Gibbs free energy
Gibbs free energy () 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
The sign of tells us whether the reaction is spontaneous:
When and have different signs, we know the spontaneity right away. When they have the same signs, we have to use to determine which one overcomes the other.