the Ag of a reaction depends both on the change in internal en ergy and on the change in entropy of the system. The change in free energy(AG)of a reaction, in contrast with the change in internal energy(AE) of a reaction, is a valuable criterion of whether it can occur spontaneously 1. A reaction can occur spontaneously only i/Ag ve 2.A system is at equilibrium and no net change can take place if Agis zero. 3. a reaction cannot occur spontaneously if AG is positive. An input of tree energy is required to drive such a reaction