Free energy

From Freepedia

For other uses, see Free energy (disambiguation).

In thermodynamics, free energy is a measure of the amount of work that can be extracted from a system. In this sense, it measures not the energy content of the system, but the "useful energy" content.

In different situations, free energy is related to internal energy in different ways. If one is measuring the internal energy of a system at constant pressure and constant temperature, Gibbs free energy is the most useful (hence its utility to solution-phase chemists). Otherwise, Helmholtz free energy is useful, since it is related to the partition function of a canonical ensemble in statistical mechanics (hence its utility to gas-phase chemists, and physicists).

Helmholtz free energy is A = U - TS, where U is the internal energy, T is the temperature, and S is the entropy.

Gibbs free energy is G = H - TS, where H is the enthalpy, T is the temperature, and S is the entropy. This takes into account work to "make space for molecules" (as H is U + PV, where P is pressure and V is volume).

The historical notation in physics and chemistry is confusing and as follows:

• In physics, free energy by default refers to Helmholtz free energy, and is notated F.
• In chemistry, free energy by default refers to Gibbs free energy, and is also notated F.

However, both fields do often require use of the 'other' free energy. Recently a compromise notation has become common, using A for the Helmholtz function, G for the Gibbs function, and avoiding F entirely.