Displacement reactions as redox reactions
4 marks · NormalChlorine water is added to a solution of potassium bromide.
State what you would see when chlorine water is added to potassium bromide solution.
Explain why this reaction is an example of a redox reaction. Refer to oxidation and reduction in your answer.
Name the oxidising agent in this reaction.
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(a) The solution turns orange/brown (due to bromine formed). · must show Colour change described: orange or brown from Br2
(b) Chlorine is reduced as it gains electrons to form chloride ions; bromide ions are oxidised as they lose electrons to form bromine. Both oxidation and reduction occur. · must show Chlorine gains electrons/reduced; bromide ions lose electrons/oxidised; reference to both processes
(c) Chlorine or Cl2 · must show Chlorine (not chloride)
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Marking points
B1 (a) · 1 mark · Observation: solution turns orange or brown (allow 'orange-brown')
M1 (b) · 1 mark · States that chlorine is reduced (gains electrons) or bromide ions are oxidised (lose electrons).
M2 (b) · 1 mark · References that both oxidation and reduction take place (or both processes named).
B2 (c) · 1 mark · Names chlorine (or Cl2) as oxidising agent.
Worked solution
a) When chlorine water is added to potassium bromide solution, bromine is produced, which is orange/brown in colour.
b) This is a redox reaction – chlorine gains electrons (is reduced) to become chloride ions, while bromide ions lose electrons (are oxidised) to become bromine. Both oxidation and reduction occur.
c) Chlorine (Cl2) is the oxidising agent because it causes bromide ions to be oxidised.