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

Explanation

The standard free energies of formation `(Delta _f G ^(@)) ` of most of the sulphides are more negative than those of ` CS_2 (CS_2 ` is, in fact, an endothermic compound ) and ` H _ 2 S `. Therefore, neither carbon nor hydrogen can reduce metal sulphides to metal. In contrast, the standard free energies of formation of oxides are much lower than that of ` SO _ 2 ` and hence oxidation of metal sulphides to metal oxides is thermodynamically favaourable. Furthemore, the free energies of formation of these metal oxides is less negative than that of CO and hence carbon can easily reduce these oxides to the corresponding metals.