structural:scholar_edition
Free energy change and spontaneous processes

According to the second law of thermodynamics, for any process that occurs in a closed system, the inequality of Clausius, Δ S > q / T, applies. For a process at constant temperature and pressure without non- PV work, this inequality transforms into Δ G < 0 {\displaystyle \Delta G<0}. Similarly, for a process at constant temperature and volume, Δ A < 0 {\displaystyle \Delta A<0}. Thus, a negative value of the change in free energy is a necessary condition for a process to be spontaneous; this is the most useful form of the second law of thermodynamics in chemistry. In chemical equilibrium at constant T and p without electrical work, d G = 0.