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]