Extraction and Uses of Metals
6 marks · NormalMetals such as aluminium and sodium are found above carbon in the reactivity series. Aluminium is a widely used metal in the modern world.
Explain why aluminium cannot be extracted from its ore by heating it with carbon.
State and explain one property of aluminium that makes it suitable for use in overhead electricity cables.
Copper is also used in electricity cables. Compare one property of copper with aluminium that affects its use in cables.
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(a) Because aluminium is more reactive than carbon, so carbon cannot reduce aluminium oxide to aluminium.
(b) Aluminium has a low density, making cables light enough to be supported on pylons; or it is a good conductor of electricity, so it can transmit electrical power efficiently; or it resists corrosion because of its oxide layer.
(c) Copper is a better conductor than aluminium, so copper cables can carry more current for a given size, but aluminium is lighter, so is preferred for overhead cables.
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Marking points
M1 (a) · 1 mark · States or explains that aluminium is more reactive than carbon.
A1 (a) · 1 mark · Links reactivity to the inability of carbon to reduce aluminium oxide (carbon cannot displace aluminium from its oxide).
B1 (b) · 1 mark · States a correct property of aluminium related to its use in cables (low density, good conductor, corrosion-resistant).
B2 (b) · 1 mark · Explains why that property is useful in cables.
B3 (c) · 1 mark · States a correct property difference between copper and aluminium (e.g., conductivity or density).
B4 (c) · 1 mark · Explains how that difference affects their use (e.g., copper is a better conductor, but aluminium is lighter for overhead cables).
Worked solution
a) Aluminium is above carbon in the reactivity series. This means aluminium is more reactive than carbon, so carbon cannot take the oxygen away from aluminium oxide by reduction. Therefore, aluminium cannot be extracted from its ore by heating it with carbon.
b) Aluminium has a low density, which makes it light. This is important for overhead electricity cables because it reduces the weight supported by pylons. Another property is aluminium is a good conductor of electricity, which is needed for efficient power transmission, and it resists corrosion, so the cables last longer outdoors.
c) Copper is a better conductor of electricity than aluminium, so copper cables can carry more current for the same thickness. However, aluminium is much lighter, which is why it is often used for overhead power cables, since less weight is suspended between pylons.