Reactions of halide ionsAQA A-Level Chemistry: Revision notes

A reducing agent donates electrons (and is oxidised). Halide ions donate an electron: 2X⁻ → X₂ + 2e⁻.

Reducing ability increases down the group: Cl⁻ < Br⁻ < I⁻.

Explanation: larger ions have the outer electron further from the nucleus and more shielded, so it is lost more easily. This is the reverse of the trend in oxidising ability of the halogens.

The trend is shown by how far halide ions reduce concentrated sulfuric acid.

All three first undergo an acid-base reaction giving a hydrogen halide: NaX + H₂SO₄ → NaHSO₄ + HX.

• Chloride: steamy white fumes of HCl. No redox, because Cl⁻ is too weak a reducing agent.
• Bromide: HBr then reduces the acid: 2HBr + H₂SO₄ → Br₂ + SO₂ + 2H₂O. Orange-brown Br₂ and colourless, choking SO₂. Sulfur goes from +6 to +4.
• Iodide: HI reduces the acid further. 2HI + H₂SO₄ → I₂ + SO₂ + 2H₂O; 6HI + H₂SO₄ → 3I₂ + S + 4H₂O; 8HI + H₂SO₄ → 4I₂ + H₂S + 4H₂O. Black solid or purple vapour (I₂), yellow solid (S) and a bad egg smell (H₂S). Sulfur goes from +6 to +4, 0 and −2.

Aqueous silver nitrate is used because silver ions give coloured precipitates with halide ions: Ag⁺(aq) + X⁻(aq) → AgX(s).

• Chloride: white AgCl
• Bromide: cream AgBr
• Iodide: yellow AgI

The solution is first acidified with dilute nitric acid to remove carbonate (and hydroxide) ions, which would otherwise form precipitates of silver carbonate (or silver oxide) and give a false positive. Nitric acid is used because hydrochloric acid would add Cl⁻ and sulfuric acid would add sulfate, which forms silver sulfate.

Because the colours are hard to judge, ammonia solution is added to the precipitates. Solubility in ammonia decreases from chloride to iodide:

• AgCl dissolves in dilute ammonia solution
• AgBr is insoluble in dilute but dissolves in concentrated ammonia solution
• AgI does not dissolve in concentrated ammonia solution

Dissolving occurs because the soluble, colourless complex ion [Ag(NH₃)₂]⁺ forms: AgCl + 2NH₃ → [Ag(NH₃)₂]⁺ + Cl⁻.

Test for each ion on its own and in a sensible order: carbonate first, then sulfate, then halide, because carbonate would interfere with the later tests.

• Group 2 cations: add NaOH(aq). Mg²⁺ gives a white precipitate insoluble in excess; hydroxide solubility increases down the group. Ba²⁺ gives a white precipitate with sulfate ions.
• Ammonium: warm with NaOH(aq); NH₃ turns damp red litmus paper blue.
• Hydroxide: turns damp red litmus paper blue.
• Carbonate: add dilute acid; effervescence of CO₂ turns limewater milky.
• Sulfate: add acidified barium chloride; white precipitate of BaSO₄.
• Halide: acidified silver nitrate, then ammonia.