Extraction and Uses of Metals

Question 1

Explain how the methods of extraction of the metals in this section are related to their positions in the reactivity series

Answer 1

For metals up to zinc in the reactivity series, the cheapest method of reducing the ore is to heat it with carbon or carbon monoxide. Carbon is cheap and can also be used as the source of heat.
Metals above zinc are usually produced by electrolysis. The metal ions are given electrons directly by the cathode. The large amounts of electricity involved make this an expensive process.
Titanium is extracted by heating the compound with a more reactive metal. This is bound to be expensive as the more reactive metal itself will have to be extracted by an expensive process first

Question 2

Describe and explain the extraction of aluminium from purified aluminium oxide by electrolysis

Answer 2

Bauxite is first treated to produce pure aluminium oxide. Aluminium is fairly reactive so has to be extracted suing electrolysis. Aluminium have a hight melting point and isn’t practical to electrolyse molten aluminium oxide so instead the aluminium oxide is dissolved in molten cryolite. The electrolyte is a solution of aluminium oxide in olden cryolite at 1000 degrees. The cell operate at about 5-6 volts but with currents up to 100,000amps. The heat generated by the huge current keeps the electrolyte molten.
Aluminium ions are attracted to the cathode and are reduce to aluminium by gaining electrons. The oxide ions are attracted to the anode and are oxidised by losing electrons.
Because of the high temperatures, the carbon anodes burn in the oxygen to form carbon dioxide. The anodes have to be replaced regularly.

Question 3

Write ionic half-equations for the reactions at the electrodes in aluminium extraction

Answer 3

Al3+ + 3e- —-> Al

2O2- —-> O2 + 2e-

Question 4

Describe and explain the main reactions involved in the extraction of iron from haematite

Answer 4

Hot waste gases at the top of the furnace are piped away and used to heat the air blast at the bottom. Coke burns in the hot air blast to form carbon dioxide. This is a strongly exothermic reaction. At high temperatures, the carbon dioxide is reduced by more carbon to form carbon monoxide, which is the main reducing agent in the furnace. The iron melts and flows to the bottom where it can be tapped off. In hotter parts, some of the iron oxide is reduced by carbon itself.
Limestone is added to the furnace to remove the impurities in the ore which would otherwise clog the furnace with solid material. The furnace is hot enough tot the limestone (calcium carbonate) to undergo thermal decomposition. It splits up into calcium oxide and carbon dioxide. This is an endothermic reaction and it is important not to add too much limestone to avoid cooling the furnace. Calcium oxide reacts with silicon dioxide to form calcium silicate which melts and trickles to the bottom of the furnace as molten slag.

Question 5

Explain the uses of aluminium and iron

Answer 5

Iron: manhole covers, guttering, drainpipes and cylinder blocks in car engines
Steel: wire, nails, car bodies, bridges