Gibbs free energy

The Gibbs free energy or Gibbs thermodynamic potential $G$ is a measure of energy of a system. It is defined as

where $A$ is the Helmholtz theorem, $P$ is the pressure, $V$ is the volume, $U$ is the internal energy, $T$ is the temperature and $S$ is the entropy.

Constant temperature and pressure#

For a system with constant pressure and temperature, $G$ never increases. In fact, from Helmholtz free energy we get

Thus the most stable state (i.e. lowest energy state) is the one where $G$ is lowest.