`Cu_(2)O` is reduced easily by C but `CaO` is not - explain.

Explanation

`Delta_(f)G_(("Cu,Cu"_(2)O))^(@)` is less negative than `Delta_(f)G_("(C,CO)")^(@)` or `Delta_(f)G_(("C,CO"_(2)))^(@)` at all temperature. Hence, `Cu_(2)O` is reduced easily by carbon. On the other hand, `Delta_(f)G_("(Ca,CaO)")^(@)` becomes less negative than `Delta_(f)G_("(C,CO)")^(@)` or `Delta_(f)G_(("C,CO"_(2)))^(@)` at very high temperature. Hence, carbon reduction of `CaO` is difficult because at high temperature, molten calcium combines with carbon to form calcium carbide `(CaC_(2))`.