Ans:

The graph of Gibbs energy ∆G^θ versus Temperature for the formation of solid oxides is shown above. This graph shows that if the ∆_fG^θ of a metal’s oxide is more negative than the ∆_fG^θ of another metal’s oxide, the former metal can reduce the latter metal’s oxide (i.e., the oxide with less negative ∆_fG^θ). For example, because ∆_fG^θ ( Al, Al₂O₃) is less negative than ∆_fG^θ ( Cu, Cu₂O), Al can easily decrease Cu₂O to Cu but Cu cannot reduce Al₂O₃. In the same way, Zn cannot reduce MgO, whereas Mg may reduce ZnO to Zn. This is due to the fact that ∆_fG^θ ( Mg, MgO ) is more negative than ∆_fG^θ ( Mg, MgO ). ( Zn,ZnO ).