3.1.5Additional lessons
Group 17 Elements with Halide Ions — IB Chemistry HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 52 of 172
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 5Displacement Reactions
Rule
- Rule A more reactive halogen oxidises the halide ion of a less reactive halogen and displaces the free halogen from solution.
Cl₂ and Br⁻
- Cl₂ and Br⁻ Cl₂(aq) + 2Br⁻(aq) → 2Cl⁻(aq) + Br₂(aq), and the solution turns from colourless to orange.
Cl₂ and I⁻
- Cl₂ and I⁻ Cl₂(aq) + 2I⁻(aq) → 2Cl⁻(aq) + I₂(aq), giving a brown solution with a black solid on standing.
Br₂ and I⁻
- Br₂ and I⁻ Br₂(aq) + 2I⁻(aq) → 2Br⁻(aq) + I₂(aq), so bromine also displaces iodine.
No reaction
- No reaction I₂ does not displace Cl⁻ or Br⁻, and Br₂ does not displace Cl⁻.
| Halogen | Colour in water | Colour in hexane |
|---|---|---|
| Chlorine | Pale green or colourless | Colourless to pale green |
| Bromine | Yellow to orange | Orange |
| Iodine | Brown | Violet |
Section 2 of 5Explaining the Trend
1
A halogen acts as an oxidising agent by gaining an electron to form a halide ion.
2
Smaller atoms with less shielding attract the incoming electron more strongly into the outer shell.
3
Oxidising ability therefore decreases down the group, in the order Cl₂ > Br₂ > I₂.
4
Reactivity of Group 17 is opposite to Group 1, because Group 1 atoms lose electrons while halogens gain them.
5
Half-equations show each step: Cl₂ + 2e⁻ → 2Cl⁻ is reduction and 2I⁻ → I₂ + 2e⁻ is oxidation.
Section 3 of 5Practical Tips
Practical Tips
Hexane layer
- Hexane layer Adding hexane and shaking helps to identify the halogen, because it dissolves in the upper organic layer with a clear colour.
Controls
- Controls Compare each mixture against a test tube of halide solution without added halogen to avoid misreading colours.
Concentration
- Concentration Dilute solutions give weaker colours, so use equal volumes and concentrations for a fair test.
Safety
- Safety Halogens are toxic and corrosive, so work in a fume cupboard or a well-ventilated room.
Section 4 of 5Worked Example
Question
- 50.0 cm³ of 0.100 mol dm⁻³ NaBr reacts with excess chlorine.
- Calculate the mass of bromine formed.
- n(Br⁻) = 0.0500 dm³ × 0.100 mol dm⁻³ = 0.00500 mol.
- From 2Br⁻ → Br₂, n(Br₂) = 0.00250 mol.
- M(Br₂) = 159.80 g mol⁻¹.
- m = 0.00250 × 159.80 = 0.3995 g.
Answer Mass of Br₂ ≈ 0.400 g
Section 5 of 5Exam 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 its compound.
- Oxidising agent A species that accepts electrons and is itself reduced.
- Halide ion A negative ion formed by a halogen atom gaining one electron.
- Redox A reaction involving both reduction and oxidation.
Common Mistakes
- Reading brown iodine solution as orange bromine without comparing the organic layer.
- Writing the product as Cl⁻ without balancing the charge in the ionic equation.
- Stating that reactivity increases down Group 17.
- Forgetting that spectator ions such as Na⁺ or K⁺ are omitted from ionic equations.
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