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Question
Predict conditions under which Al might be expected to reduce MgO.
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Solution 1
Above 1350°C, the standard Gibbs free energy of formation of Al2O3 from Al is less than that of MgO from Mg. Therefore, above 1350°C, Al can reduce MgO.
Solution 2
The equations for the formation of the two oxides are:
\[\ce{\frac{4}{3} Al_{(s) + O2_{(s)} -> \frac{2}{3} Al2O3_{(s)}\]
\[\ce{2Mg_{(s)} + O2_{(s)} -> 2MgO_{(s)}\]
If we look at the plots for the formation of the two oxides of the Ellingham diagram, we find that they intersect at certain points. The corresponding value of ΔG° becomes zero for the reduction of MgO by Al metal.
\[\ce{2MgO_{(s)} + \frac{4}{3} Al_{(s)} <=> 2Mg_{(s)} + \frac{2}{3}Al2O3_{(s)}}\]
This means that the reduction of MgO by Al metal can occur below this temperature. Aluminium (Al) metal can reduce MgO to Mg above this temperature because Δ°G for Al2O3 is less as compared to that of MgO.
\[\ce{3MgO_{(s)} + 2Al_{(s)} ->[(>1665 K)] Al2O3_{(s)} + 3Mg_{(s)}}\]
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