Gibbs free energy

Gibbs free energy (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

Delta G = Delta H - TDelta S.

The sign of Delta G tells us whether the reaction is spontaneous:

• Delta G < 0 => spontaneous
• Delta G > 0 => non-spontaneous

When Delta H and Delta S have different signs, we know the spontaneity right away. When they have the same signs, we have to use Delta G to determine which one overcomes the other.