willard Gibbs defined a new function, which is called Gibbs free energy (Gibbs potential) G. The maximum useful work is the amount of energy produced, is given by the decrease in another thermodynamic property known as the Gibbs free energy.
Gibbs Free Energy Formula is independent of pressure and temperature and can be stated as
Gibbs free energy is a single valued function and is an extensive property. Since by definition H = U + PV.Gibbs Free Energy Formula
can be written as
Change in Gibbs free energy for a finite system is expressed as
At constant temperature and pressure the equation becomes