A measure of the maximum useful work in a system that can be obtained, defined by:

where T is the absolute temperature, H is the enthalpy, and S is the entropy. A quantitative measure as to how near or far a potential reaction is from occurring can be determined from the change in Gibbs free energy ΔG. Negative values of ΔG indicate that change can occur spontaneously whereas positive values indicate that energy is required to be supplied for (p. 169) the reaction to occur. It is also known as Gibbs energy or Gibbs function after American physicist Josiah Willard Gibbs (1839–1903). The Helmholtz free energy is defined as F = U–TS where U is the internal energy. For a reversible isothermal process, the change Helmholtz free energy represents the useful work available. It is named after German physicist and physiologist Hermann von Helmholtz (1821–94).