Question

Why is the extraction of copper from pyrites more difficult than that from its oxide ore through reduction?

Answer 1

The standard free energy of formation (${\Delta }_f{G}^0$) of $C{u}_{2}S$ is lesser than those of $C{S}_2$ and $H_{2}S$. So, $Cu{S}_2$ (pyrites) can not be reduced by carbon or hydrogen.

$2C{u}_{2}S+C⟶4Cu+C{S}_2$

$2C{u}_{2}S+H_2⟶2Cu+H_{2}S$

However, the ${\Delta }_f{G}^0$ of copper oxide is greater than that of $C{O}_2$.

Therefore, the sulphide ore is first converted to the oxide by roasting and then reduced.

$2C{u}_{2}S+3O_2\underrightarrow{Roasting}2C{u}_{2}O+S{O}_2$

$2C{u}_{2}O+C\underrightarrow{heating}4Cu+C{O}_2$