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Helmholtz and Gibbs energies predict spontaneity.
G = H − TSGibbs free energy G = H − TS decides spontaneity at constant T and P: processes with ΔG < 0 happen. Helmholtz F = U − TS applies at constant volume.
Ice melts above 0 °C where TΔS beats ΔH.
Equilibrium has ΔG = 0.
In more detail
Free energy combines energy and entropy to indicate which processes are spontaneous at constant temperature. The Helmholtz free energy is F = U − TS, and the Gibbs free energy is G = H − TS.
Real-world uses
- Chemists predict reaction spontaneity using Gibbs free energy.
- Battery performance is determined by free energy changes.
- Chemists use Gibbs free energy to decide whether a reaction will happen spontaneously at a given temperature and pressure.
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Frequently asked questions
When is a process spontaneous?
When the free energy change is negative at constant temperature and pressure for Gibbs.
Why is free energy useful?
It combines energy and entropy into one criterion.
When is a reaction spontaneous?
When the change in Gibbs free energy is negative at the given temperature and pressure.
Related topics in Heat & Thermodynamics
Entropy · Entropy change · Statistical entropy · Third law · Maxwell relations · Clausius–Clapeyron equation