2.3.2Physical Chemistry
Chemical Properties of Group 7 — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 4 parts Lesson 61 of 102
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.
Part 1 of 4Oxidising Power
A halogen reacts by gaining one electron to form a halide ion: X₂ + 2e⁻ → 2X⁻.
Gaining electrons
- Gaining electrons Halogens are oxidising agents because they accept electrons and are reduced to halide ions.
Trend
- Trend Oxidising power decreases down the group, so chlorine is a stronger oxidising agent than bromine, and bromine stronger than iodine.
Explanation
- Explanation Smaller atoms have the incoming electron closer to the nucleus with less shielding, so the attraction for it is greater.
Displacement
- Displacement A more reactive halogen oxidises the halide ion of a less reactive halogen and displaces that halogen from solution.
Part 2 of 4Displacement Reactions
Displacement Reactions
Organic solvent test
- Organic solvent test Adding cyclohexane makes the halogen easier to identify: chlorine pale green, bromine orange-red, iodine violet.
Reading the results
- Reading the results A colour change shows that a new halogen has formed, which proves the added halogen is the stronger oxidising agent.
Spectator ions
- Spectator ions Potassium ions are not involved, so ionic equations show only the halogen and halide ions.
| Halogen added | Halide solution | Observation | Reaction |
|---|---|---|---|
| Chlorine water | Potassium bromide | Colourless to orange | Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂ |
| Chlorine water | Potassium iodide | Colourless to brown | Cl₂ + 2I⁻ → 2Cl⁻ + I₂ |
| Bromine water | Potassium iodide | Orange to brown | Br₂ + 2I⁻ → 2Br⁻ + I₂ |
| Bromine water | Potassium chloride | No reaction | Br₂ is a weaker oxidising agent than Cl₂ |
Part 3 of 4Writing the Equations
1
Identify the more reactive halogen, which is higher in the group and therefore the stronger oxidising agent.
2
Write the half-equation for reduction of the added halogen, for example Cl₂ + 2e⁻ → 2Cl⁻.
3
Write the half-equation for oxidation of the halide ion, for example 2Br⁻ → Br₂ + 2e⁻.
4
Add the half-equations and cancel electrons to give Cl₂(aq) + 2Br⁻(aq) → 2Cl⁻(aq) + Br₂(aq).
5
State the oxidation state changes: chlorine goes from 0 to −1 (reduced) and bromine from −1 to 0 (oxidised).
Part 4 of 4Exam Checklist & Tips
2.3.2 – Key Points for the Exam
- Displacement reaction A reaction in which a more reactive element displaces a less reactive one from its compound or ion.
- Oxidising agent A substance that gains electrons and is reduced, causing another species to be oxidised.
- Halide ion A negative ion formed by a halogen, such as Cl⁻, Br⁻ or I⁻.
- Ionic equation An equation that shows only the species that change during the reaction.
Common Mistakes
- Saying that bromine is more reactive than chlorine because it is larger; reactivity falls down the group.
- Writing a displacement equation with the halogens as single atoms rather than diatomic molecules.
- Forgetting to describe the colour change as observed in aqueous solution, not just the product name.
- Stating that fluorine gets reduced less easily than iodine, when it is the strongest oxidising agent.
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