2.5.4Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols
Other Reactions of the Halogens — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 4 parts Lesson 36 of 53 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 4Halides with Concentrated Sulfuric Acid
These reactions give toxic gases, so they are done in a fume cupboard, and they show that reducing power increases down the group.
Chloride
- Chloride NaCl(s) + H₂SO₄(l) → NaHSO₄(s) + HCl(g), giving steamy fumes with no redox change.
Bromide
- Bromide HBr forms first, but it reduces sulfuric acid: 2HBr + H₂SO₄ → Br₂ + SO₂ + 2H₂O, giving orange-brown fumes.
Iodide
- Iodide HI reduces sulfuric acid further: 8HI + H₂SO₄ → 4I₂ + H₂S + 4H₂O, giving purple fumes, black solid and a bad-egg smell.
Sulfur products
- Sulfur products Sulfur is reduced from +6 in H₂SO₄ to +4 in SO₂, 0 in S or -2 in H₂S.
Trend
- Trend Iodide is the strongest reducing agent, then bromide, then chloride, because larger ions lose electrons more easily.
Why bromide differs
- Why bromide differs Bromide ions are stronger reducing agents than chloride ions,
- so they reduce concentrated sulfuric acid while chloride ions cannot.
| Halide | Main products | Observation |
|---|---|---|
| Chloride | HCl | Steamy white fumes |
| Bromide | HBr, Br₂, SO₂ | Orange-brown fumes, choking gas |
| Iodide | I₂, H₂S, S | Purple fumes, black solid, bad-egg smell |
Section 2 of 4Silver Nitrate Test for Halides
1
Acidify the sample with dilute nitric acid to remove carbonates and other ions that could precipitate.
2
Add silver nitrate solution: Ag⁺(aq) + X⁻(aq) → AgX(s).
3
Record the colour: AgCl is white, AgBr is cream and AgI is pale yellow.
4
Add dilute ammonia solution, in which only silver chloride dissolves.
5
Add concentrated ammonia, which also dissolves silver bromide, while silver iodide stays insoluble.
6
Silver halides darken in light as silver metal forms, so the observations should be made promptly.
Section 3 of 4Hydrogen Halides
Hydrogen Halides
With ammonia
- With ammonia NH₃(g) + HCl(g) → NH₄Cl(s) forms white smoke.
In water
- In water HCl(g) → H⁺(aq) + Cl⁻(aq) gives a strongly acidic solution.
Fluoride
- Fluoride Silver fluoride is soluble, so the silver nitrate test does not work for fluoride ions.
Predicting astatine
- Predicting astatine HAt would behave like HI, with At⁻ the least stable and most strongly reducing halide.
Reading the test
- Reading the test A white precipitate that dissolves in dilute ammonia indicates chloride, but one needing concentrated ammonia indicates bromide.
Section 4 of 4Exam Checklist & Tips
2.5.4 – Key Points for the Exam
- Reducing agent A species that donates electrons and is oxidised.
- Halide test Acidified silver nitrate followed by ammonia to identify chloride, bromide and iodide.
- Fume cupboard A ventilated enclosure that removes toxic gases.
- Hydrogen halide A compound HX formed from hydrogen and a halogen.
Common Mistakes
- Saying chloride ions reduce concentrated sulfuric acid; they do not, so no SO₂ forms.
- Forgetting to acidify with nitric acid before adding silver nitrate.
- Using hydrochloric acid to acidify, which adds chloride ions and gives a false positive.
- Confusing the colours of AgBr (cream) and AgI (pale yellow).
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