Question:
Why is the extraction of copper from pyrites more difficult than that from its oxide ore through reduction?
Solution:
The Gibbs free energy of formation (ΔfG) of Cu2S is less than that of 
and
. Therefore, H2 and C cannot reduce Cu2S to Cu.
On the other hand, the Gibbs free energy of formation of 
is greater than that of
. Hence, C can reduce Cu2O to Cu.
$\mathrm{C}_{(s)}+\mathrm{Cu}_{2} \mathrm{O}_{(s)} \longrightarrow 2 \mathrm{Cu}_{(s)}+\mathrm{CO}_{(g)}$
Hence, the extraction of copper from its pyrite ore is difficult than from its oxide ore through reduction.