Question

(a) 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'.
(i) Which solution is used to react with bauxite as a first step in obtaining pure aluminium oxide?
(ii) The aluminium oxide for the electrolysis of aluminium is obtained by heating aluminium hydroxide. Write the equation for this reaction.
(iii) Name the element which serve both as the anode and the cathode in the extraction of aluminium.
(iv) Write the equation for the reaction that occurs at the cathode during the extraction of aluminium by electrolysis.
(v) Give the equation for the reaction which occurs at the anode when aluminium is purified by electrolysis.

Answer 1

(i) Sodium hydroxide solution (NaOH) is used to obtain pure aluminium oxide (Al₂O₃) from bauxite. The equations for the first step of process is as follows:

$$\begin{gathered} {\text{Al}}_2{\text{O}}_3.{\text{2H}}_{2}O+\text{2NaOH}\stackrel{\text{423-523 K}}{\to }{\text{2NaAlO}}_2+{\text{H}}_2\text{O} \\ \text{Bauxite} \end{gathered}$$

(ii) Aluminium hydroxide on heating forms aluminium oxide and water. The equation is as follows:
${\text{2Al(OH)}}_3\stackrel{1100-{1200}^{0}C}{\to }{\text{Al}}_2{\text{O}}_3+{\text{3H}}_2\text{O}$

(iii) The anode and cathode used in the extraction of aluminium are made of carbon.

(iv) Aluminium metal is produced at the cathode during electrolysis of alumina. The equation is as follows:

Al³⁺ + 3e⁻ → Al

(v) At anode, initially Oxygen gas is produced which oxidises carbon of the anode to form carbon dioxide gas. The equation is as follows:
$$\begin{gathered} {\text{2O}}^{2-}-{\text{4e}}^{-}\to {\text{O}}_2 \\ {\text{C + O}}_2\to {\text{CO}}_2 \end{gathered}$$