A student is investigating the chemistry of halogens extracted from the sea. They have a solution containing iodide ions (I⁻) and want to confirm whether a more reactive halogen can displace iodine from this solution. They then wish to identify the iodine produced using a named reagent. Describe how the student could carry out this displacement reaction and identify the iodine formed, explaining why the displacement occurs in terms of reactivity.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Iodine can be obtained from solutions containing iodide ions by reacting them with a more reactive halogen. The reactivity order of the halogens decreases down Group 7 of the periodic table.
Model answer (5 marks)
Add a more reactive halogen such as chlorine or bromine to the iodide solution. The halogen oxidises the iodide ions and displaces iodine:
2 I⁻ + Cl₂ → Cl⁻ + Cl⁻ + I₂
The solution immediately turns a brown–orange colour, indicating the formation of iodine. To confirm the presence of iodine, the student adds a few drops of starch solution. In the presence of iodine the starch turns a deep blue‑black colour.
The displacement occurs because chlorine or bromine are higher in the halogen reactivity series than iodine; a more reactive halogen can displace a less reactive halogen from its salt solution.
2 I⁻ + Cl₂ → Cl⁻ + Cl⁻ + I₂
The solution immediately turns a brown–orange colour, indicating the formation of iodine. To confirm the presence of iodine, the student adds a few drops of starch solution. In the presence of iodine the starch turns a deep blue‑black colour.
The displacement occurs because chlorine or bromine are higher in the halogen reactivity series than iodine; a more reactive halogen can displace a less reactive halogen from its salt solution.
Examiner tips
- Use the correct chemical equation and show the colour change; mention the reactivity series; state the colour change of starch with iodine; keep the answer concise and use exam terminology.
- Use the word ‘displacement’ and explain it in terms of reactivity order; include the colour change of the iodine solution and the blue‑black starch test.
Common mistakes
- Writing the wrong reaction (e.g. I₂ + Cl₂ → ICl + Cl₂)
- Forgetting to mention the colour change of the iodine solution
- Using the wrong reagent to test for iodine (e.g. phenolphthalein instead of starch)
Mark scheme (5 marks)
- A more reactive halogen (e.g. chlorine or bromine) is added to the solution containing iodide ions
- The solution turns brown/orange, showing that iodine has been produced
- Starch solution is used to identify/confirm the presence of iodine
- Starch turns blue-black in the presence of iodine
- Displacement occurs because chlorine/bromine is more reactive than iodine and so displaces it from the solution / a more reactive halogen can displace a less reactive halogen from its salt solution
Key terms in this question
displacement reaction · halogen · iodide ions · reactivity
Related
- All OCR A-Level Chemistry B: Salters (H433) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
More Elements from the sea questions
- Bromine can be extracted from sea water. Explain how bromine is obtained from se…
- A company extracts iodine from seaweed on an industrial scale. After harvesting …
- Iodine is an element that can be extracted from seaweed. A student carries out a…
- A coastal chemical plant extracts bromine from sea water. The plant operator not…