3.1.5Additional lessons
Group 17 Elements with Halide Ions — IB Chemistry SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 41 of 98
Part 1 of 6The Halogens
Group 17 elements are diatomic non-metals that each need one more electron to fill their valence shell.
Fluorine
- A pale yellow gas that is the most reactive halogen and is not used in school experiments.
Chlorine
- A yellow-green gas that dissolves in water to give a very pale yellow-green solution known as chlorine water.
Bromine
- A red-brown liquid that gives an orange to brown solution in water.
Iodine
- A dark grey solid that gives a brown solution in water and a violet solution in a non-polar solvent such as hexane.
Part 2 of 6Displacement Reactions
1
A more reactive halogen displaces a less reactive halogen from a solution of its halide.
2
Chlorine added to potassium bromide solution turns it orange as bromine forms: Cl₂(aq) + 2Br⁻(aq) → 2Cl⁻(aq) + Br₂(aq).
3
Chlorine added to potassium iodide solution turns it brown as iodine forms: Cl₂(aq) + 2I⁻(aq) → 2Cl⁻(aq) + I₂(aq).
4
Bromine added to potassium iodide solution turns it brown: Br₂(aq) + 2I⁻(aq) → 2Br⁻(aq) + I₂(aq).
5
Bromine added to potassium chloride shows no reaction, because bromine is less reactive than chlorine.
| Halogen added | Cl⁻(aq) | Br⁻(aq) | I⁻(aq) |
|---|---|---|---|
| Cl₂(aq) | no reaction | orange (Br₂) | brown (I₂) |
| Br₂(aq) | no reaction | no reaction | brown (I₂) |
| I₂(aq) | no reaction | no reaction | no reaction |
Part 3 of 6Explaining the Trend
Explaining the Trend
Oxidising agent
- In each displacement the halogen gains electrons, so the more reactive halogen is the stronger oxidising agent.
Down the group
- Atoms get larger and the incoming electron is further from the nucleus and more shielded.
Electron affinity
- The attraction for an extra electron decreases, so reactivity decreases from chlorine to iodine.
Redox view
- The halide ion is oxidised to the halogen as it loses its electron.
Part 4 of 6Practical Points
Colours
- Using hexane helps to identify halogensbromine is orange-red and iodine is violet in this layer.
Reading results
- A colour change shows that a new halogen has formed, so a displacement reaction occurred.
Safety
- Chlorine and bromine are toxic, so work in a fume cupboard and use dilute solutions.
Writing equations
- Spectator ions such as K⁺ are left out of ionic equations.
Part 5 of 6Worked Example
Question Predict whether a reaction occurs when bromine water is added to sodium chloride solution, and justify your answer.
- Compare reactivitychlorine is above bromine in Group 17.
- For displacement the added halogen must be more reactive than the halogen in the halide.
- Bromine is less reactive than chlorine, so it cannot oxidise Cl⁻ to Cl₂.
Answer No reaction occurs because bromine is less reactive than chlorine.
Part 6 of 6Exam Checklist & Tips
3.1.5 – Key Points for the Exam
- Displacement reaction A reaction in which a more reactive element replaces a less reactive one from a compound.
- Halide ion A negative ion of Group 17: F⁻, Cl⁻, Br⁻ or I⁻.
- Oxidising agent A species that gains electrons and oxidises another species.
- Spectator ion An ion that does not take part in the reaction.
Common Mistakes
- Writing the halogens as monatomic, for example Cl instead of Cl₂.
- Saying iodine is the most reactive halogen.
- Confusing the colour of iodine in water (brown) with that in hexane (violet).
- Predicting that a less reactive halogen displaces a more reactive one.
Last updated:
\n