Which Way Is That System Going? The Gibbs Free Energy
So far we have treated the first and second laws separately, but in a real system both energy conservation and entropy play a role. The role of entropy in determining the spontaneous direction of a system was derived for an isolated system that exchanged neither energy nor material with its surroundings. There are few biological systems of interest that fulfill the requirements of this isolation. We need a state variable that will indicate the direction and equilibrium position of a system that undergoes radical changes in energy (usually measured as enthalpy) and entropy together. This state function exists. It is the Gibbs free energy, G.