A level Chemistry · Edexcel 9CH0 · Topic 4B
Group 7 (the halogens)
Key points
- Down Group 7: colour darkens (pale yellow F₂, green Cl₂, red-brown Br₂, grey/purple I₂), boiling points rise and oxidising power falls.
- Displacement: a more reactive halogen oxidises a less reactive halide, e.g. Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂.
- Disproportionation: Cl₂ + H₂O ⇌ HCl + HClO; cold dilute NaOH gives NaCl + NaClO (bleach); hot alkali gives chlorate(V).
- Halide reducing power increases down the group because larger ions lose electrons more easily.
- With concentrated H₂SO₄: NaCl gives only HCl; NaBr gives HBr then Br₂ and SO₂; NaI gives HI, I₂, SO₂, S and H₂S.
- Hydrogen halides form steamy fumes in moist air, dissolve to give acids and react with ammonia to give white smoke of NH₄X.
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The halogens are reactive non-metals that exist as diatomic molecules. Their oxidising power falls down the group, while their halide ions become better reducing agents: these two opposite trends explain most of their chemistry.
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