Chlorine pulls electrons from bromide and iodide ions; iodine cannot take them back. The same falling oxidising power shows up when the halogens react with hydrogen and when the hydrogen halides are heated.
Halogens are oxidising agents: they gain electrons to form halide ions. Oxidising power decreases down the group (Cl2 > Br2 > I2) because larger atoms attract an incoming electron less strongly. So a halogen displaces any halogen below it from a solution of its halide. The halogens react with hydrogen less and less vigorously down the group, and the thermal stability of the hydrogen halides decreases from HCl to HI because the H–X bond gets longer and weaker.
Add each halogen to each halide solution in the simulator. Shake with a little hexane to make the colour clearer.
| Add | to KCl(aq) | to KBr(aq) | to KI(aq) |
|---|---|---|---|
| Cl2(aq) | no change | orange (Br2) | brown (I2) |
| Br2(aq) | no reaction | no change | brown (I2) |
| I2(aq) | no reaction | no reaction | no change |
Compare how the halogens react with hydrogen, then heat each hydrogen halide in the simulator.
| Reaction with H2 | Conditions | H–X bond energy / kJ mol−1 | On heating HX |
|---|---|---|---|
| H2 + F2 → 2HF | explosive, even cold and dark | 562 | very stable |
| H2 + Cl2 → 2HCl | explosive in sunlight | 431 | stable |
| H2 + Br2 → 2HBr | needs heat and a catalyst | 366 | slight decomposition |
| H2 + I2 ⇌ 2HI | slow and reversible on heating | 299 | purple I2 with a hot rod |
Oxidising power decreases down the group, so iodine cannot oxidise chloride or bromide ions. When explaining displacement, identify the halogen as the oxidising agent and the halide ion as the reducing agent. For thermal stability, refer to the H–X bond energy, not to intermolecular forces.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Which is the strongest oxidising agent?
Chlorine water is added to potassium iodide solution. What is seen?
Which hydrogen halide decomposes most easily on heating?
In Cl2 + 2Br− → 2Cl− + Br2, which species is oxidised?
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
Why does the oxidising power of the halogens decrease down the group?
Which mixture would show a colour change?
A hot glass rod is plunged into a test tube of hydrogen iodide. What is seen and why?
Iodine water is added to a mixture, and when shaken with hexane the upper layer is purple. Which statement is correct?
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