Question

The following is an extract from 'Metals in the Service of Man, Alexander and Street/Pelican 1976':
'Alumina (Aluminium oxide) has a very high melting point of over 2000 C so that it cannot readily be liquefied. However, conversion of alumina to aluminium and oxygen, by electrolysis, can occur when it is dissolved in some other substance'.
(a) Which solution is used to react with bauxite as a first step in obtaining pure aluminium oxide?
(b) The aluminium oxide for the electrolytic extraction of aluminium is obtained by heating aluminium hydroxide. Write the balanced chemical equation for this reaction.
(c) Name the element which serves both as anode and the cathode in the extraction of aluminium.
(d) Write the balanced chemical equation for the reaction that occurs at the cathode during the extraction of aluminium by electrolysis.
(e) Give the balanced chemical equation for the reaction which occurs at the anode when aluminium is purified by electrolysis.

Answer 1

(a)Reaction of a hot solution of sodium hydroxide with bauxite is the first step in obtaining pure aluminium oxide.
(b) The aluminium oxide that is required for the electrolytic extraction of aluminium is obtained by heating aluminium hydroxide. The equation for this reaction is
$$\begin{gathered} 2\mathrm{Al}(\mathrm{OH}{)}_3\stackrel{\mathrm{heated}}{\to }{\mathrm{Al}}_2{\mathrm{O}}_3+3{\mathrm{H}}_2\mathrm{O} \\ \mathrm{Aluminium}\mathrm{hydroxide}\mathrm{Aluminium}oxide \end{gathered}$$
(c) Carbon serves both as the anode and the cathode in the extraction of aluminium.
(d) During the extraction of aluminium, the aluminium metal is deposited at the cathode. Aluminium ions accept electrons and get reduced to aluminium metal at the cathode. The equation for the reaction is
Reaction at cathode: ${\mathrm{Al}}^{3+}+3{\mathrm{e}}^{-}\stackrel{}{\to }\mathrm{Al}$
(e) During the extraction of aluminium by electrolysis, oxygen gas is liberated at the anode. Oxygen ions lose electrons to form oxygen gas at the anode.
Reaction at anode: $$\begin{gathered} {\mathrm{O}}^{2-}\stackrel{}{\to }\mathrm{O}+2{\mathrm{e}}^{-} \\ \mathrm{O}+\mathrm{O}\stackrel{}{\to }{\mathrm{O}}_2 \end{gathered}$$