Question

In order to obtain one tonne of aluminium, the following inputs are required:
4 tonnes of bauxite, 150 kg of sodium hydroxide and 600 kg of graphite.
The aluminium compound in bauxite is aluminium oxide and the main impurity is iron (III) oxide. Aluminium is obtained by the electrolysis of aluminium oxide dissolved in cryolite.
(a) When bauxite is treated with sodium hydroxide solution, what happens to:
(i) the aluminium oxide?
(ii) the iron (III) oxide?
(b) (i) Name the process used for the purification of bauxite.
(ii) Write the equation to show the action of heat on aluminium hydroxide.
(c) (i) Write the formula of cryolite.
(ii) Write down the word which correctly completes the following sentence:
"By dissolving aluminium oxide in cryolite, a __________ (conducting/non-conducting) solution is produced.
(iii) Why is so much graphite required for this electrolytic process?
(iv) Write the equation for the reaction which takes place at the cathode.
(d) In construction work, why is the alloy of aluminium, duralumin used rather than pure aluminium.

Answer 1

(a) When bauxite is treated with sodium hydroxide solution:
(i) The aluminium oxide (alumina) gets dissolved in the solution and forms sodium aluminate.
$$\begin{gathered} {\mathrm{Al}}_2{\mathrm{O}}_3.2{\mathrm{H}}_2\mathrm{O}+2\mathrm{NaOH}\stackrel{\mathrm{Heat}}{\to }2{\mathrm{NaAlO}}_2+3{\mathrm{H}}_2\mathrm{O} \\ \mathrm{Alumina}\mathrm{Sodium}\mathrm{hydroxide}\mathrm{solu}.\mathrm{Sodium}\mathrm{aluminate} \end{gathered}$$

(ii) Iron (III) oxide, which is present as an impurity, does not dissolve in sodium hydroxide solution and is separated by filtration.

(b) (i) Baeyer's process is used for the purification of aluminium.

(ii) ​Aluminium hydroxide is heated strongly at around 1100 ^oC to 1200 ^oC. This results in the decomposition of aluminium hydroxide to aluminium oxide (alumina).
$$\begin{gathered} 2\mathrm{Al}(\mathrm{OH}{)}_3\stackrel{\mathrm{Heat}}{\to }{\mathrm{Al}}_2{\mathrm{O}}_3+3{\mathrm{H}}_2\mathrm{O} \\ \mathrm{Aluminium}\mathrm{hydroxide}\mathrm{Aluminium}\mathrm{oxide}\mathrm{Water} \end{gathered}$$

(c) (i) The formula for cryolite is​ ${\mathrm{Na}}_3{\mathrm{AlF}}_6$. ​
(ii) On dissolving aluminium oxide in cryolite, a conducting solution is produced.
(iii) A large amount of graphite is required for this electroytic process because the oxygen produced at the anode reacts with (oxidises) the carbon (graphite) anode, liberating carbon monoxide and carbon dioxide gas. Thus, it needs to be frequently replaced.

$$\begin{gathered} 2\mathrm{C}+{\mathrm{O}}_2\stackrel{}{\to }2\mathrm{CO} \\ \mathrm{Carbon}\mathrm{Oxygen}\mathrm{Carbon}\mathrm{monoxide}\mathrm{gas} \\ (\mathrm{liberated}\mathrm{at}\mathrm{anode}) \\ \mathrm{C}+{\mathrm{O}}_2\stackrel{}{\to }{\mathrm{CO}}_2 \\ \mathrm{Carbon}\mathrm{Oxygen}\mathrm{Carbon}\mathrm{dioxide}\mathrm{gas} \\ (\mathrm{liberated}\mathrm{at}\mathrm{anode}) \end{gathered}$$

(iv) The equation for the reaction that takes place at the cathode during the extraction of aluminium is as follows:
${\mathrm{Al}}^{3+}+3{\mathrm{e}}^{-}\stackrel{}{\to }\mathrm{Al}$
Aluminium that is formed at the cathode is in a molten state and settles at the bottom of the cell. It is collected from time to time. The aluminium obtained in this process is 99% pure.

(d) Duralumin, which is an alloy of aluminium, has several advantages over pure aluminium. It is light like aluminium but also stronger. Duralumin is highly resistant to corrosion as compared to aluminium. The tensile strength of duralumin is very high when compared to aluminium. Due to these advantages, it is preferred over aluminium in construction work.