# Group 7 — Halogens (A-Level)
At A-Level, halogen chemistry extends beyond GCSE displacement reactions to include halide ion tests, disproportionation reactions, the chemistry of chlorine with water and alkali, and the reducing power of halide ions with concentrated sulfuric acid.
1. Halide Ion Tests
Test for Cl⁻, Br⁻, and I⁻ using silver nitrate solution:
- Add dilute nitric acid (to prevent false positives from carbonates/sulfates)
- Add silver nitrate solution (AgNO₃)
| Halide | Precipitate | Colour | Solubility in NH₃ |
|---|---|---|---|
| Cl⁻ | AgCl | White | Soluble in dilute NH₃ |
| Br⁻ | AgBr | Cream | Soluble in concentrated NH₃ |
| I⁻ | AgI | Yellow | Insoluble in NH₃ |
The ammonia test helps distinguish between the precipitates when colour is ambiguous.
2. Reactions of Halides with Concentrated Sulfuric Acid
This reveals the reducing power of halide ions. Reducing power increases down the group: Cl⁻ < Br⁻ < I⁻.
Chloride + conc. H₂SO₄
- White steamy fumes of HCl
- Simple acid-base reaction (no redox — Cl⁻ is not a strong enough reducing agent)
Bromide + conc. H₂SO₄
Then Br⁻ reduces H₂SO₄:
- Orange/brown fumes (Br₂) and choking gas (SO₂)
Iodide + conc. H₂SO₄
I⁻ is a strong reducing agent and reduces H₂SO₄ further:
Products: purple vapour (I₂), yellow solid (S), rotten eggs smell (H₂S)
3. Chlorine with Water and Alkali
Chlorine in Water
HOCl (chloric(I) acid / hypochlorous acid) is a powerful oxidising agent — it kills bacteria. This is why chlorine is used in water treatment.
Disproportionation
This reaction is a disproportionation — chlorine is simultaneously oxidised and reduced:
- Cl₂ (0) → Cl⁻ in HCl (−1) — reduction
- Cl₂ (0) → Cl in HOCl (+1) — oxidation
Chlorine with Cold Dilute NaOH
NaOCl = sodium hypochlorite (bleach). Another disproportionation.
Chlorine with Hot Concentrated NaOH
NaClO₃ = sodium chlorate(V). Cl goes from 0 to −1 and +5.
4. Uses of Chlorine and Chlorine Compounds
- Water treatment — kills bacteria (Cl₂ or NaOCl)
- Bleach (NaOCl) — disinfectant and whitening
- PVC production — chloroethene polymerisation
- Chlorine in swimming pools
Ethical Considerations
- Chlorine in water prevents deadly diseases (cholera, typhoid)
- But chlorine can react with organic matter to form chlorinated hydrocarbons (potential carcinogens)
- Benefits generally outweigh risks
Worked Example: Halide Test
Question: A solution is tested with acidified silver nitrate. A cream precipitate forms that dissolves in concentrated ammonia but not dilute ammonia. Identify the halide.
Cream precipitate = AgBr → the halide is bromide (Br⁻). AgBr dissolves in concentrated NH₃ but not dilute.
Worked Example: Disproportionation
Question: Identify the disproportionation in:
Chlorine starts at oxidation state 0. In NaCl, Cl is −1 (reduced). In NaOCl, Cl is +1 (oxidised). The same element is both oxidised and reduced = disproportionation.
6. Practice Questions
- Describe how to test for chloride, bromide, and iodide ions using silver nitrate.
- Write equations for the reaction of NaBr with concentrated H₂SO₄.
- Explain why NaI with conc. H₂SO₄ produces more reduction products than NaCl.
- Define disproportionation and give an example from chlorine chemistry.
- Explain why chlorine is added to drinking water. State one concern.
Want to check your answers and get step-by-step solutions?
7. Exam Tips
- Silver nitrate test: remember the ammonia solubility for distinguishing halides
- Halide reducing power increases down the group: Cl⁻ < Br⁻ < I⁻
- Disproportionation = same element both oxidised AND reduced
- Know the difference between chlorine with cold vs hot NaOH
Summary
- Halide test: AgNO₃ → white (Cl⁻), cream (Br⁻), yellow (I⁻)
- Reducing power increases down Group 7: I⁻ > Br⁻ > Cl⁻
- Cl⁻ + conc. H₂SO₄ → HCl only; Br⁻ → Br₂ + SO₂; I⁻ → I₂ + SO₂ + S + H₂S
- Disproportionation: Cl₂ + water/NaOH
- Chlorine in water treatment: kills bacteria via HOCl