A student adds a small amount of bromine water to two test tubes. To the first test tube, the student adds potassium chloride solution. To the second test tube, the student adds potassium iodide solution. Describe what the student would observe in each test tube and explain these observations in terms of displacement reactions.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
In the tube with potassium chloride the bromine remains orange – no colour change is seen because bromine cannot displace chloride ions; chlorine is more reactive than bromine.
In the tube with potassium iodide the orange bromine turns brown/dark brown as iodine is produced – bromine displaces iodide because bromine is more reactive than iodine.
These are displacement reactions: a more reactive halogen displaces a less reactive halogen from its aqueous salt, showing the relative reactivity of the halogens.
In the tube with potassium iodide the orange bromine turns brown/dark brown as iodine is produced – bromine displaces iodide because bromine is more reactive than iodine.
These are displacement reactions: a more reactive halogen displaces a less reactive halogen from its aqueous salt, showing the relative reactivity of the halogens.
Examiner tips
- Use the command words ‘describe’ and ‘explain’ – describe the colour change, explain with the reactivity series.
- Mention the relative reactivity of Cl > Br > I to justify the displacement.
- Show that no reaction occurs with KCl because Br cannot displace Cl.
- Show that Br displaces I because Br > I.
Mark scheme (5 marks)
- No colour change / reaction in the test tube containing potassium chloride solution
- Bromine cannot displace chloride ions because chlorine is more reactive than bromine / bromine is less reactive than chlorine
- The test tube containing potassium iodide turns brown / dark brown (or iodine is produced causing a colour change)
- Bromine displaces iodide ions from potassium iodide solution because bromine is more reactive than iodine
- A more reactive halogen can displace a less reactive halogen from an aqueous solution of its salt / displacement reactions show the relative reactivity of halogens
Key terms in this question
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →
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