Summary — What This Chapter Covers
- Cl₂ pale green gas, Br₂ red-brown liquid, I₂ grey-black solid (purple vapour). Volatility decreases down the group as id-id forces increase.
- X–X bond strength: Cl–Cl > Br–Br > I–I.
- Oxidising power: Cl₂ > Br₂ > I₂. Reactivity with hydrogen decreases down the group.
- Thermal stability of HX: HCl > HBr > HI, because the H–X bond gets weaker.
- Reducing power of halide ions: I⁻ > Br⁻ > Cl⁻, shown by their reactions with concentrated H₂SO₄.
- Silver halides: AgCl white, AgBr cream, AgI yellow; tell them apart with aqueous ammonia.
- Chlorine disproportionates in cold and in hot NaOH, and kills bacteria in water via HOCl and ClO⁻.
1. Physical Properties of the Group 17 Elements
Colour and volatility
Cl₂, Br₂, I₂
| Halogen | State at room temperature | Colour | Boiling point / °C |
|---|
| Chlorine, Cl₂ | Gas | Pale green (yellow-green) | −34 |
| Bromine, Br₂ | Liquid | Red-brown liquid, orange-brown vapour | 59 |
| Iodine, I₂ | Solid | Grey-black solid, purple vapour | 184 |
Volatility decreases down the group. The molecules have more electrons, so the instantaneous dipole–induced dipole forces between them are stronger and more energy is needed to separate them.
Bond strength
Why the X–X bond weakens
| Bond | Cl–Cl | Br–Br | I–I |
|---|
| Bond energy / kJ mol⁻¹ | 242 | 193 | 151 |
Down the group the atoms are larger, so the bond is longer and the bonding pair is further from the nuclei and more shielded. The attraction between the nuclei and the shared pair is weaker, so the bond energy falls.
Practice Question
Explain why iodine is a solid at room temperature but chlorine is a gas.
2. Chemical Properties of the Halogens and Hydrogen Halides
Halogens as oxidising agents
Cl₂ > Br₂ > I₂
A halogen acts as an oxidising agent by gaining electrons: X₂ + 2e⁻ → 2X⁻. Down the group the atoms are larger and the incoming electron is further from the nucleus and more shielded, so it is attracted less: oxidising power decreases.
A more reactive halogen displaces a less reactive one from a solution of its halide:
| KCl(aq) | KBr(aq) | KI(aq) |
|---|
| Cl₂(aq) | – | Orange solution: Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂ | Brown solution: Cl₂ + 2I⁻ → 2Cl⁻ + I₂ |
| Br₂(aq) | No reaction | – | Brown solution: Br₂ + 2I⁻ → 2Br⁻ + I₂ |
| I₂(aq) | No reaction | No reaction | – |
Adding a non-polar solvent (e.g. cyclohexane) helps: Br₂ is orange and I₂ is purple in it.
Reactions with hydrogen
H₂ + X₂ → 2HX
| Halogen | Reaction with hydrogen |
|---|
| Cl₂ | Explosive in sunlight (or a flame): H₂ + Cl₂ → 2HCl |
| Br₂ | Needs heating (about 300 °C) with a platinum catalyst: H₂ + Br₂ → 2HBr |
| I₂ | Slow and incomplete even with heat and a catalyst: H₂ + I₂ ⇌ 2HI |
Reactivity decreases down the group: the halogens are weaker oxidising agents and the H–X bond formed is weaker, so less energy is released (less exothermic).
Thermal stability of hydrogen halides
Bond strength decides
| Bond | H–Cl | H–Br | H–I |
|---|
| Bond energy / kJ mol⁻¹ |