11.3 Some reactions of the halide ions

A halide ion is a reducing agent when it donates an electron and is oxidised to its halogen: 2X⁻ → X₂ + 2e⁻. Reducing power increases from fluoride to iodide.

The larger, more shielded ions hold their outer electrons less strongly. Iodide therefore loses an electron more readily, while fluoride is difficult to oxidise in aqueous solution.

Bromide reduces chlorine water, producing bromine: Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂. The reverse displacement is not spontaneous under the same conditions.

This is the opposite trend to halogen oxidising power. Always state whether the species is gaining or losing electrons.

Aqueous halide ions form precipitates with silver ions: Ag⁺ + X⁻ → AgX. The precipitate colour and its response to aqueous ammonia provide a structured identification test.

Chloride, bromide and iodide give different AgX observations. Concentrated sulfuric acid gives a second line of evidence: halide ions reduce sulfuric acid to different extents, so balanced equations and oxidation states matter.

Add nitric acid then silver nitrate to an unknown. A pale precipitate that dissolves in dilute ammonia is consistent with AgCl; use the whole observation sequence rather than colour alone.

Ammonia is not just a washing step, and precipitate colour alone is not a complete identification. Keep test conditions and ionic equations explicit.