2.5.2Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols
Halogen Displacement Reactions — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 5 parts Lesson 34 of 53 Unit quiz
Part 1 of 5The Principle
A more reactive halogen displaces a less reactive halogen from a solution of its halide.
The Principle
Oxidising power
- The more reactive halogen is the better oxidising agent, so it takes electrons from the halide ions of a weaker one.
Order
- The order of oxidising power is Cl₂ > Br₂ > I₂, so chlorine can displace both bromide and iodide.
No reaction
- A less reactive halogen does not displace a more reactive halide, so I₂ does nothing with Br⁻ or Cl⁻.
Redox
- The halogen is reduced and the halide ion is oxidised, so each displacement is a redox reaction.
Part 2 of 5Observations
Chlorine with bromide
- Cl₂(aq) + 2KBr(aq) → 2KCl(aq) + Br₂(aq), and the solution turns yellow-orange.
Chlorine with iodide
- Cl₂(aq) + 2KI(aq) → 2KCl(aq) + I₂(aq), and the solution turns brown.
Bromine with iodide
- Br₂(aq) + 2KI(aq) → 2KBr(aq) + I₂(aq), and the solution again turns brown.
Organic solvent
- Shaking with cyclohexane gives an orange layer for bromine and a purple layer for iodine, which are easier to identify.
Ionic equation
- Cl₂(aq) + 2Br⁻(aq) → 2Cl⁻(aq) + Br₂(aq) removes the spectator K⁺ ions.
| Halogen added | Halide solution | Observation |
|---|---|---|
| Chlorine | Bromide | Yellow-orange (Br₂) |
| Chlorine | Iodide | Brown (I₂) |
| Bromine | Iodide | Brown (I₂) |
| Bromine | Chloride | No reaction |
Part 3 of 5Using Oxidation Numbers
1
Write the oxidation numbers: chlorine in Cl₂ is 0 and in Cl⁻ is -1, so chlorine has been reduced.
2
Bromine in Br⁻ is -1 and in Br₂ is 0, so the bromide ion has been oxidised.
3
Chlorine is therefore the oxidising agent and bromide the reducing agent.
4
The potassium ion stays at +1, so it is a spectator and is left out of the ionic equation.
5
Predict reactions by asking whether the added halogen is above the halide's halogen in the group.
Part 4 of 5Worked Example
Question
- State whether a reaction occurs when bromine water is added to potassium chloride solution, and whether it occurs with potassium iodide solution.
- Write the ionic equation if there is a reaction.
- Bromine is below chlorine in Group 7, so it is a weaker oxidising agent than chlorine.
- It cannot oxidise chloride ions, so there is no reaction with KCl.
- Bromine is above iodine, so it can oxidise iodide ions to iodine.
- The solution turns from orange to brown as iodine forms.
Answer
- No reaction with chloride;
- with iodide: Br₂(aq) + 2I⁻(aq) → 2Br⁻(aq) + I₂(aq).
Part 5 of 5Exam Checklist & Tips
2.5.2 – Key Points for the Exam
- Displacement reaction A reaction in which a more reactive element takes the place of a less reactive one in a compound.
- Halide ion A negative ion formed by a halogen, such as Cl⁻, Br⁻ or I⁻.
- Cyclohexane A non-polar organic solvent that dissolves halogens and gives clearer colours.
- Spectator ion An ion that does not change during a reaction.
Common Mistakes
- Forgetting the colour of the halogen in organic solvent, such as writing brown for iodine instead of purple.
- Writing the ionic equation with K⁺ ions included.
- Predicting that iodine displaces bromide because it is larger.
- Describing chlorine as oxidised when it is actually reduced.
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