Thermodynamic potentials
System	Potential	Conditions	Equilibrium
Isolated system	Entropy (negative) Negentropy		dS = 0 (S = max)
(add)	Internal energy (dS = 0, dV = 0)	Quantities of extensity constant	dU = 0 (U = min)
Closed isentropic isobaric system	Enthalpy (dS = 0, dP = 0)	Entropy, pressure, and amount of substance constant	dH = 0 (H = min)
Closed isochoric isothermal system	Helmholtz free energy (dT = 0, dV = 0)	Temperature, volume, and amount of substance constant	dF = 0 (F = min)
Closed isobaric isothermal system	Gibbs free energy (dT = 0, dP = 0) Note: (freely running)	Temperature, pressure, and amount of substance constant	dG = 0 (G = min)
Open isobaric isothermal system	Gibbs free energy (dT = 0, dP = 0) Addition factors: (chemical potential: μ) (turnover rate)	Temperature and pressure constant; amount of substance varies	dG = 0 (G = min)

In thermodynamics, thermodynamic potential is the name given to a function whose minimum gives the equilibrium state of a system subject to specific constraints. [1] Among the most often encountered thermodynamic potentials cited include:

● Negentropy | -S | isolated system.
● Internal energy | U | quantities of extensity constant.
● Helmholtz free energy | U – TS | temperature, volume, and amount of substance constant.
● Gibbs free energy | U + PV – TS | temperature, pressure, and amount of substance constant.
● Enthalpy | U + PV | entropy, pressure, and amount of substance constant.

Stated verbally, the conception of thermodynamic potential provides for a description of the direction of evolution of physical systems. Through the second law, the science of thermodynamics states that a system evolves in the direction that minimizes an appropriate thermodynamic potential, for example the "negative of entropy" (neg-entropy) for isolated systems, or the Gibbs free energy at constant pressure and temperature. [2]

Chemical reactions
The application of the thermodynamic concept of potential to the kinetics of chemical reactions (transition state theory) provides a criterion for selecting the optimal pathway for a transition, usually the pathway with the transition state of the lowest free energy. In this perspective, by providing a direction for systems to evolve and an optimal pathway, thermodynamics offers a way for answering why things happen the way they do. [2]

Humor
The following is a thermodynamics humor T-Shirt made by Zazzle creator Word & UnWords, showing Willard Gibbs 1873 ‘available energy’ (AB) graph: [3]

See also
● Coupling

References
1. Perrot, Pierre. (1998). A to Z of Thermodynamics, Oxford: Oxford University Press.
2. Di-Cera, Enrico. (2000). Thermodynamics in Biology, (pgs. 3-4). Oxford: Oxford University Press.
3. What’s your thermodynamic potential? (T-Shirt) – Zazzle.com.