Ellingham diagram

An Ellingham diagram is a graph showing the temperature dependence of the stability for compounds. This analysis is usually used to evaluate the ease of reduction of metal oxides and sulfides. These diagrams were first constructed by Harold Ellingham in 1944. In metallurgy, the Ellingham diagram is used to predict the equilibrium temperature between a metal, its oxide, and oxygen — and by extension, reactions of a metal with sulfur, nitrogen, and other non-metals. The diagrams are useful in predicting the conditions under which an ore will be reduced to its metal. The analysis is thermodynamic in nature and ignores reaction kinetics. Thus, processes that are predicted to be favourable by the Ellingham diagram can still be slow. 4 3 Cr ( s ) + O 2 ( g ) ⟶ 2 3 Cr 2 O 3 {displaystyle {ce {4/3 Cr(s) + O2(g) -> 2/3 Cr2O3}}}     (1) 4 3 Al ( s ) + O 2 ( g ) ⟶ 2 3 Al 2 O 3 {displaystyle {ce {4/3 Al(s) + O2(g) -> 2/3 Al2O3}}}     (2) 2 3 Cr 2 O 3 ( s ) + 4 3 Al ( s ) ⟶ 2 3 Al 2 O 3 + 4 3 Cr {displaystyle {ce {2/3 Cr2O3(s) + 4/3 Al(s) -> 2/3 Al2O3 + 4/3 Cr}}}     (3) An Ellingham diagram is a graph showing the temperature dependence of the stability for compounds. This analysis is usually used to evaluate the ease of reduction of metal oxides and sulfides. These diagrams were first constructed by Harold Ellingham in 1944. In metallurgy, the Ellingham diagram is used to predict the equilibrium temperature between a metal, its oxide, and oxygen — and by extension, reactions of a metal with sulfur, nitrogen, and other non-metals. The diagrams are useful in predicting the conditions under which an ore will be reduced to its metal. The analysis is thermodynamic in nature and ignores reaction kinetics. Thus, processes that are predicted to be favourable by the Ellingham diagram can still be slow. Ellingham diagrams are a particular graphical form of the principle that the thermodynamic feasibility of a reaction depends on the sign of ΔG, the Gibbs free energy change, which is equal to ΔH − TΔS, where ΔH is the enthalpy change and ΔS is the entropy change. The Ellingham diagram plots the Gibbs free energy change (ΔG) for each oxidation reaction as a function of temperature. For comparison of different reactions, all values of ΔG refer to the reaction of the same quantity of oxygen, chosen as one mole O (​1⁄2 mol O2) by some authors and one mole O2 by others. The diagram shown refers to 1 mole O2, so that for example the line for the oxidation of chromium shows ΔG for the reaction ​4⁄3 Cr(s) + O2(g) → ​2⁄3 Cr2O3(s), which is ​2⁄3 of the molar Gibbs energy of formation ΔGf°(Cr2O3, s). In the temperature ranges commonly used, the metal and the oxide are in a condensed state (solid or liquid), and oxygen is a gas with a much larger molar entropy. For the oxidation of each metal, the dominant contribution to the entropy change (ΔS) is the removal of ​1⁄2 mol O2, so that ΔS is negative and roughly equal for all metals. The slope of the plots d Δ G / d T = − Δ S {displaystyle dDelta G/dT=-Delta S} is therefore positive for all metals, with ΔG always becoming more negative with lower temperature, and the lines for all the metal oxides are approximately parallel. Since these reactions are exothermic, they always become feasible at lower temperatures. At a sufficiently high temperature, the sign of ΔG may invert (becoming positive) and the oxide can spontaneously reduce to the metal, as shown for Ag and Cu. For oxidation of carbon, the red line is for the formation of CO: C(s) + ​1⁄2 O2(g) → CO(g) with an increase in the number of moles of gas, leading to a positive ΔS and a negative slope. The blue line for the formation of CO2 is approximately horizontal, since the reaction C(s) + O2(g) → CO2(g) leaves the number of moles of gas unchanged so that ΔS is small. As with any chemical reaction prediction based on purely thermodynamic grounds, a spontaneous reaction may be very slow if one or more stages in the reaction pathway have very high activation energies EA.