Ellingham diagram

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In metallurgy, the Ellingham diagram is used to predict the equilibrium temperature between a metal, its oxide and oxygen.

The Ellingham diagram plots the Gibbs free energy change (ΔG) for the oxidation reaction versus the temperature. In the temperature ranges commonly used, the metal and the oxide are in a condensed state (liquid or solid) and the oxygen gaseous, therefore the ΔG of the oxidation becomes smaller with higher temperature, and thus the reaction slower. At a sufficiently high temperature, the reaction may invert, and the oxide spontaneously reduce to metal.

If two metals are present, two equilibriums have to be considered, so that the metal with the smaller reaction ΔG reduces, the other oxidizes.

In industrial processes, the reduction of metal oxides is obtained using carbon, which is available cheaply in reduced form (as coal).

Moreover, when carbon reacts with oxygen it forms gaseous composts carbon monoxide and carbon dioxide, therefore the dynamics of its oxidation is different from that for metals: its oxidation has a larger ΔG with higher temperatures. Using this property, reduction of metals may be performed as a double redox reaction at relatively low temperature.