Thermodynamic Potentials

[!summary] Scope Thermodynamic potentials are scalar functions used to describe the equilibrium state of a system under specific control variables.

Potential	Symbol	Definition	Natural Variables	Use Case
Internal Energy	$U$	$U$	$S, V$	Isolated systems
Enthalpy	$H$	$U + PV$	$S, P$	Isobaric adiabatic
Helmholtz Energy	$A$	$U - TS$	$T, V$	Isothermal isochoric
Gibbs Energy	$G$	$H - TS$	$T, P$	Isothermal isobaric

Spontaneity Criteria

For a process at constant $T$ and $P$ (most chemical/engineering contexts), spontaneity requires: $\Delta G < 0$ Equilibrium occurs when $\Delta G = 0$.

Relationships

• Related: Internal Energy, Enthalpy, Chemical Potential
• Derivation: First Law of Thermodynamics, Second Law of Thermodynamics

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