Bromine can be extracted from sea water. Explain how bromine is obtained from sea water using chlorine, and explain why chlorine is able to displace bromine from the solution.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Sea water contains bromide ions at low concentrations. The chemical industry extracts bromine from sea water on a large scale by reacting the sea water with chlorine gas.
Model answer (5 marks)
Chlorine gas is bubbled through sea water that contains bromide ions.
A displacement reaction takes place: 2Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂.
The bromide ions are oxidised to bromine, while chlorine is reduced to chloride ions.
Chlorine is more reactive than bromine; it sits higher in the halogen reactivity series.
A more reactive halogen can displace a less reactive halogen from the salt solution because it gains electrons more readily.
A displacement reaction takes place: 2Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂.
The bromide ions are oxidised to bromine, while chlorine is reduced to chloride ions.
Chlorine is more reactive than bromine; it sits higher in the halogen reactivity series.
A more reactive halogen can displace a less reactive halogen from the salt solution because it gains electrons more readily.
Examiner tips
- Use the command word ‘explain’ – give a clear sequence of steps and the reason for each step. Mention the displacement reaction and the change in oxidation states. Show the relative reactivity of chlorine and bromine.
- common_mistakes
- :
- Forgetting to state the displacement reaction. Confusing the direction of electron flow or the oxidation states. Not linking the reactivity series to why chlorine displaces bromine.
Mark scheme (5 marks)
- Chlorine is added to / bubbled through sea water containing bromide ions
- A displacement reaction occurs, producing bromine / bromide ions are oxidised to bromine
- Chlorine is more reactive than bromine
- Chlorine is higher in the halogen reactivity series / halogens become less reactive going down Group 7
- A more reactive halogen can displace a less reactive halogen from a solution of its salt / chlorine can gain electrons more easily than bromine so it displaces bromide ions
Related
- All OCR A-Level Chemistry B: Salters (H433) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Elements from the sea questions
- A student is investigating the chemistry of halogens extracted from the sea. The…
- 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…