Group VII Displacement Reactions
Section: 8. The Periodic Table | Syllabus: Cambridge IGCSE Chemistry (0620)
Halogen Displacement Reactions
A more reactive halogen will displace a less reactive halogen from a solution of its halide. Since reactivity decreases down the group (Cl > Br > I), these reactions provide direct evidence for the reactivity order.
more reactive halogen + less reactive halide → less reactive halogen + more reactive halide
Worked Examples
| Halogen added | Halide solution | Reaction? | Observation |
|---|---|---|---|
| Chlorine | potassium bromide | yes | colourless → orange (Br₂ forms) |
| Chlorine | potassium iodide | yes | colourless → brown (I₂ forms) |
| Bromine | potassium iodide | yes | colourless → brown (I₂ forms) |
| Bromine | potassium chloride | no | no colour change - bromine is less reactive than chlorine |
| Iodine | potassium bromide/chloride | no | no colour change - iodine is the least reactive of the three |
Equations
Cl₂(aq) + 2KI(aq) → 2KCl(aq) + I₂(aq) ionic: Cl₂(aq) + 2I⁻(aq) → 2Cl⁻(aq) + I₂(aq)
Why It Happens
This is a redox reaction: the added halogen is reduced (gains electrons), displacing the halide ion, which is oxidised (loses electrons) to become the free halogen.
- Cl₂ + 2e⁻ → 2Cl⁻ (chlorine reduced)
- 2I⁻ → I₂ + 2e⁻ (iodide oxidised)
The more reactive halogen is the stronger oxidising agent, so it "wins" the electrons.
Worked Example: Confirming the Halogen Produced
- Question: chlorine water is added to potassium iodide solution and the mixture turns brown. Describe how you could confirm iodine has been produced. [2]
- Answer: shake the mixture with an organic solvent such as cyclohexane and allow the layers to separate [1]; a purple layer confirms iodine is present (bromine would instead give an orange layer) [1]
Only a more reactive halogen displaces a less reactive one; the colour of the organic layer identifies which halogen was produced
Common Mistakes
- ❌ Assuming a displacement reaction always occurs when two halogens mix
- ✓ check the reactivity order first (Cl > Br > I) - a less reactive halogen added to a more reactive halide gives NO reaction
- ❌ Forgetting which species is oxidised and which is reduced
- ✓ the halogen added is reduced (gains electrons); the halide ion already in solution is oxidised (loses electrons) to form the free halogen
Interactive revision notes, videos and practice questions load below.