This article discusses the thermodynamics term of "free energy", see Free energy (disambiguation) for other possible uses.
In thermodynamics, the term free energy denotes either of two related concepts of importance. They express the total amount of energy which is used up or released during a chemical reaction. Both attempt to capture that part of the total energy of a system which is available for "useful work" and is hence not stored in "useless random thermal motion". As a system undergoes changes, its free energy will decrease.
When a system of molecules undergoes change, whether chemical reaction or changes in physical states such as phase changes, there are two tendencies driving the changes:
If E represents the energy, T the temperature, and S the entropy, these two tendencies can be combined by stating that the expression
- E - TS, the Helmholtz function
- H - TS = E + PV - TS, the Gibbs function.