Explain why chlorine is more reactive than bromine, and predict what would be observed when chlorine water is added to a solution of potassium bromide.

AQA A-Level Chemistry (7405) — 3.2.3 Group 7(17), the halogens · Explain · 5 marks · View as Markdown

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

The halogens in Group 7 react by gaining one electron to form a negative ion. Their reactivity decreases down the group.

Model answer (5 marks)

Chlorine atoms are smaller than bromine atoms, so the incoming electron is attracted more strongly to the nucleus in chlorine. Consequently chlorine is more reactive than bromine. When chlorine water is added to a potassium bromide solution, chlorine displaces bromine:
Cl₂ + 2KBr → 2KCl + Br₂. The solution turns orange/brown as bromine is produced.

Examiner tips

  • Mention atomic size and electron attraction to justify reactivity. State the displacement reaction and the colour change. Use the correct balanced equation.
  • common_mistakes
  • :
  • Confusing reactivity order (saying bromine is more reactive). Omitting the colour change or the displacement reaction. Using the wrong balanced equation or missing the 2:2 stoichiometry.

Mark scheme (5 marks)

  1. Chlorine atoms are smaller than bromine atoms (or chlorine has fewer electron shells than bromine)
  2. The incoming electron is attracted more strongly to the nucleus in chlorine (or the nucleus has a greater attraction for the gained electron in chlorine)
  3. Chlorine displaces bromine from the potassium bromide solution (a displacement reaction occurs)
  4. The solution turns orange / brown (bromine is produced)
  5. The ionic equation / word equation shows chlorine reacting with potassium bromide to form potassium chloride and bromine (or correct reference to Cl₂ + 2KBr → 2KCl + Br₂)

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