In 1875 J. Willard Gibbs defined the spontaneity of a reaction in terms of its ability to perform "useful work" (in principle or practice). This ability to do work is determined, according to Gibbs, by the change in free energy during a chemical change, ΔG. This quantity gives the maximum amount of work obtainable from a spontaneous reaction (OR it tells how much work must be done to make a reaction spontaneous).

The sign of ΔG in an indicator of whether a reaction will be spontaneous or not.

• when ΔG < 0, the reaction is spontaneous (it is capable of doing useful work)
• when ΔG = 0, the system is at equilibrium
• when ΔG > 0, the reaction is non-spontaneous (work must come from the surroundings to make the reaction happen)