Aluminium is a reactive metal, yet it is widely used for making drink cans and aircraft parts because it resists corrosion. Explain why aluminium resists corrosion even though it is a reactive metal, and state why more reactive metals such as sodium cannot be extracted by the same method used to extract aluminium.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Aluminium is the most abundant metal in the Earth's crust. It is found in its ore, bauxite, as aluminium oxide. Aluminium is extracted from aluminium oxide by electrolysis. Sodium is even more reactive than aluminium.
Model answer (4 marks)
Aluminium reacts with oxygen in the air to form a thin, continuous layer of aluminium oxide on its surface.
This oxide layer adheres strongly to the metal and is impermeable to oxygen and water, so further oxidation is prevented.
Sodium is more reactive than aluminium; it lies higher in the reactivity series.
Because of this high reactivity, sodium cannot be reduced by carbon in a smelting process and must be produced by electrolysis instead of the carbon reduction used for aluminium.
This oxide layer adheres strongly to the metal and is impermeable to oxygen and water, so further oxidation is prevented.
Sodium is more reactive than aluminium; it lies higher in the reactivity series.
Because of this high reactivity, sodium cannot be reduced by carbon in a smelting process and must be produced by electrolysis instead of the carbon reduction used for aluminium.
Examiner tips
- Show the formation of the oxide layer and its protective effect; use terms ‘passivation’ and ‘impermeable’. Explain that sodium’s higher reactivity means carbon cannot reduce it – mention the reactivity series. Use the word ‘electrolysis’ for sodium’s extraction method.
- common_mistakes
- :
- Confusing the protective layer with a general oxide film; not linking it to passivation. Saying sodium can be extracted by smelting or by the same method as aluminium. Using vague terms like ‘reactive’ without specifying the reactivity series.
Mark scheme (4 marks)
- Aluminium reacts with oxygen in the air to form a layer of aluminium oxide on its surface
- This oxide layer prevents further reaction / stops oxygen (and water) reaching the metal underneath
- Sodium is too reactive / higher in the reactivity series than aluminium, so it cannot be reduced by carbon
- Sodium (like aluminium) must be extracted by electrolysis instead
Key terms in this question
Related
- All WJEC GCSE Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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