The chemical properties of the halogen elements and the hydrogen halides

Need to know

What you need to know

• This trend in oxidising power leads to a series of displacement reactions. A more reactive halogen will displace a less reactive halide ion from its salt solution.
• $Cl_2(aq) + 2Br^-(aq) \rightarrow 2Cl^-(aq) + Br_2(aq)$ (A colourless solution turns orange/brown)
• $Br_2(aq) + 2I^-(aq) \rightarrow 2Br^-(aq) + I_2(aq)$ (An orange/brown solution turns a darker brown)
• Reactions such as $I_2(aq) + 2Cl^-(aq)$ do not occur as iodine is a weaker oxidising agent than chlorine.

Explanation

Halogens: A Tale of Reactivity

1. X₂ + 2e⁻ → 2X⁻ — oxidising power ↓ down group (F₂ most reactive).
2. H–X bond enthalpy peaks at HF then ↓ — HF anomalously strong H-bonds.
3. HX(aq) acid strength: HF < HCl < HBr < HI.
4. Disproportionation: Cl₂ + OH⁻ → Cl⁻ + ClO⁻ (+ water) cold dilute.