Enthalpy and Helmholtz thermodynamic potentials
The Legendre transforms H=U+pV and F=U−TS change the natural state variables: dH=T dS+V dp and dF=−S dT−p dV at fixed composition.
Key knowledge
The Legendre transforms H=U+pV and F=U−TS change the natural state variables: dH=T dS+V dp and dF=−S dT−p dV at fixed composition. At fixed temperature and volume, equilibrium minimizes F among allowed states. For a closed system with only pressure-volume work, unchanged bulk kinetic/gravitational potential energies and constant external pressure equal to both equilibrium endpoint pressures, heat supplied equals the enthalpy change.
Values and equations
H=U+pV; F=U−TS; dH=T dS+V dp; dF=−S dT−p dV
- H,F,U
- Enthalpy, Helmholtz energy and internal energy J
- p
- Pressure Pa
- V
- Volume m³
- T
- Temperature K
- S
- Entropy J K⁻¹
Conditions and scope
- Simple compressible equilibrium state functions with fixed amount and composition; extra work coordinates require additional terms. The heat–enthalpy process rule requires ΔKE=ΔPE=0 and work p_extΔV at the stated constant pressure; mechanically quasistatic constant-p processes satisfy the pressure condition.
- The equilibrium-minimum rule assumes the specified thermal reservoir and constraints; H here means enthalpy, not a Hamiltonian.
Sources · 4
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Richard Fitzpatrick / University of Texas at Austin4 sources
- Thermodynamics and Statistical Mechanics (2016): EnthalpyRichard Fitzpatrick / University of Texas at AustinCourse notes
- Thermodynamics and Statistical Mechanics (2016): Laws of thermodynamicsRichard Fitzpatrick / University of Texas at AustinCourse notes
- Thermodynamics and Statistical Mechanics (2016): Helmholtz free energyRichard Fitzpatrick / University of Texas at AustinCourse notes
- Thermodynamics and Statistical Mechanics (2016): Equilibrium of constant-temperature systemRichard Fitzpatrick / University of Texas at AustinCourse notes
Evidence and review
Thermodynamics and Statistical Mechanics (2016): Enthalpy: Equations 6.90–6.99: transform and state differential. The finite heat identity is separately derived by combining H=U+pV with the inspected first law at fixed external/end pressure; this page does not itself display that process rule. Thermodynamics and Statistical Mechanics (2016): Laws of thermodynamics: First-law internal-energy balance. Combining ΔU=Q−p_extΔV with Δ(pV)=p_extΔV at the stated endpoints gives Q=ΔH only with unchanged bulk kinetic/potential energies and no other work. Thermodynamics and Statistical Mechanics (2016): Helmholtz free energy: Equations 6.100–6.109: transform and differential. Thermodynamics and Statistical Mechanics (2016): Equilibrium of constant-temperature system: Fixed temperature/volume free-energy minimum.
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