Ellingham Diagram - Salient Features

Salient Features

1. Curves in the Ellingham diagrams for the formation of metallic oxides are straight lines with a positive slope.
2. The lower the position of a metal in the Ellingham diagram, the greater is the stability of its oxide. For example, the Ellingham diagram for Al is found to be below Fe₂O₃.
3. The greater the gap between any two lines, the greater the efficiency of the reducing agent.
4. Stability of metallic oxides decrease with increase in temperature. Highly unstable oxides like Ag₂O and HgO easily undergo thermal decomposition.
5. The formation enthalpy of carbon dioxide (CO₂) is almost a temperature-independent constant, while that of carbon monoxide (CO) has negative slope. According to Boudouard reaction, carbon monoxide is the dominant compound in higher temperatures, and the higher the temperature, the more efficient reductant carbon monoxide also is.
6. A substance whose Gibbs free energy of formation is lower (ΔG line lower on diagram) at given temperature, will reduce one whose free energy of formation is higher on the diagram. Hence metallic aluminum can reduce iron from iron oxide into metallic iron, aluminum itself oxidizing into aluminum oxide. (This reaction is employed in thermite.)
7. The intersection of two lines imply the equilibrium of oxidation and reduction reaction between two substances. Reduction with using a certain reductant is possible at the intersection point and higher temperatures where the ΔG line of the reductant is lower on diagram than the metallic oxide to be reduced. At the point of intersection the Gibbs energy is 0(zero), below this point the Gibbs energy is <0 and the oxides are stable,while above the point of intersection the Gibbs energy is >0 and so, the oxides are unstable.