Thermodynamic free energy

A thermodynamic state function that measures energy available to perform work under specified constraints. Helmholtz free energy applies at constant temperature and volume; Gibbs free energy applies at constant temperature and pressure.

Entropy: A state function measuring energy dispersal and the number of microscopic arrangements compatible with a macroscopic state. Its contribution sets how much energy is unavailable for work at a given temperature.
Chemical equilibrium: A state in which forward and reverse chemical reactions occur at equal rates, leaving macroscopic composition unchanged. At fixed temperature and pressure, equilibrium minimizes Gibbs free energy.
Helmholtz free energy: The thermodynamic potential F = U − TS, useful for systems at constant temperature and volume. It is the constant-volume counterpart to Gibbs free energy.
Second law of thermodynamics: The law that total entropy does not decrease in an isolated system undergoing a spontaneous process. Free-energy decrease under fixed constraints expresses the second law for the system and surroundings.
Temperature: A physical quantity that determines the direction of heat transfer and characterizes thermal state. Temperature scales the entropic term in both Helmholtz and Gibbs free energy.
Gibbs free energy of reaction: The change in Gibbs free energy associated with a chemical reaction at specified conditions. Its sign indicates whether a reaction can proceed spontaneously at constant temperature and pressure.
Gibbs free energy: The thermodynamic potential G = H − TS, useful for systems at constant temperature and pressure. It is the constant-pressure counterpart to Helmholtz free energy.
Thermodynamic stability: Resistance of an equilibrium state to small perturbations, assessed through thermodynamic potentials and their curvature. A stable equilibrium minimizes the appropriate free energy under its constraints.
Pressure: Force exerted per unit area by a system on its surroundings. Constant-pressure conditions lead to Gibbs free energy and its volume-work term.
Electrochemical potential: The chemical potential of a charged species including its electrical potential-energy contribution. Electrochemical gradients store free energy that can drive ion transport and cellular work.

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Thermodynamic limitRelated: Its limiting value per volume or particle determines equilibrium bulk properties.