Question

(i) Name the solution used to react with Bauxite as a first step in obtaining pure aluminium oxide, in the Baeyer's process.
(ii) Write the equation for the reaction where the aluminium oxide for the electrolytic extraction of aluminium is obtained by heating aluminium hydroxide.
(iii) Name the compound added to pure alumina to lower the fusion temperature during the electrolytic reduction of alumina.
(iv) Write the equation for the reaction that occurs at the cathode during the extraction of aluminium by electrolysis.
(v) Explain why it is preferable to use a number of graphite electrodes as anode instead of a single electrode, during the above electrolysis.

Answer 1

(i) The first step it concentration of ore to oxide. For that caustic alkali i.e. NaOH is used.

(ii) $Al(OH{)}_3$ is heated at a temperature of ${1100}^{\circ }C$ to obtain$A{l}_2{O}_3$.

$2Al(OH{)}_3\stackrel{{1100}^{\circ }}{\to }A{l}_2{O}_3+3H_{2}O$

(iii) Cryolite ($N{a}_{3}Al{F}_6$) is added the lower the fusion temperature.

(iv) At the cathode pure aluminium metal is obtained.

$A{l}^{3+}+3e^{-}\to Al$

(v) During this process oxygen is evolved which reacts with rod and forms carbon dioxide and hence the rod gradually gets exhausted. Hence it is favorable to use number of graphite rods so that the process can go on for a longer time.