The Halogens
8 marks · HardThis question is about the reactions between halogens and halide ions.
Chlorine water is added to a solution of sodium bromide.
Write the ionic equation for the reaction that takes place.
Explain why chlorine is able to displace bromine from sodium bromide solution but not the other way around.
The reaction between chlorine and sodium bromide is a redox reaction.
For this reaction:
Identify which species is oxidised and which is reduced.
Use your knowledge of electron transfer to explain your answer to part (c)(i).
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) Cl2(aq) + 2Br^-(aq) → 2Cl^-(aq) + Br2(aq) · must show balanced equation, correct ions
(b) Chlorine is more reactive than bromine because it is a stronger oxidising agent and can displace bromide ions from solution; bromine cannot displace chloride ions because it is less reactive. · must show relative reactivity, oxidising ability
(c(i)) Bromide ions (Br^-) are oxidised; chlorine (Cl2) is reduced. · must show identify oxidised and reduced species
(c(ii)) Bromide ions lose electrons (are oxidised) to form bromine, while chlorine molecules gain electrons (are reduced) to form chloride ions. · must show electron loss for Br^-, electron gain for Cl2
Show mark scheme and worked solution
Marking points
A1 (a) · 2 marks · Fully correct balanced ionic equation with all species and charges shown.
B1 (b) · 1 mark · Correctly states chlorine is more reactive than bromine (or equivalent).
B2 (b) · 1 mark · Explains in terms of oxidising agent ability and/or why bromine cannot displace chloride.
C1 (c(i)) · 1 mark · Correctly identifies Br^- is oxidised.
C2 (c(i)) · 1 mark · Correctly identifies Cl2 is reduced.
D1 (c(ii)) · 1 mark · Explains oxidation as loss of electrons by Br^- ions.
D2 (c(ii)) · 1 mark · Explains reduction as gain of electrons by Cl2 molecules.
Worked solution
a) The balanced ionic equation is:
b) Chlorine is more reactive than bromine. This means it is a stronger oxidising agent and can remove electrons from bromide ions, causing bromine to be displaced as bromine gas. Bromine cannot displace chloride ions because it is less reactive and not a stronger oxidising agent than chlorine.
c) i) In this reaction, the bromide ions (Br^-) are oxidised to bromine (Br2), and chlorine (Cl2) is reduced to chloride ions (Cl^-).
ii) Bromide ions lose electrons (are oxidised) to form bromine molecules:
Chlorine molecules gain these electrons (are reduced) to form chloride ions:
This shows that bromide ions are oxidised, and chlorine is reduced in the same process.