Why is the extraction of copper from pyrites more difficult than that from its oxide ore through reduction?

Explanation

The standard free energy ` ( Delta _ f G ^(@)) ` of formation of `Cu_ 2S ` is more - ve than those of `CS_ 2 ` and `H_ 2 S (CS_ 2 ` is , in fact, an endothermic compound). Therefore, neither carbon nor hydrogen can reduce ` Cu_ 2 S ` to Cu metal.

In contrast, ` Delta _ f G^(@) ` of `Cu_ 2 O ` is less - ve than that of CO and hence carbon can easily reduce ` Cu_ 2 O ` to Cu
` " " Cu_ 2 O (s) + C (s) to 2 Cu (s) + CO (g) `
It is because of this reason that the extraction of copper from pyrite is difficult than from its oxide ore through reduction.
Alternatively, `Delta G ^(@)//T ` line for CO has a - ve slope and there is no compound CS analogous to CO with a steep negative ` Delta G^(@)//T ` line. Thus, carbon is a good reducing agent for oxides but a poor reucing agent for sulphides. Therefore, sulphides are normally roasted in air to from oxides before reducing them with carbon.