Extraction and Uses of Metals
6 marks · NormalAluminium and iron are both important industrial metals but are extracted by different methods. The table below gives some data about their extraction processes:
| Metal | Extraction Method | Relative Energy Needed (arbitrary units per tonne metal) |
|---|---|---|
| Aluminium | Electrolysis of Al2O3 (in cryolite) | 35 |
| Iron | Reduction of Fe2O3 in a blast furnace (with carbon) | 6 |
Use the data to calculate how many times more energy is needed to extract 1 tonne of aluminium by electrolysis than to extract 1 tonne of iron by reduction with carbon.
Explain, in terms of the reactivity series, why aluminium is extracted by electrolysis and not by reduction with carbon like iron.
Give one reason, based on its physical properties, why aluminium is used for electricity cables instead of iron.
Aluminium is often alloyed for use in cables. Explain why alloys are used instead of pure metals for this purpose.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) 5.83 times · must show Correct calculation: 35/6 = 5.83 (accept 6 to 2 s.f. or a rounded value)
(b) — · must show Aluminium is more reactive than carbon, Cannot be reduced by carbon, Extraction by electrolysis required for metals above carbon
(c) Low density OR good electrical conductivity · must show Physical property that makes aluminium suitable (low density, good conductivity, resists corrosion)
(d) — · must show Alloys are harder/stronger than pure metals, Atoms in alloy have different sizes, distort lattice, prevent layer sliding
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Selects correct operation: divides energy needed for aluminium by energy needed for iron (35/6).
A1 (a) · 1 mark · Correct numerical answer to at least 2 decimal places: 5.83 (accept 6 if rounded appropriately). (follow-through)
B1 (b) · 1 mark · States that aluminium is more reactive than carbon (or above carbon in the reactivity series).
B2 (b) · 1 mark · Explains that because aluminium is more reactive, carbon cannot reduce its oxide: must use electrolysis.
B3 (c) · 1 mark · States a relevant property (e.g., 'aluminium is a good conductor of electricity' or 'aluminium is low density').
B4 (d) · 1 mark · Explains alloys are harder/stronger than pure metals due to distorted lattice preventing layers sliding.
Worked solution
a) To find how many times more energy is needed to extract aluminium than iron:
So, electrolysis of aluminium uses about 5.83 times more energy per tonne than extracting iron by carbon reduction.
b) Aluminium is above carbon in the reactivity series, so it is more reactive than carbon. This means carbon cannot remove the oxygen from aluminium oxide by reduction. Therefore, aluminium must be extracted by electrolysis instead of by reduction with carbon (the method used for iron).
c) Aluminium is used for electricity cables because it is a good conductor of electricity (or, because it has a low density, making cables lighter).
d) Alloys are used instead of pure aluminium for cables because alloys are harder and stronger than pure metals. The differently sized atoms in alloys distort the layers and prevent them from sliding easily, improving strength for structural use.