Why is it advantageous to roast a sulphide ore to the oxide before reduction ?

Explanation

The standard free energies of formation, `Delta_(f) G^(Ө)` of most of the sulphides are more negative than those of `CS_(2)` and `H_(2) S` (`CS_(2)`, in fact, is an endothermic compound). Therefore, neither carbon nor hydrogen can reduce metal sulphides to metal. In constrast, the standard free energies of formation of oxides ar much lower than that of `SO_(2)` and hence oxidation of metal sulphides to metal oxides is thermodynamically favourable. Moreover, the free energyies of formation of these metal oxides are less negative than that of `CO` and hence carbon can easily reduce these oxides to the corresponding metals.