Chlorine, bromine and iodine are all halogens in Group 7 of the periodic table. Describe and explain the trend in reactivity of the halogens as you go down Group 7, and explain what happens when chlorine water is added to a solution of potassium bromide.

Pearson Edexcel International GCSE Chemistry (4CH1) — 2.2 Group 7 (halogens) · Describe and explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Reactivity decreases down Group 7 because each successive halogen atom is larger and has a lower effective nuclear charge, making it harder to gain an electron and form the halide ion.

When chlorine water is added to a potassium bromide solution, chlorine displaces bromine from the bromide ion:

Cl₂ + 2 KBr → 2 KCl + Br₂

The solution turns orange‑brown as bromine is produced.

Examiner tips

  • Use the word ‘decreases’ and explain size/effective nuclear charge. Show the displacement equation and the colour change. Link the displacement to the relative reactivity of the halogens.

Common mistakes

  • Saying reactivity increases down the group. Forgetting that the reaction produces Br₂. Using the wrong equation (e.g. Cl₂ + Br⁻ → Cl⁻ + Br₂).

Mark scheme (4 marks)

  1. Reactivity decreases going down Group 7
  2. Each halogen atom gains one electron to form a halide ion, and atoms become larger down the group so gaining an electron becomes more difficult / less easy
  3. Chlorine displaces bromide ions from potassium bromide solution / a displacement reaction occurs
  4. The solution turns orange/brown (as bromine is produced)

Key terms in this question

halogens · reactivity

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