Gibbs-energy change distinguishes thermodynamic favourability from reaction speed.
Key knowledge
Gibbs energy is G = H − TS. At constant temperature and pressure, a process in a closed system with only pressure-volume work is thermodynamically favoured when its Gibbs-energy change is negative. This criterion does not specify how quickly the process proceeds.
Values and equations
ΔG = ΔH − TΔS
ΔG
Change in Gibbs energy J
ΔH
Change in enthalpy J
T
Absolute temperature K
ΔS
Change in entropy J K⁻¹
Conditions and scope
The stated direction criterion assumes fixed temperature and pressure and only pressure-volume work.
Use the actual states and composition; standard-state ΔG° is not interchangeable with ΔG under arbitrary conditions.
The initial and final states have the same absolute temperature T.
For the stated spontaneity test, pressure is also fixed and the closed system performs only pressure-volume work.
Sources · 4
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Paul Flowers, Klaus Theopold, Richard Langley and William R. Robinson — OpenStax, Rice University2 sources
Textbook thermodynamic derivation and the distinction between spontaneity and rate; no fresh experimental proof.
2026-10-10 · English Gibbs definition and constant-temperature/pressure criterion checked against sections 16.1 and 16.4, with work constraints stated.
Foundational review
2026-10-10 · Fresh primary-source inspection of the complete compact record, including every statement, equation, quantity and material condition, followed by independent AI editorial review. This check does not certify all linked lessons or every passage of every cited publication.
Sources from other places
Agreement within this scope
G=H−TS and the constant-temperature/pressure direction criterion, distinct from rate.
Thermodynamic favourability does not determine reaction speed.
In a reversible reaction at chemical equilibrium, the forward and reverse reaction rates are equal. Reactant and product concentrations remain constant under unchanged conditions, although reactions continue in both directions.
Gibbs energy is G = H − TS. At constant temperature and pressure, a process in a closed system with only pressure-volume work is thermodynamically favoured when its Gibbs-energy change is negative. This criterion does not specify how quickly the process proceeds.
Equilibrium and Gibbs-energy criteria are connected under the specified thermodynamic constraints.
The Nernst equation relates a reversible cell potential to its standard potential, temperature and reaction quotient. It follows from the thermodynamic relations between reaction Gibbs energy and reversible electrical work; it does not include current-dependent voltage losses.
Reversible cell potential is related to reaction Gibbs energy.
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.
Gibbs energy is the existing complementary potential; no duplicate Gibbs definition is introduced.
Pure phases coexist at common T and p when their molar Gibbs energies are equal. Along a first-order coexistence curve, dp/dT=Δs_m/Δv_m=L_m/(TΔv_m), with phase differences taken in the same direction and L_m=TΔs_m.
Equality of phase chemical potentials and their equilibrium differentials give the coexistence slope.
The molar Gibbs energy of a reaction at its current composition is ΔᵣG = ΔᵣG° + RT ln Q. At equilibrium it vanishes, giving ΔᵣG° = −RT ln K. The relation concerns thermodynamic direction and equilibrium, not reaction speed.
The reaction relation specializes Gibbs-energy accounting.