In thermodynamics, free energy (TR:862) (LH:8) (TL:870) is the total energy of a system, less the bound energy (TS) (Helmholtz, 1882); a synonym for "available energy" (Gibbs, 1876); also called "reaction energy" (Haber, 1909).
The term "free energy" can refer either to Helmholtz energy, namely the available energy or free energy of an isothermal isochoric system, or the Gibbs energy, the available energy or free energy of an isothermal isobaric system.
The following are quotes:
- “The heat of a reaction is not the true thermodynamic criterion for chemical reaction. On the contrary, the change of free energy, the capacity of the system to do chemical, electrical or mechanical work, is the correct measure of the driving force of a reaction.”
- — Hugh Taylor (1942), A Treatise on Physical Chemistry (pgs. 449-59 
- Helmholtz, Hermann. (1882). “On the Thermodynamics of Chemical Processes”, in: Physical Memoirs Selected and Translated from Foreign Sources, 1:43-97. Physical Society of London, Taylor and Francis, 1888.
- Taylor, Hugh; Glasstone, Samuel. (1942). A Treatise on Physical Chemistry, Volume One: Atomistics and Thermodynamics (pgs. 449-50). Van Nostrand.
- Free energy – Hmolpedia 2020.