Reactions of the Halogens — Group 7: The Halogens | IGCSE Chemistry

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 6% of your exam marks.

stable

Low

Stable6%

Displacement reactions and physical property trends tested consistently.

Halogens with metals: metal halides

Halogens react with many metals to form ionic metal halide salts
Each halogen atom gains one electron to form a with a 1− charge (F⁻, Cl⁻, Br⁻, I⁻)
The formula of the metal halide depends on the charge on the metal ion:
- Group 1 metals form M⁺ ions, giving compounds like NaCl
  - 2 Na(s) + Cl₂(g) → 2 NaCl(s)
- Group 2 metals form M²⁺ ions, giving compounds like CaBr₂
  - Ca(s) + Br₂(l) → CaBr₂(s)
- Iron forms FeX₃ with chlorine and bromine
  - 2 Fe(s) + 3 Cl₂(g) → 2 FeCl₃(s)
Reactivity falls down the group, so the same metal reacts more slowly with bromine or iodine than with chlorine

Halogens with non-metals: hydrogen halides

With non-metal partners, the halogens form simple molecular covalent compounds
With hydrogen, every halogen gives a hydrogen halide:
- H₂(g) + F₂(g) → 2 HF(g) (very vigorous, even in the dark and cold)
- H₂(g) + Cl₂(g) → 2 HCl(g) (needs UV light or heat)
- H₂(g) + Br₂(g) → 2 HBr(g) (slower, needs heat)
- H₂(g) + I₂(g) → 2 HI(g) (slow, partial reaction)
The vigour of the reaction falls down the group — the same pattern as for the metals

Displacement reactions of the halogens

A happens when a more reactive halogen pushes a less reactive halogen out of an aqueous solution of its halide salt
The reactivity order in Group 7 is: chlorine > bromine > iodine
Chlorine in water (pale green) displaces both bromine and iodine:
- Cl₂(aq) + 2 KBr(aq) → 2 KCl(aq) + Br₂(aq) — solution turns orange
- Cl₂(aq) + 2 KI(aq) → 2 KCl(aq) + I₂(aq) — solution turns brown
Bromine in water (orange) displaces iodine:
- Br₂(aq) + 2 KI(aq) → 2 KBr(aq) + I₂(aq) — solution turns brown
Iodine cannot displace either chlorine or bromine because it is the least reactive
These reactions are sometimes called single replacement reactions

Exam tip

Displacement reactions: predict, observe, conclude

What comes up: you are given two halide solutions and asked to explain how mixing them demonstrates the reactivity order — including the observation for each pair.

Write (four marks): (1) mix the two solutions; (2) state the observation — for bromine added to potassium iodide the solution turns brown (iodine is produced), for bromine added to potassium chloride there is no reaction/no colour change; (3) bromine displaces iodine because iodine is a less reactive halogen; (4) conclude the overall reactivity order from the results.

Watch out: the mark scheme penalises (deducts a mark) for confusing the "-ine" and "-ide" endings — for example, writing "chloride displaces bromide" instead of "chlorine displaces bromide ions". Also, the mark scheme accepts "red-brown" as the colour of the displaced bromine, but the credited colour for the displaced iodine (in solution) is brown, not "purple" or "black".

Add →	Cl₂(aq)	Br₂(aq)	I₂(aq)
KCl(aq)	—	No reaction	No reaction
KBr(aq)	Solution turns orange (Br₂ formed)	—	No reaction
KI(aq)	Solution turns brown (I formed)

Exam tip

Test for chlorine gas

What comes up: "Describe a test for chlorine gas" (2 marks).

Write (two marks): (1) hold a piece of damp blue litmus paper in the gas; (2) the litmus paper is bleached and turns white.

Watch out: the mark scheme accepts "damp blue litmus paper turns red and then bleaches" for M1 only (the bleaching must still appear as a separate point for M2). Writing just "turns red" without the bleaching scores M1 only and misses M2. The mark scheme ignores whether you test the gas or the solution — test the gas with the paper, not the liquid.