Chlorine gas is bubbled through potassium bromide solution. The colourless solution turns orange-brown. Explain this colour change in terms of oxidation and reduction, identifying which species is oxidised and which is reduced. Give the ionic equation for the reaction.

AQA A-Level Chemistry (7405) — 3.1.7 Oxidation, reduction and redox equations · Explain · 5 marks · View as Markdown

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

Chlorine is a more reactive halogen than bromine. When chlorine is bubbled through potassium bromide solution, a displacement reaction occurs and the solution turns orange-brown.

Model answer (5 marks)

Chlorine is a stronger oxidising agent than bromine. When Cl₂ is bubbled through KBr, the bromide ions give up electrons (are oxidised) to the chlorine molecules, which gain electrons (are reduced). The bromide ions are oxidised to bromine atoms that combine to give Br₂, the orange‑brown species that colours the solution.

Ionic equation:
Cl₂ + 2 Br⁻ → 2 Cl⁻ + Br₂

Examiner tips

  • Show the electron transfer to emphasise redox; mention the colour change is due to Br₂; write the balanced ionic equation exactly as in the mark scheme

Common mistakes

  • Writing the wrong products (e.g. Br⁻ instead of Br₂); omitting the electron transfer description; using the wrong stoichiometry in the ionic equation

Mark scheme (5 marks)

  1. Bromide ions are oxidised (lose electrons)
  2. Chlorine molecules are reduced (gain electrons)
  3. Bromine molecules (Br₂) are formed and are responsible for the orange-brown colour
  4. Correct ionic equation: Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂
  5. The reaction is described as redox because oxidation and reduction occur simultaneously

Key terms in this question

oxidation · reduction · ionic equation

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