free energy

Energy available to do work at constant temperature (decreases in spontaneous processes, minimum at equilibrium).

Gibbs free energy (constant temperature and pressure):

$$G=H-TS$$

Helmholtz free energy (constant temperature and volume):

$$A=U-TS$$

where:

• $H$: enthalpy,
• $U$: internal energy,
• $T$: temperature (K),
• $S$: entropy.