The spontaneity of reaction is based upon the negative value of ΔG. AG is based upon T,ΔS and ΔH according to following equation (Gibbs-Helmholtz equation) ΔG=ΔH - TΔS If the magnitude of ΔH - TΔS is negative, then the reaction is spontaneous, when TΔS > ΔH and ΔH and ΔS are + ve, then ΔG is negative.