or d G < 0. For a similar process at constant temperature and volume, the change in Helmholtz free energy must be negative, Δ Δ A < 0 . Thus, a negative value of the change in free energy (G or A) is a necessary condition for a process to be spontaneous. This is the most useful form of the second law of thermodynamics in chemistry, where free-energy changes can be calculated from tabulated enthalpies of formation and standard molar entropies of reactants and products. The chemical equilibrium condition at constant T and p without electrical work is d G = 0.