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In thermodynamics, the Helmholtz free energy (or Helmholtz energy) is a thermodynamic potential that measures the useful work obtainable from a closed thermodynamic system at a constant temperature . The change in the Helmholtz energy during a process is equal to the maximum amount of work that the system can perform in a thermodynamic process ...
The definition of the Gibbs function is = + where H is the enthalpy defined by: = +. Taking differentials of each definition to find dH and dG, then using the fundamental thermodynamic relation (always true for reversible or irreversible processes): = where S is the entropy, V is volume, (minus sign due to reversibility, in which dU = 0: work other than pressure-volume may be done and is equal ...
For example, the Gibbs free energy change is used when considering processes that occur under constant pressure and temperature conditions, whereas the Helmholtz free energy change is used when considering processes that occur under constant volume and temperature conditions. The value and even the sign of both free energy changes can depend ...
Historically, the term 'free energy' has been used for either quantity. In physics, free energy most often refers to the Helmholtz free energy, denoted by A (or F), while in chemistry, free energy most often refers to the Gibbs free energy. The values of the two free energies are usually quite similar and the intended free energy function is ...
The Helmholtz free energy is in ISO/IEC standard called Helmholtz energy [1] or Helmholtz function. It is often denoted by the symbol F, but the use of A is preferred by IUPAC, [5] ISO and IEC. [6] These five common potentials are all potential energies, but there are also entropy potentials.
The maximum work is thus regarded as the diminution of the free, or available, energy of the system (Gibbs free energy G at T = constant, P = constant or Helmholtz free energy F at T = constant, V = constant), whilst the heat given out is usually a measure of the diminution of the total energy of the system (internal energy).
For a similar process at constant temperature and volume, the change in Helmholtz free energy must be negative, <. Thus, a negative value of the change in free energy (G or A) is a necessary condition for a process to be spontaneous. This is the most useful form of the second law of thermodynamics in chemistry, where free-energy changes can be ...
Gibbs free energy: G: J Specific Gibbs free energy g: J/kg Gibbs free entropy: Ξ: J/K Grand / Landau potential: Ω: J Heat capacity (constant pressure) C p: J/K Specific heat capacity (constant pressure) c p: J/(kg·K) Heat capacity (constant volume) C v: J/K Specific heat capacity (constant volume) c v: J/(kg·K) Helmholtz free energy