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11.2 The chemical properties of the halogen elements and the hydrogen halides

Syllabus
9701–2028–2029
Topic
11.2
Level
AS

Halogen oxidising power decreases down Group 17

A halogen is an oxidising agent when it gains electrons and is reduced: X₂ + 2e⁻ → 2X⁻. Relative oxidising power decreases F₂ > Cl₂ > Br₂ > I₂.

Halogen added to a halide solution Spontaneous displacement among Cl, Br and I
Cl₂ oxidises Br⁻ and I⁻
Br₂ oxidises I⁻, but not Cl⁻
I₂ oxidises neither Cl⁻ nor Br⁻

Cl2+2Br−→2Cl−+Br2\mathrm{Cl_2 + 2Br^- \rightarrow 2Cl^- + Br_2}

Br2+2I−→2Br−+I2\mathrm{Br_2 + 2I^- \rightarrow 2Br^- + I_2}

Down the group, atomic radius and shielding increase. The nucleus attracts an incoming electron less strongly, so formation of X⁻ becomes less favourable and X₂ is a weaker oxidising agent.

In a displacement equation, the more powerful halogen oxidising agent is reduced to X⁻, while the displaced halide is oxidised to its element. Do not reverse this order or confuse halogen oxidising power with halide reducing power, which increases down the group.

Reactions with hydrogen become less vigorous down Group 17

H2(g)+X2(g)→2HX(g)\mathrm{H_2(g) + X_2(g) \rightarrow 2HX(g)}

Halogen Reaction with H₂ Relative behaviour
F₂ H₂ + F₂ → 2HF explosive even in cool, dark conditions
Cl₂ H₂ + Cl₂ → 2HCl explosive in bright light / sunlight
Br₂ H₂ + Br₂ → 2HBr slow reaction on heating
I₂ H₂ + I₂ ⇌ 2HI requires heating and forms an equilibrium mixture

Relative reactivity is F₂ > Cl₂ > Br₂ > I₂. Down the group, greater radius and shielding reduce the ability of X₂ to gain electrons during compound formation, so progressively more demanding conditions are needed and the reaction is less vigorous.

This outcome concerns forming gaseous hydrogen halides directly from the elements. Do not replace it with a comparison of aqueous acid strengths; HF being a weak acid in water does not make F₂ unreactive toward H₂.

Hydrogen halide thermal stability decreases as H–X bonds weaken

Hydrogen halide Relative H–X bond strength Relative thermal stability
HF strongest highest
HCl weaker lower
HBr weaker again lower again
HI weakest lowest

2HX(g)⇌H2(g)+X2(g)\mathrm{2HX(g) \rightleftharpoons H_2(g) + X_2(g)}

From F to I, the halogen atom is larger, so the H–X bond is longer and overlap between the hydrogen 1s orbital and the halogen orbital becomes less effective. Attraction in the bond weakens, bond enthalpy falls, and less heat is required to decompose HX.

HI therefore decomposes on heating more readily than HBr, which decomposes more readily than HCl; HF is most resistant. A complete explanation names bond length or orbital overlap, then bond strength, then ease of thermal decomposition.

Thermal stability is not boiling point, volatility or aqueous acid strength. It asks how readily the covalent H–X bond breaks when heated.