Gibbs free energy

Gibbs free energy (G\displaystyle G) is energy that is available to do useful work. It can be thought of as a balance between enthalpy and entropy that accurately represents their roles in determining the spontaneity of a reaction. The change in free energy is defined by

ΔG=ΔHTΔS\displaystyle \Delta{} G = \Delta{} H - T \Delta{} S.

The sign of ΔG\displaystyle \Delta{} G tells us whether the reaction is spontaneous:

When ΔH\displaystyle \Delta{} H and ΔS\displaystyle \Delta{} S have different signs, we know the spontaneity right away. When they have the same signs, we have to use ΔG\displaystyle \Delta{} G to determine which one overcomes the other.