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.