The Halogens
5 marks · EasyIodine is unable to displace bromine from potassium bromide solution. Use knowledge of redox and electronic structure to answer the following:
Explain, in terms of electron transfer, what occurs to the bromide ions in a displacement reaction with chlorine.
State whether iodine acts as an oxidising agent or reducing agent in displacement reactions, and explain your answer.
Using information about electronic configuration, explain why chlorine is more reactive than bromine.
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) Bromide ions lose electrons to chlorine (are oxidised to bromine molecules).
(b) Iodine acts as an oxidising agent because it accepts electrons from other ions (although it is weaker than chlorine or bromine).
(c) Chlorine is more reactive than bromine because its atoms are smaller so the extra electron joins a shell closer to the nucleus, where it is more strongly attracted. The process forms a Cl− ion more easily than a Br− ion can form. This stronger attraction for incoming electrons makes chlorine a better oxidising agent than bromine.
Common mistakes: Confusing oxidising with reducing agent; Incorrect electron transfer direction; Incorrect role of nucleus or electron distance
Show mark scheme and worked solution
Marking points
M1 (a(b)) · 1 mark · Correct statement about bromide ions losing electrons (oxidation).
C1 (a(b)) · 1 mark · Clear, correct explanation that bromine forms as bromide ions are oxidised.
B1 (a(c)) · 1 mark · Iodine correctly identified as (weak) oxidising agent with explanation.
M2 (a(c)) · 1 mark · States that chlorine’s extra electron joins a shell closer to the nucleus.
C2 (a(c)) · 1 mark · Explains the stronger attraction of nucleus for new electron makes Cl more reactive.
Worked solution
a) In a displacement reaction with chlorine, each bromide ion loses an electron to chlorine, so is oxidised to form Br2 molecules.
b) Iodine acts as an oxidising agent as it accepts electrons from other ions, although less effectively than chlorine or bromine.
c) Chlorine is more reactive than bromine because its atoms are smaller. The added electron joins a shell closer to the nucleus and is more strongly attracted. This makes it easier for chlorine to form a Cl− ion, so it is a stronger oxidising agent than bromine.