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|Internal Energy]], [[enthalpy|Enthalpy]], Chemical Potential
• Derivation: [[first-law|First Law of Thermodynamics]], [[second-law|Second Law of Thermodynamics]]

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