Iodine is an element that can be extracted from seaweed. A student carries out a series of reactions to obtain pure iodine from seaweed and then tests the iodine they have made. Describe how iodine can be identified in the laboratory, and explain why chlorine is able to displace iodine from a solution of potassium iodide during its extraction.

OCR A-Level Chemistry B: Salters (H433) — ES Elements from the sea · Describe and Explain · 5 marks · View as Markdown

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

Seaweed contains iodide ions. To extract iodine, the seaweed is burned to ash, the ash is dissolved in water, and chlorine water is added to the solution of iodide ions. The iodine produced can then be identified using a simple chemical test.

Model answer (5 marks)

Iodine can be identified by adding a few drops of starch solution to the sample. The iodine reacts with the starch to give a dark blue‑black colour.

Chlorine is a more reactive halogen than iodine. In the reactivity series of halogens chlorine lies above iodine, so it is a stronger oxidising agent. When chlorine gas is bubbled through a solution of potassium iodide, the chlorine atoms gain electrons more readily than the iodide ions. Consequently the iodide ions are oxidised to iodine (2 I⁻ → I₂ + 2 e⁻) while the chlorine is reduced to chloride ions. The more reactive halogen therefore displaces the less reactive halogen from its salt solution.

Examiner tips

  • Use the exact test – starch solution gives blue‑black colour. Explain the displacement in terms of the reactivity series and electron gain. Show the half‑reaction for iodide oxidation.
  • common_mistakes
  • :
  • Saying iodine turns the solution colourless. Forgetting that chlorine is a better oxidiser. Using the wrong half‑reaction (e.g. I₂ → 2 I⁻).

Mark scheme (5 marks)

  1. Iodine turns starch solution blue-black (accept: dark blue/blue-black colour with starch).
  2. Chlorine is a more reactive (non-metal) than iodine / chlorine is higher in the reactivity series of halogens than iodine.
  3. More reactive halogens displace less reactive halogens from their salt solutions / a more reactive halogen can take the place of a less reactive one.
  4. Chlorine gains electrons more easily than iodine / chlorine atoms are better at gaining electrons (accept: chlorine is a better oxidising agent).
  5. Iodide ions lose electrons to form iodine molecules / iodide ions are oxidised to iodine (I⁻ → I₂ / 2I⁻ → I₂ + 2e⁻).

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