Overview
This lecture covers the key facts about Group 7 (the halogens) for the AQA Chemistry specification, including their properties, reactions, and important qualitative tests.
Halogen Properties and Trends
- Halogens (Group 7) are non-metals found on the right side of the periodic table.
- Fluorine is a pale yellow gas, chlorine is a pale green gas, bromine is a browny-orange liquid, and iodine is a grey solid.
- Boiling points and molecular/atomic mass increase down the group due to stronger van der Waals forces.
- Electronegativity decreases down the group due to increasing atom size and electron shielding.
Reactivity and Displacement Reactions
- More reactive halogens displace less reactive halide ions from solutions.
- Reactivity decreases down the group (F₂ > Cl₂ > Br₂ > I₂).
- Chlorine displaces bromide (orange solution) and iodide (brown solution); bromine displaces iodide (brown solution).
- No reaction occurs if the halogen is less reactive than the halide ion.
Bleach and Disproportionation
- Bleach (sodium chlorate(I), NaClO) is made by reacting chlorine with cold sodium hydroxide.
- Disproportionation: chlorine is both oxidized and reduced in the reaction.
- Uses of bleach: water treatment, bleaching paper/fabric, cleaning.
Chlorination of Water
- Chlorine reacts with water, forming HCl and chloric(I) acid (HClO), a disproportionation reaction.
- Chloric(I) acid kills bacteria in drinking water and swimming pools.
- Sunlight breaks down chloric acid, so swimming pool chemicals must be replaced regularly.
- Advantages: kills pathogens, long-lasting, prevents bad taste/odours.
- Disadvantages: toxic, can irritate respiratory system, possible formation of carcinogenic byproducts.
Halide Ions as Reducing Agents
- Halide ions (Cl⁻, Br⁻, I⁻) become better reducing agents down the group due to increased ionic radius and shielding.
- Iodide is a stronger reducing agent than bromide or chloride.
Reaction with Concentrated Sulfuric Acid
- Chlorides: only give HCl (white fumes), not a redox reaction.
- Bromides: produce HBr, then reduce H₂SO₄ to SO₂ (orange Br₂ vapour, choking gas).
- Iodides: produce HI, then reduce H₂SO₄ to SO₂, S (yellow solid), and H₂S (rotten egg smell).
Halide Ion Test with Silver Nitrate
- Add dilute nitric acid, then silver nitrate: forms precipitates.
- Chloride: white (AgCl), Bromide: cream (AgBr), Iodide: yellow (AgI).
- Further test: add ammonia—AgCl dissolves in dilute, AgBr in concentrated, AgI is insoluble.
Additional Qualitative Tests
- Group 2 ions: flame tests (Ca²⁺ dark red, Sr²⁺ red, Ba²⁺ green).
- Ammonium ion: add NaOH and warm, damp red litmus turns blue if NH₃ is released.
- Carbonate: add acid, CO₂ produced turns limewater cloudy.
- Sulfate: add HCl (removes carbonates) then BaCl₂, white BaSO₄ precipitate if sulfate present.
- Test in the order: carbonate → sulfate → halide.
Key Terms & Definitions
- Halogen — Group 7 non-metals with similar chemical properties.
- Displacement reaction — When a more reactive halogen replaces a less reactive halide ion.
- Disproportionation — A reaction where the same element is oxidized and reduced.
- Reducing agent — A substance that donates electrons and is itself oxidized.
- Halide ion — The negative ion of a halogen (Cl⁻, Br⁻, I⁻).
- Precipitate — An insoluble solid formed in a solution during a reaction.
Action Items / Next Steps
- Practice writing electron configurations and half-equations for halogen reactions.
- Memorize the colours and solubility patterns of silver halide precipitates.
- Review practical procedures for qualitative ion tests.