(a) Give two reasons why fused alumina is mixed with cryolite in the electrolytic extraction of aluminium. (b) Write an equation for the reaction of sodium ...
(a) Give two reasons why fused alumina is mixed with cryolite in the electrolytic extraction of aluminium.
(b) Write an equation for the reaction of sodium hydroxide with bauxite.
(a) Why fused alumina is mixed with cryolite
Cryolite dissolves the alumina and lowers its melting point from about 2050 °C to about 950 °C, greatly reducing the energy and cost of the extraction.
The molten cryolite increases the electrical conductivity of the melt, allowing electrolysis to proceed efficiently.
(b) Reaction of sodium hydroxide with bauxite
The amphoteric alumina in bauxite dissolves in hot concentrated sodium hydroxide to form soluble sodium aluminate: \[Al_2O_{3(s)} + 2NaOH_{(aq)} \rightarrow 2NaAlO_{2(aq)} + H_2O_{(l)}\]
Cryolite dissolves the alumina and lowers its melting point from about 2050 °C to about 950 °C, greatly reducing the energy and cost of the extraction.
The molten cryolite increases the electrical conductivity of the melt, allowing electrolysis to proceed efficiently.
(b) Reaction of sodium hydroxide with bauxite
The amphoteric alumina in bauxite dissolves in hot concentrated sodium hydroxide to form soluble sodium aluminate: \[Al_2O_{3(s)} + 2NaOH_{(aq)} \rightarrow 2NaAlO_{2(aq)} + H_2O_{(l)}\]