The standard free enegy `(trianglel_(f)G^(ɵ))` of formation of `Cu_2S` is more negative than those of `CS_2` and `H_2S(CS_2` is in fact, an endothermic compound). Therefore neither corbon nor hydrogen can reduce `Cu_2S` to `Cu` metal.
`Cu_2S+H_2to2Cu+H_2S,2Cu_2S+Cto4Cu+CS_2` in contrast `triangle_(f)G^(ɵ)` of `Cu_2O` is less `-ve` than that of CO and hence carbon can easily reduce `Cu_2O` to `Cu`
`Cu_2O+Cto2Cu+CO(g)`
Hence the extraction of copper from pyrite is difficult than from its oxide ore through reduction.
Alternatively, `(triangle_(f)G^(ɵ))/(T)` line for CO has a negative slope and there is no compound CS analogous to CO with a steep negative `(triangle_(f)G^(ɵ))/(T)` line. Thus carbon is a good reducing agent for oxides but a poor reducing agent for sulphides. Therefore sulphides are normally roasted in air to form oxides before reducing them with Carbon.
