Gibbs free energy a thermodynamic potential that measures the "useful"or process-initiating work obtainable from an isothermal,isobaric thermodynamic system. the chemical potential that is minimized when a system reaches equilibrium at constant pressure and temperature A convenient criterion of spontaneity for processes with constant pressure and temperaturethe chemical potential that is minimized when a system reaches equilibrium at constant pressure and temperature A convenient criterion of spontaneity for processes with constant pressure and temperature a thermodynamic potential that measures the "useful" or process‐initiating work obtainable from an isothermal, isobaric thermodynamic system. Gibbs free energy