A diagram used to show the conditions under which a metal oxide can be reduced to a metal. The standard Gibbs free energy of formation of the oxide is considered, for example,
M+½O2 → MO
This value, ΔG⦵, is plotted against temperature. In general, the result is a straight line. In some cases, there is an abrupt change in the line's slope at a point because of a phase change. The value of ΔG⦵ for the reducing agent is also plotted. For example, if the reducing agent is carbon forming carbon dioxide, it is ΔG⦵ for the reaction
C+O2 → CO2
Reduction can occur in the range of temperatures in which the carbon curve is lower than the metal curve. The diagram was devised by the physical chemist H. J. T. Ellingham.