Thermodynamic potentials
coupled bulbs (left)Isolated system
Entropy (negative)

dS = 0
(S = max)
System (left)(add)Internal energy
(dS = 0, dV = 0)
Quantities of extensity constantdU = 0
(U = min)
System (left)Closed isentropic isobaric systemEnthalpy
(dS = 0, dP = 0)
Entropy, pressure, and amount of substance constantdH = 0
(H = min)
Battery (vertical left) nClosed isochoric isothermal systemHelmholtz free energy
(dT = 0, dV = 0)

Temperature, volume, and amount of substance constant dF = 0
(F = min)
Social systems (verticle right)Closed isobaric isothermal
Gibbs free energy
(dT = 0, dP = 0)

(freely running)
Temperature, pressure, and amount of substance constantdG = 0
(G = min)
Social systems (verticle right)Open isobaric isothermal systemGibbs free energy
(dT = 0, dP = 0)

Addition factors:
(chemical potential:
(turnover rate)
Temperature and pressure constant; amount of substance variesdG = 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]


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

What's your thermodynamic potential?

See also

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) –

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