Halogen displacement reactions are redox processes where the more reactive halogen acts as an oxidizing agent. It gains electrons (is reduced) to form halide ions, while the less reactive halide ion loses electrons (is oxidized) to form the diatomic halogen.
The oxidizing power decreases down the group because atomic radius increases and electron shielding becomes more significant. This makes it harder for the nucleus of a larger halogen atom to attract and capture an incoming electron into its outer shell.
The ionic equation for a typical displacement reaction is: , where is the more reactive halogen and is the less reactive halogen being displaced.
| Halogen | Aqueous Solution Color | Organic Solvent Color |
|---|---|---|
| Chlorine () | Very pale green / Colorless | Colorless |
| Bromine () | Orange / Yellow | Orange |
| Iodine () | Brown | Purple / Violet |
Always check the reactivity order: Before writing an equation, verify that the standalone halogen is higher in Group 7 than the halide ion in the salt. If it is lower, write 'No Reaction'.
Master the ionic equations: Examiners frequently ask for ionic equations rather than full chemical equations. Remember to omit the 'spectator ions' (like or ) that do not change during the reaction.
Identify the Oxidizing Agent: The halogen that starts as a molecule () is always the oxidizing agent because it 'pulls' electrons away from the halide ions.
Purple is the Key: If a question mentions a 'purple layer' after adding an organic solvent, it is a definitive indicator that iodine () has been produced.
Confusing Halogens and Halides: Students often mix up the names. Remember that 'Halogens' are the diatomic molecules () and 'Halides' are the negatively charged ions ().
Incorrect Color Assignment: A common mistake is describing the aqueous iodine solution as purple. In water, iodine is brown; it only turns purple when dissolved in an organic solvent.
Predicting Impossible Reactions: Do not assume a reaction occurs just because two substances are mixed. For example, adding to results in no change because iodine is less reactive than chlorine.
