Main Page | See live article | Alphabetical index 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 This article is from Wikipedia. All text is available under the terms of the GNU Free Documentation License.