Answer is: - delta G.The change in Gibbs free energy (ΔG), at constant temperature and pressure, is: ΔG=ΔH−TΔS.ΔH is the change in enthalpy.ΔS is change in entropy.T is temperature of the system.When ΔG is negative, a reaction (occurs without the addition of external energy) will be spontaneous (exergonic).