AQA A Level chemistry - AS Unit 2: Section 3.2.5 Group 7(17), the Halogens - Trends in the oxidising abilities of the halogens

3.2.5 Group 7(17), the Halogens - Trends in the oxidising abilities of the halogens

Students should:

understand that the ability of the halogens (from fluorine to iodine) to oxidise decreases down the group (e.g. the displacement reactions with halide ions in aqueous solution)

Displacement reaction of halogens

Reactivity of the halogens decreases going down the group and the more reactive halogen will displace a less reactive halogen from a solution of its ions. This is also a redox reaction.

The halogens, as elements, are oxidising agents as they can easily remove an electron from another species, becoming negatively charged halide ions in turn.

Cl₂ + 2e 2Cl^-

This means that a halogen can oxidise a less reactive halide ion.

If we ignore fluorine for the moment (it's such a powerful oxidising agent can even oxidise even water), Cl₂(aq) is the most powerful oxidising agent of the halogens. It will oxidise both bromide ions and iodide ions.

Cl₂ + 2Br^- 2Cl^- + Br₂

Cl₂ + 2I^- 2Cl^- + I₂

Hence, chlorine water (a solution of chlorine in water) + bromide ions turns orange due to formation of bromine in solution. Bromine, however, will only oxidise iodide ions.

Br₂ + 2I^- 2Br^- + I₂

Bromine water (a solution of bromine in water) + iodide ions turns brownish due to formation of iodine in solution.

Iodine solution is an oxidising agent, but it is not strong enough to oxidise either chloride or bromide ions.

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