Halogen Displacement Reactions
The Schoolyard Bully — when a stronger halogen steals the show
🏫 The Schoolyard Bully Analogy
Step 1 — The happy pair: Potassium (K⁺) is holding hands with Iodide (I⁻). They are perfectly happy. The solution is completely colourless — you can't see either of them.
Step 2 — The bully arrives: Chlorine water is added. Chlorine shouts: "I'm more reactive than Iodine! That Potassium is MINE!"
Step 3 — The displacement: Chlorine kicks Iodide out and grabs Potassium. Iodide is now alone as I₂ molecules — and they are BROWN. The whole solution turns brown instantly.
What if the bully is weak? If Iodine tries to bully Potassium Chloride — it can't. Chloride is more reactive. Iodine gives up. No reaction.
🔬 The Key Rule
A more reactive halogen will displace a less reactive halogen from an aqueous solution of its halide salt. The reactivity decreases down Group VII:
// The Chemistry at Particle Level
Cl₂(aq) + 2KI(aq) → 2KCl(aq) + I₂(aq)
Colourless → Colourless + Pale green → Colourless + BROWN
KEY: Halide ions (Cl⁻, Br⁻, I⁻) are COLOURLESS in solution.
KEY: Halogen molecules (Cl₂, Br₂, I₂) are COLOURED.
⚠️ HAZARD: Chlorine water (Cl₂(aq)) — Toxic, oxidising. Always use in a fume cupboard. Avoid inhalation. Cl₂ gas is pale green and has a pungent smell.
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