Gibbs free energy
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Gibbs free energy is a measure of the maximum available work that can be derived from any system under conditions of constant temperature (T) and pressure (P). G is a thermodynamic "state function", i.e., an equilibrium property that depends only upon the conditions - such as T, P and electrical, magnetic and gravitational fields - imposed on the system being considered, and not on that system's past history. Since absolute G values cannot be determined, changes in G as a system goes from one state to another become the main focus of attention. These DG ("delta-G") values are highly informative. If DG = ( Gfinal state - Ginitial state) is negative, the process observed liberates energy: it will occur spontaneously and can be harnessed to do useful work. For chemical changes, tabulated standard free energy values can be used to predict the direction and energy yield of a particular reaction. For example, it is easy to calculate that if one burns a mole (114 g) of isoctane to carbon dioxide and water, a total of 5226 kJ (kilojoules) of Gibbs free energy will be released, i.e., DG = -5226 kJ/mol. This large negative value predicts a spontaneous process that proceeds completely to products. Performed in an internal combustion engine, about one-third of the DG will be recovered. A substantially larger fraction could be extracted by a fuel cell. J. Willard Gibbs (1839-1903) first defined the free energy function that bears his name in the landmark theoretical papers of 1876 and 1878 that have led most authorities to rank among America's greatest native-born scientist.
Further reading
Gibbs free energy, HyperPhysics.
