Metals above carbon in the reactivity series
3 marks · NormalAluminium is extracted by electrolysis, while iron is extracted by reduction with carbon. The extraction of 1 tonne of aluminium requires 14,000 kWh of electricity at a cost of £0.12 per kWh. The extraction of 1 tonne of iron requires 500 kWh of electricity at the same cost per kWh. Calculate the difference in electricity cost between extracting 1 tonne of aluminium and 1 tonne of iron, and explain one reason why aluminium is extracted by electrolysis rather than by carbon reduction.
Calculate the electricity cost to extract 1 tonne of aluminium and 1 tonne of iron.
Explain one reason why aluminium is extracted using electrolysis rather than by reduction with carbon.
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(a) Aluminium: £1,680; Iron: £60; Difference: £1,620
(b) Aluminium is more reactive than carbon, so it cannot be extracted by carbon reduction.
Common mistakes: Using wrong energy usage values; Forgetting to multiply cost per kWh; Giving a factually wrong reason about reactivity
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Marking points
M1 (a) · 1 mark · Correct method to calculate cost (energy x price per kWh) for each metal.
A1 (a) · 1 mark · Correct final electricity cost for both (Aluminium: £1,680; Iron: £60), and correct difference (£1,620). (follow-through)
B1 (b) · 1 mark · States that aluminium is more reactive than carbon (or that carbon cannot reduce aluminium oxide, so electrolysis is used).
Worked solution
a) Electricity cost:
b) Aluminium is more reactive than carbon, so carbon cannot reduce aluminium oxide. Therefore, aluminium must be extracted by electrolysis.