Thermodynamic potentials

In thermodynamics, four different quantities, measured in units of energy, are called thermodynamic potentials:

• 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

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. (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.

External link

• http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/thepot.html