Question

What are the features of Ellingham diagram? What is its significance?

Answer 1

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Ellingham Diagrams

• These diagrams represent the changes in the Gibbs free energy with respect to temperature during the formation of oxides of various elements.
• These diagrams are helpful in predicting the feasibility of a thermal reduction of an ore.

Features of Ellingham Diagrams

• The slope of the curve for the formation of a metal oxide is positive because the change in the Gibbs free energy becomes less negative with increase in temperature, as Δ​S is negative for the reaction.
• Each plot is a straight line except when there is some change in the phase that results in a large change in entropy. As a result, the slope of the curve also changes.
• Below the $∆G=0$ line, the oxides are stable as their free energy changes for formation are negative.
• Above a temperature when the plot crosses the $∆G=0$ line, the oxides become unstable and decompose on their own into metal and oxygen.
• A metal is able to reduce the oxides of only those metals that lie above it in the Ellingham diagram. In such cases, the Gibbs free energy change for the process becomes negative by an amount that is equal to the difference between the two plots at that particular temperature. Hence, the reduction process becomes feasible.

Significance of Ellingham Diagrams

• The positive slope of a metal oxide indicates that its stability increases with increase in temperature.
• A decrease in the stability of metal oxides is due to an increase in the ΔG^o value.
• A sudden change in the graph shows a phase change.
• The negative slope of CO indicates that it becomes more stable with increase in temperature.
• These diagrams enable the selection of a suitable reducing reagent for a given metal oxide.