Iron can be protected from rusting by a process called sacrificial protection, in which a more reactive metal is attached to the iron. Explain why zinc is suitable for use as a sacrificial metal to protect iron, and describe what happens to the zinc over time.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Iron structures such as ships' hulls and underground pipelines are vulnerable to corrosion when exposed to water and oxygen.
Model answer (4 marks)
Zinc is higher in the reactivity series than iron, so it is more easily oxidised.
When zinc is attached to iron it is oxidised first, forming Zn²⁺ and releasing electrons.
The electrons flow to the iron, reducing the iron surface and preventing its oxidation.
Thus zinc corrodes preferentially, acting as a physical barrier that keeps water and oxygen away from the iron.
Over time the zinc is gradually consumed by corrosion and must be replaced to maintain protection.
When zinc is attached to iron it is oxidised first, forming Zn²⁺ and releasing electrons.
The electrons flow to the iron, reducing the iron surface and preventing its oxidation.
Thus zinc corrodes preferentially, acting as a physical barrier that keeps water and oxygen away from the iron.
Over time the zinc is gradually consumed by corrosion and must be replaced to maintain protection.
Examiner tips
- State zinc’s higher reactivity and its preferential oxidation
- Explain electron flow and reduction of iron
- Mention the physical barrier effect
- Note that zinc is consumed and must be replaced
Common mistakes
- Confusing the direction of electron flow
- Failing to mention that zinc is consumed
- Using vague terms like "protects" without explaining the mechanism
Mark scheme (4 marks)
- Zinc is more reactive than iron (in the reactivity series)
- Zinc is oxidised / corrodes preferentially instead of the iron
- Zinc creates a physical barrier, preventing water and oxygen from reaching the iron
- The zinc is gradually used up / corroded over time and must be replaced
Key terms in this question
Related
- All Pearson Edexcel International GCSE Chemistry (4CH1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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