Question

In order to obtain 1 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.

1. Write the formula of cryolite.
2. Write down the word which correctly completes the following sentence.
   By dissolving aluminium oxide in cryolite, a ______ (conducting/non-conducting) solution is produced.
3. Why is so much graphite required for the electrolytic process?
4. Write the equation for the reaction which takes place at the cathode.
5. What is cathode made up of?

Answer 1

1. The formula of the cryolite is \[\ce{Na3AlF6}\].
2. By dissolving aluminium oxide in cryolite, a conducting solution is produced.
3. Thick graphite rods are used as the anode. The anode has to be replaced from time to time, as it gets oxidised by evolved oxygen.
4. Reaction at the cathode:
   \[\ce{4Al^3^+ +12e^- ->4Al}\]
5. The cathode is made of carbon.