2.3.2Organic Chemistry
Halogen Displacement Reactions — Edexcel AS Chemistry Revision Notes & Topical Questions
5 min read 4 parts Lesson 11 of 42
Section 1 of 4The Principle
Rule
- A more reactive halogen will displace a less reactive halogen from an aqueous solution of its halide.
Cause
- A stronger oxidising agent takes electrons from the halide ion, so the halide ion is oxidised to the halogen.
Order
- Oxidising power runs Cl₂ > Br₂ > I₂, so chlorine displaces both bromine and iodine.
No reaction
- A less reactive halogen cannot oxidise a more stable halide ion, so iodine added to chloride ions gives no change.
Redox
- Each displacement is a redox reaction in which the added halogen is reduced and the halide ion is oxidised.
Section 2 of 4Observations
Observations
Chlorine + bromide
- The solution turns from colourless to orange as bromine forms, showing that bromide ions were oxidised.
Chlorine + iodide
- The solution turns brown or dark red-brown as iodine forms, and a black solid may appear.
Bromine + iodide
- The solution turns brown as iodine forms, because bromine is the stronger oxidising agent.
Organic solvent
- Shaking with cyclohexane makes the halogens easier to identify: bromine appears orange and iodine appears violet.
Chlorine solution
- Chlorine in water is pale green, which can be difficult to see against other colours.
| Halide solution | Cl₂(aq) | Br₂(aq) | I₂(aq) |
|---|---|---|---|
| KCl | No reaction | No reaction | No reaction |
| KBr | Orange (Br₂) | No reaction | No reaction |
| KI | Brown (I₂) | Brown (I₂) | No reaction |
Section 3 of 4Writing Equations
1
Full equation for chlorine and potassium bromide: Cl₂(aq) + 2KBr(aq) → 2KCl(aq) + Br₂(aq).
2
Full equation for chlorine and potassium iodide: Cl₂(aq) + 2KI(aq) → 2KCl(aq) + I₂(aq).
3
Full equation for bromine and potassium iodide: Br₂(aq) + 2KI(aq) → 2KBr(aq) + I₂(aq).
4
Remove the spectator K⁺ ions to give the ionic equation, for example Cl₂(aq) + 2Br⁻(aq) → 2Cl⁻(aq) + Br₂(aq).
5
Split into half-equations: Cl₂ + 2e⁻ → 2Cl⁻ is reduction and 2Br⁻ → Br₂ + 2e⁻ is oxidation.
Section 4 of 4Exam Checklist & Tips
2.3.2 – Key Points for the Exam
- Displacement A reaction in which a more reactive element takes the place of a less reactive one in a compound.
- Oxidation Loss of electrons, or an increase in oxidation number.
- Reduction Gain of electrons, or a decrease in oxidation number.
- Spectator ion An ion that is present but unchanged and so is left out of an ionic equation.
- Halide ion A negative ion formed from a halogen atom, such as Cl⁻, Br⁻ or I⁻.
Common Mistakes
- Reversing the order and predicting that iodine displaces chlorine or bromine from their halide solutions.
- Leaving K⁺ in the ionic equation instead of cancelling it as a spectator ion.
- Writing Br⁻ as the product of the oxidation: the halide ions are oxidised to the halogen, not reduced.
- Quoting the colour of the halogen without saying whether it is in water or in an organic solvent.
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