Displacement reaction (halogens)

A halogen displacement reaction is one in which a more reactive halogen takes the place of a less reactive halogen in its salt solution, releasing the less reactive halogen. Malay: tindak balas penyesaran · Chinese: 置换反应.

This reaction is the practical test of the halogen reactivity order. Because reactivity falls down Group 17, chlorine is more reactive than bromine, which is more reactive than iodine. When you add a more reactive halogen to a solution containing the halide ions of a less reactive one, the more reactive halogen pulls the electrons away and takes the place in the salt, setting the less reactive halogen free. The freed halogen usually shows itself by a colour change in the solution.

Example. Adding chlorine water to colourless potassium bromide solution turns it orange, because chlorine displaces bromine: Cl₂ + 2KBr → 2KCl + Br₂. Chlorine also displaces iodine from potassium iodide, turning the solution brown. Bromine can displace iodine, but iodine cannot displace either chlorine or bromine, because iodine is the least reactive of the three.

Confusion to avoid. The direction only goes one way: a more reactive halogen displaces a less reactive one, never the reverse. Do not write iodine displacing chlorine. Keep this displacement of a non-metal apart from the displacement of a metal from its salt by a more reactive metal, the idea is the same, but here it is halogens and halides.

In the Periodic Table of Elements chapter the displacement reaction turns the abstract reactivity trend of the halogens into something you can see in a test tube, which is why it appears so often as evidence for the order of the group. If you can name the starting colours, the final colours and write the balanced equation, you have the full answer such a question is looking for.