The standard free energy (∆f G°) of formation of Cu2S is greater than those of CS2 and H2S (CS2 is infact, an endothermic compound) Therefore neither carbon nor hydrogen can reduce Cu2S to Cu metal.
Cu2S + H2 > 2Cu + H2S
2Cu2S + C > 4Cu + CS2
In contrast, ∆fG° of Cu2O is much lower than that of CO and hence carbon can easily reduce Cu2O to Cu.
Cu2O (S) + C(S) → 2Cu (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.
