Extraction and Uses of Metals
6 marks · NormalAluminium is an important metal that is mainly obtained from its ore using electrolysis. Aluminium alloys are used in many products because of their useful properties.
Explain how aluminium is extracted from aluminium oxide using electrolysis. Include details of the role of cryolite and what happens at the electrodes.
Define what an alloy is.
Explain why alloys such as aluminium alloys are harder than pure aluminium.
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(a) Electrolysis is used because aluminium is more reactive than carbon so it cannot be reduced by carbon. Aluminium oxide is dissolved in molten cryolite to lower the melting point and save energy. During electrolysis, aluminium ions gain electrons at the cathode to form aluminium metal (Al^{3+} + 3e^- \to Al), and oxide ions lose electrons at the anode to form oxygen (2O^{2-} \to O_2 + 4e^-).
(b) A mixture of a metal with other metals or elements, usually designed to have improved properties.
(c) In an alloy, atoms of different sizes disrupt the regular metal lattice, making it harder for layers of atoms to slide over each other. This makes the alloy harder than pure aluminium, where layers can slide easily.
Common mistakes: Omitting the function of cryolite; Not including both electrodes' reactions; Confusing pure metals with alloys; Saying alloys are harder just 'because they are mixed'
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Marking points
M1 (a) · 1 mark · States that aluminium is extracted by electrolysis from aluminium oxide because it is more reactive than carbon; or says carbon cannot reduce aluminium oxide.
M2 (a) · 1 mark · Mentions cryolite lowers the melting point of aluminium oxide or acts as a solvent.
A1 (a) · 1 mark · Correctly describes cathode and/or anode reactions (Al^{3+} + 3e^- \to Al at cathode; 2O^{2-} \to O_2 + 4e^- at anode), or describes that aluminium forms at cathode and oxygen at anode.
B1 (b) · 1 mark · Correct definition of alloy as mixture of metal with other metals or elements.
M3 (c) · 1 mark · Mentions that different sized atoms disturb the regular arrangement/lattice.
A2 (c) · 1 mark · Explains that this disruption prevents layers from sliding, making alloy harder than pure aluminium.
Worked solution
a) Aluminium is too reactive to be extracted from its oxide using carbon reduction, so electrolysis is used. Aluminium oxide (Al2O3) is dissolved in molten cryolite. This lowers the temperature needed for the process, saving energy. During electrolysis, aluminium ions move to the cathode, gain 3 electrons each, and form aluminium metal:
Oxide ions move to the anode, lose electrons, and form oxygen gas:
b) An alloy is a mixture of a metal with other metals or carbon.
c) In pure aluminium, atoms are all the same size and arranged in layers that can slide over each other easily. In an alloy, the different sized atoms disrupt this regular lattice, making it hard for the layers to slide. This makes the alloy harder than pure aluminium.