Explain how pure aluminium oxide is obtained from bauxite.

Explain why sodium hexafluoroaluminate, \({\text{N}}{{\text{a}}_{\text{3}}}{\text{Al}}{{\text{F}}_{\text{6}}}\), (cryolite) is added to the aluminium oxide before electrolysis takes place to produce aluminium.

State the half-equations for the reactions taking place at the positive and negative electrodes during the production of aluminium by electrolysis.

Positive electrode (anode):

Negative electrode (cathode):

Before the introduction of the electrolytic method by Hall and Héroult in the 1880s it was very difficult to obtain aluminium metal from its ores. Suggest one way in which it was achieved.

The worldwide production of aluminium by electrolysis makes a significant impact on global warming. Suggest two different ways in which the process increases the amount of carbon dioxide in the atmosphere.

(bauxite) is reacted with (concentrated) sodium hydroxide/NaOH (solution at high temperature);

forms sodium aluminate / \({\text{Al(OH}}{{\text{)}}_3} + {\text{O}}{{\text{H}}^ - } \to {\text{Al(OH)}}_4^ - \);

Accept both ionic and non-ionic equations and different, correct representations of the aluminate ion (Al(OH )₄^–, AlO₂^–).

solution is filtered / insoluble impurities removed (by filtration);

reaction reversed by cooling / diluting solution / adding water;

Accept passing CO₂ through the solution.

mixture seeded with alumina crystals;

pure hydroxide precipitated / \({\text{Al(OH)}}_4^ -  \to {\text{Al (OH}}{{\text{)}}_3} + {\text{O}}{{\text{H}}^ - }\);

Accept both ionic and non-ionic equations and different, correct representations of the aluminate ion (Al(OH)₄^–, AlO₂^–).

(pure) \({\text{Al(OH}}{{\text{)}}_3}\) heated / \({\text{2Al(OH}}{{\text{)}}_3} \to {\text{A}}{{\text{l}}_2}{{\text{O}}_3} + {\text{3}}{{\text{H}}_2}{\text{O}}\);

Award [1 max] for “Alumina is soluble in alkali, but impurities are not” / OWTTE.

Ignore state symbols.

melting point of the cryolite solution is much lower than the melting point of alumina/Al₂O₃ / it lowers the melting point (of the mixture);

Do not allow lowers melting point of aluminium.

Do not allow lowers required/operating temperature.

Accept improves conductivity of the electrolyte/aluminium oxide.

Positive electrode (anode):

\({\text{2}}{{\text{O}}^{2 - }} \to {{\text{O}}_2} + {\text{4}}{{\text{e}}^ - }/{{\text{O}}^{2 - }} \to \frac{1}{2}{{\text{O}}_2} + {\text{2}}{{\text{e}}^ - }/{\text{C}} + {\text{2}}{{\text{O}}^{2 - }} \to {\text{C}}{{\text{O}}_2} + {\text{4}}{{\text{e}}^ - }\);

Negative electrode (cathode):

\({\text{A}}{{\text{l}}^{3 + }} + {\text{3}}{{\text{e}}^ - } \to {\text{Al}}\);

Allow e instead of e^–.

Accept multiples of the correct equations, such as 2Al³⁺ + 6e^– \( \to \) 2Al .

Award [1 max] if correct equations but at wrong electrodes.

Ignore state symbols.

by reduction with a more reactive metal/metal above Al in electrochemical

series/ECS/reactivity series / OWTTE;

Accept equations for displacement reactions of Al₂O₃ with more reactive metals.

graphite/carbon electrodes converted/oxidized (into CO₂);

the fossil fuels used to provide energy/transport (produce CO₂);

This question was probably the worst answered question on the whole paper. In the first section many candidates confused the purification process with the electrolytic extraction and answers that scored any marks were rare. Many candidates knew the reasons for the addition of cryolite, but it was unusual to find both electrode equations correct and balanced. Hardly any had the lateral thinking skills to suggest displacement by a more reactive metal as a possible way of obtaining aluminium, but most students knew of at least one way in which aluminium production resulted in the emission of carbon dioxide.

This question was probably the worst answered question on the whole paper. In the first section many candidates confused the purification process with the electrolytic extraction and answers that scored any marks were rare. Many candidates knew the reasons for the addition of cryolite, but it was unusual to find both electrode equations correct and balanced. Hardly any had the lateral thinking skills to suggest displacement by a more reactive metal as a possible way of obtaining aluminium, but most students knew of at least one way in which aluminium production resulted in the emission of carbon dioxide.

This question was probably the worst answered question on the whole paper. In the first section many candidates confused the purification process with the electrolytic extraction and answers that scored any marks were rare. Many candidates knew the reasons for the addition of cryolite, but it was unusual to find both electrode equations correct and balanced. Hardly any had the lateral thinking skills to suggest displacement by a more reactive metal as a possible way of obtaining aluminium, but most students knew of at least one way in which aluminium production resulted in the emission of carbon dioxide.

This question was probably the worst answered question on the whole paper. In the first section many candidates confused the purification process with the electrolytic extraction and answers that scored any marks were rare. Many candidates knew the reasons for the addition of cryolite, but it was unusual to find both electrode equations correct and balanced. Hardly any had the lateral thinking skills to suggest displacement by a more reactive metal as a possible way of obtaining aluminium, but most students knew of at least one way in which aluminium production resulted in the emission of carbon dioxide.

This question was probably the worst answered question on the whole paper. In the first section many candidates confused the purification process with the electrolytic extraction and answers that scored any marks were rare. Many candidates knew the reasons for the addition of cryolite, but it was unusual to find both electrode equations correct and balanced. Hardly any had the lateral thinking skills to suggest displacement by a more reactive metal as a possible way of obtaining aluminium, but most students knew of at least one way in which aluminium production resulted in the emission of carbon dioxide.