# Thermodynamic potential

 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.  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. 

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. 

Humor

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