Explain why chlorine is able to displace bromide ions from a solution of potassium bromide, but iodine is not able to displace bromide ions from the same solution.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
1. Chlorine is more reactive than bromine.
2. A more reactive halogen displaces a less reactive halogen from its salt solution.
3. Chlorine atoms gain electrons more easily than bromine atoms because the outer shell is closer to the nucleus and has less shielding.
4. Iodine cannot displace bromide ions because iodine is less reactive than bromine and gains electrons less easily.
2. A more reactive halogen displaces a less reactive halogen from its salt solution.
3. Chlorine atoms gain electrons more easily than bromine atoms because the outer shell is closer to the nucleus and has less shielding.
4. Iodine cannot displace bromide ions because iodine is less reactive than bromine and gains electrons less easily.
Examiner tips
- Use the term ‘more reactive’ to show understanding of halogen reactivity order.
- Explain the displacement reaction mechanism briefly.
- Mention electron affinity and shielding to justify reactivity differences.
- Show the logical link between reactivity and displacement ability.
Common mistakes
- Confusing ‘reactive’ with ‘stable’ or ‘noble’ halogens.
- Failing to mention the displacement reaction step.
- Using vague terms like ‘stronger’ without explaining why.
Mark scheme (4 marks)
- Chlorine is more reactive than bromine (or iodine is less reactive than bromine)
- A more reactive halogen displaces a less reactive halogen from its salt solution (displacement reaction)
- Chlorine atoms gain electrons more easily than bromine atoms (because the outer shell is closer to the nucleus / less shielding)
- Iodine cannot displace bromide ions because iodine is less reactive than bromine / iodine gains electrons less easily than bromine
Key terms in this question
Related
- All Cambridge International IGCSE Chemistry (0620) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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