Question

Aluminium is extracted from its chief ore, bauxite. The ore is first purified and then the metal is extracted from it by electrolytic reduction.

1. Write three balanced equations for the purification of bauxite by Hall’s process.
2. Name a chemical used for dissolving aluminium oxide. In which state of sub-division is the chemical used?
3. Write an equation for the reaction which takes place at the anode during the extraction of aluminium by the electrolytic process.
4. Mention one reason for the use of aluminium in thermite welding. Which particular property of cast iron makes it unsuitable for the construction of bridges?

Answer 1

a. \[\ce{ Na3AlF6 <=> 3Na+ + Al3+ + 6F-}\]

\[\ce{CaF2 <=> Ca2+ + 2F-}\]

\[\ce{Al2O3 <=> 2Al3+ + 3O^{-}_{2}}\]

b. The chemical used for dissolving aluminium oxide is cryolite. It is used in the molten state.

c. At anode: \[\ce{6O^{2-} - 12e- -> 6[O]}\]

\[\ce{3[O] + 3[O] -> 3O2}\]

The anode is oxidised to carbon monoxide, which further forms carbon dioxide

\[\ce{2C + O2 -> 2CO}\]

\[\ce{2CO + O2 -> 2CO2}\]

d. Aluminium has a high affinity for oxygen. It readily removes oxygen from oxides of less reactive metals.