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Thermodynamic potentials
In thermodynamics, four quantities, measured in units of energy, are called thermodynamic potentials:
| Internal energy | U | The energy needed to create a system |
| Helmholtz free energy | F = U - TS | Also represented by A |
| Enthalpy | H = U + PV | |
| Gibbs free energy | G = U + PV - TS |
where T = temperature, S = entropy, P = pressure, V = volume
| Contents |
Differential definitions
The following differential relations hold for the four potentials:
| dU | = | TdS | - | PdV | |
| dF | = | - | SdT | - | PdV |
| dH | = | TdS | + | VdP | |
| dG | = | - | SdT | + | VdP |
If we write the above four equations generally as
Then it is seen that
yielding expressions for T, P, S, and V in terms of derivatives of the potentials
Furthermore, mathematically we have
which gives:
which are known as Maxwell's relations
Chemical reactions
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. Δ denotes the change in the potential and at equilibrium the change will be zero.
| 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.
External links
References
- Lewis, Gilbert Newton; Randall, Merle; Revised by Pitzer, Kenneth S. & Brewer, Leo "Thermodynamics" 2nd Editon, New York, NY USA: McGraw-Hill Book Co. 1961.
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