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Thermodynamic potentials
Thermodynamic potentials are quantities which appear in thermodynamics. They are measured in units of energy, and there are four of them:
- Internal energy[?] U, which is the energy needed to create a system
- Helmholtz free energy F, given by F=U-TS
- Enthalpy H, given by H=U+pV
- Gibbs free energy G, given by G=U+pV-TS
where T = temperature, S = entropy, p = pressure, V = volume
Changes in these quantities are useful for assessing the degree to which a chemical reaction will proceed. The relevant quantity depends on the reaction conditions, as shown in the following table. (NB "Δ" denotes the change in a quantity.)
| Constant V | Constant p | |
|---|---|---|
| Constant S | ΔU | ΔH |
| Constant T | ΔF | ΔG |
Most commonly one considers reactions at constant p and T, so the Gibbs free energy is the most useful potential in studies of chemical reactions.
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