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) |

**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.