Aluminium…

You should read the (very slightly eccentric) post from a few years ago – Foiled Again. It might also be worthwhile checking the other posts in the series – Elements.

Aluminium is the most abundant metal element in the Earth’s crust. Despite this, no known plant or animal uses aluminium compounds to perform any biological function. Whereas iron is in blood haemoglobin; calcium phosphate is in bones; sodium and potassium are essential for nerve activity; aluminium has no apparent use. Fortunately, given how plentiful it is on Earth, it doesn’t seem to be particularly toxic either.

Since aluminium is quite reactive, it is never found naturally in its element form – it is always combined with other elements as a compound. Usually it is combined with oxygen to form aluminium oxide (Al₂O₃) or silicon and oxygen to make aluminium silicate (Al₂SiO₅). It cannot be extracted from its ore (bauxite) in a blast furnace because aluminium is much higher in affinity for oxygen than carbon (or carbon monoxide). Most aluminium is produced by electrolysis.

The bauxite ore is first washed with hot sodium hydroxide solution, which dissolves out the aluminium (forming sodium aluminate) but leaves other impurities (usually iron oxide) alone. Aluminium oxide (alumina) crystallizes out of the solution and this is used for the next stage. A salt called cryolite is melted in an electrolysis cell (950 °C) and the alumina dissolves into this liquid, splitting into positively charged aluminium ions and negatively charged oxide ions. This is the same process that happens when an ionic compound dissolves in water.

A high current is applied to the cell via the two carbon electrodes. The positively charged aluminium ions are attracted to the negatively charged cathode where it gains electrons, becoming liquid aluminium that can be siphoned out. The negatively charged oxygen ions travel to the positively charged carbon anode and lose their extra electrons. At this high temperature some of the oxygen then reacts with the carbon anode forming carbon dioxide. This eventually wears away the anode which will need to be replaced every few weeks.

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Questions…

1. Aluminium is the most abundant metal in the Earth’s crust yet today’s market price for steel is $50 per tonne but the price for aluminium is $339 per tonne. What accounts for this difference?
2. How would you separate a mixture between unreacted iron oxide and hot sodium aluminate solution?
3. Suggest an ionic compound that dissolves in water. Into what ions does it split when in solution?
4. Why does salt solution conduct electricity but pure water does not?
5. What gas was probably being released by the coke can reacting with the hydrochloric acid in the second video?
6. Why is it unusual for a metal to be amphoteric (react with acids and alkalis)?