Question

Explain giving equation, what happens when:
(a) ZnCO₃ is heated in the absence of air?
(b) a mixture of Cu₂O and Cu₂S is heated?

Answer 1

(a) When zinc carbonate is heated in the absence of air, it is converted to zinc oxide with the evolution of carbon dioxide gas. This is one of the methods used to convert concentrated metal carbonates to metal oxides during extraction of metals. This method is called calcination. The given zinc carbonate is nothing but calamine ore of zinc, which gets converted to zinc oxide on calcination.

$$\begin{gathered} {\mathrm{ZnCO}}_3\stackrel{\mathrm{Calcination}}{\to }\mathrm{ZnO}+{\mathrm{CO}}_2 \\ \mathrm{Zinc}\mathrm{carbonate}\mathrm{Zinc}\mathrm{oxide}\mathrm{carbon}\mathrm{dioxide} \\ (\mathrm{Calamine}ore) \end{gathered}$$

(b) When a mixture of ${\mathrm{Cu}}_2\mathrm{O}$ and ${\mathrm{Cu}}_2\mathrm{S}$ is heated, they react together to give copper metal and sulphur dioxide gas. This is one of the methods used to reduce sulphide ore of less reactive copper to copper metal. Copper glance, ${\mathrm{Cu}}_2\mathrm{S}$, is roasted in the presence of air to get converted to ${\mathrm{Cu}}_2\mathrm{O}$. ${\mathrm{Cu}}_2\mathrm{O}$ then reacts with non-oxidised ${\mathrm{Cu}}_2\mathrm{S}$ ore in the absence of air to form copper metal.

$$\begin{gathered} 2{\mathrm{Cu}}_2\mathrm{O}+{\mathrm{Cu}}_2\mathrm{S}\stackrel{\mathrm{Heated}}{\to }6\mathrm{Cu}+{\mathrm{SO}}_2 \\ \mathrm{Copper}\left(\mathrm{I}\right)\mathrm{Copper}\left(\mathrm{I}\right)\mathrm{Copper}\mathrm{Sulphur} \\ \mathrm{oxide}\underset{}{\mathrm{sulphide}\mathrm{metal}\underset{}{}\underset{}{\mathrm{d}\mathrm{ioxide}}} \end{gathered}$$