When chlorine water is added to a solution of potassium bromide, the solution turns orange-brown. When chlorine water is added to a solution of potassium iodide, the solution turns brown. Explain these observations in terms of displacement reactions and the relative reactivity of the halogens involved.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Chlorine, bromine and iodine are halogens in Group 7 of the periodic table. Their reactivity decreases down the group. Displacement reactions between halogens and halide solutions can be used to establish the order of reactivity of the halogens.
Model answer (5 marks)
Chlorine water reacts with potassium bromide:
1. Cl₂ + 2 KBr → 2 KCl + Br₂
2. Chlorine is more reactive than bromine, so it displaces bromine from the solution.
3. The orange‑brown colour observed is the Br₂ produced.
Chlorine water reacts with potassium iodide:
4. Cl₂ + 2 KI → 2 KCl + I₂
5. Chlorine is more reactive than iodine, so it displaces iodine, giving the brown/dark‑brown I₂.
1. Cl₂ + 2 KBr → 2 KCl + Br₂
2. Chlorine is more reactive than bromine, so it displaces bromine from the solution.
3. The orange‑brown colour observed is the Br₂ produced.
Chlorine water reacts with potassium iodide:
4. Cl₂ + 2 KI → 2 KCl + I₂
5. Chlorine is more reactive than iodine, so it displaces iodine, giving the brown/dark‑brown I₂.
Examiner tips
- Use the correct displacement equations, showing the halogen being displaced. Mention the relative reactivity (Cl > Br > I) to justify the displacement. Explain the colour change as the product halogen.
- common_mistakes
- :
- Writing the wrong halogen as the product (e.g., saying Br₂ is produced in the KI test). Forgetting to state that chlorine is more reactive than the halogen it displaces. Not linking the colour change to the produced halogen.
Mark scheme (5 marks)
- Chlorine displaces bromine from potassium bromide solution
- Chlorine is more reactive than bromine, so it can displace bromine
- The orange-brown colour is due to bromine being produced
- Chlorine displaces iodine from potassium iodide solution
- Chlorine is more reactive than iodine, so it can displace iodine, which is the brown/dark brown product formed
Key terms in this question
displacement · reactivity · halogen
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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