11.3.1—Reactivity of halide ions as reducing agents

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.