If two metals are present, two equilibria have to be considered. The blue line for the formation of CO 2 is approximately horizontal, since the reaction C(s) + OĢ(g) → CO 2(g) leaves the number of moles of gas unchanged so that Δ S is small.Īs 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 E A. At a sufficiently high temperature, the sign of Δ G may invert (becoming positive) and the oxide can spontaneously reduce to the metal, as shown for Ag and Cu.įor oxidation of carbon, the red line is for the formation of CO: C(s) + 1⁄ 2 OĢ(g) → CO(g) with an increase in the number of moles of gas, leading to a positive Δ S and a negative slope. Since these reactions are exothermic, they always become feasible at lower temperatures. The slope of the plots d Δ G / d T = − Δ S is therefore positive for all metals, with Δ G always becoming more negative with lower temperature, and the lines for all the metal oxides are approximately parallel. For the oxidation of each metal, the dominant contribution to the entropy change (Δ S) is the removal of 1⁄ 2 mol OĢ, so that ΔS is negative and roughly equal for all metals. 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. the line for the oxidation of chromium shows Δ G for the reaction 4⁄ 3 Cr(s) + Oģ(s), which is 2⁄ 3 of the molar Gibbs energy of formation Δ G f°( Cr The diagram shown refers to 1 mole OĢ, so that e.g. For comparison of different reactions, all values of Δ G refer to the reaction of the same quantity of oxygen, chosen as one mole O ( 1⁄ 2 mol OĢ by others.
#ELLINGHAM DIAGRAMS FREE#
The Ellingham diagram plots the Gibbs free energy change (Δ G) for each oxidation reaction as a function of temperature. Thus, processes that are predicted to be favourable by the Ellingham diagram can still be slow.Įllingham diagram for several metals giving the free energy of formation of metal oxides and the corresponding oxygen partial pressure at equilibrium.Įllingham diagrams are a particular graphical form of the principle that the thermodynamic feasibility of a reaction depends on the sign of Δ G, the Gibbs free energy change, which is equal to Δ H − TΔ S, where Δ H is the enthalpy change and Δ S is the entropy change. The analysis is thermodynamic in nature and ignores reaction kinetics. The diagrams are useful in predicting the conditions under which an ore will be reduced to its metal. 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. These diagrams were first constructed by Harold Ellingham in 1944. This analysis is usually used to evaluate the ease of reduction of metal oxides and sulfides. JSTOR ( December 2021) ( Learn how and when to remove this template message)Īn Ellingham diagram is a graph showing the temperature dependence of the stability of compounds.Unsourced material may be challenged and removed.įind sources: "Ellingham diagram" – news Please help improve this article by adding citations to reliable sources. This article needs additional citations for verification.