Free Energy in thermodynamics is energylike property or state function of a system in thermodynamic equilibrium--it has the dimensions of energy and its value is determined by the state of the system and not by its history--expressed in two forms: the Helmholtz free energy, A, sometimes called work function, and the Gibbs free energy, G, sometimes F. If E is the internal energy of the system, PV the pressure-volume product, and TS the temperature-entropy product, then A = E - TS and G = E + PV - TS. Free energy is an extensive property; i.e., the magnitude depends on the amount of the substance present in a given thermodynamic state.
The changes in free energy, A or G, are useful in evaluating certain thermodynamic processes. In a reversible process, the work under constant temperature and constant volume is equal to the change in the Helmholtz free energy, A, and the work under constant temperature and constant pressure is equal to the change in the Gibbs free energy, G.
Changes in free energy can be used to judge whether certain transformations of state can occur spontaneously. Under certain conditions of constant temperature and volume, the transformation of state will occur spontaneously, slowly or rapidly, if the Helmholtz free energy of the final state is smaller than that of the initial state; that is, if the difference A between the final and the initial state is negative. Under conditions of constant temperature and pressure, the transformation of state will occur if the change in the Gibbs free energy, G, is negative.
The term "Free energy device" has a number of meanings:
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