Question

Explain the reasons:

1. In the electrolytic reduction of alumina, the graphite anode is gradually consumed.
2. Roasting is carried out on sulphide ores and not on carbonate ores.
3. Carbon can reduce lead oxide but not aluminium oxide.

Answer 1

(i) In the electrolytic reduction of alumina molten aluminium is produced at the cathode and oxygen gas is evolved at the anode.

${\mathrm{Al}}_2{\mathrm{O}}_3\left(\mathrm{aq}\right)\stackrel{\mathrm{Electrolysis}}{\to }{\mathrm{Al}}^{3+}\left(\mathrm{aq}\right)+{\mathrm{O}}^{2-}\left(\mathrm{aq}\right)$

At cathode: The aluminium gets reduced by the gain of electrons.

$2{\mathrm{Al}}^{3+}\left(\mathrm{aq}\right)+6{\mathrm{e}}^{-}\to 2\mathrm{Al}\left(\mathrm{s}\right)$

At anode: The oxide ion gets oxidized by losing the electrons.

$$\begin{gathered} 6{\mathrm{O}}^{2-}\left(\mathrm{aq}\right)-12{\mathrm{e}}^{-}\to 6\left[\mathrm{O}\right]\left(\mathrm{g}\right) \\ 3\mathrm{O}\left(\mathrm{g}\right)+3\mathrm{O}\left(\mathrm{g}\right)\to 3{\mathrm{O}}_2\left(\mathrm{g}\right) \end{gathered}$$

• The oxygen gas evolved at the anode reacts with graphite anodes and forms carbon monoxide which further forms carbon dioxide.

$$\begin{gathered} 2\mathrm{C}\left(\mathrm{g}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\to 2\mathrm{CO}\left(\mathrm{g}\right) \\ 2\mathrm{CO}\left(\mathrm{g}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\to 2{\mathrm{CO}}_2\left(\mathrm{g}\right) \end{gathered}$$

• The anodes thus burn away. Therefore, they must be replaced from time to time.

(ii) Roasting is a process of strongly heating the concentrated ore to a high temperature in excess of air to convert it into a metal oxide from metal sulphides.

$2\mathrm{ZnS}\left(\mathrm{s}\right)+3{\mathrm{O}}_2\left(\mathrm{g}\right)\to 2\mathrm{ZnO}\left(\mathrm{s}\right)+2{\mathrm{SO}}_2\left(\mathrm{g}\right)\uparrow$

Carbonate is converted into oxide by loss of ${\mathrm{CO}}_2$ in the absence of air when heated strongly.

${\mathrm{ZnCO}}_3\left(\mathrm{s}\right)\stackrel{∆}{\to }\mathrm{ZnO}\left(\mathrm{s}\right)+{\mathrm{CO}}_2\left(\mathrm{g}\right)$

(iii) Aluminium oxide is a very stable compound because of its greater affinity toward oxygen. It can not be reduced by carbon, carbon monoxide or hydrogen.

Lead oxide can be easily reduced to metal lead by carbon because lead is less active than aluminium according to activity series.

$$\begin{gathered} \mathrm{PbO}\left(\mathrm{s}\right)+\mathrm{C}\left(\mathrm{s}\right)\to \mathrm{Pb}\left(\mathrm{s}\right)+\mathrm{CO}\left(\mathrm{g}\right) \\ \mathrm{lead}\mathrm{carbon}\mathrm{lead}\mathrm{carbon} \\ \mathrm{oxide}\mathrm{metal}\mathrm{monoxide} \end{gathered}$$