Reactions of the halogens
5 marks · HardThis question is about the reactivity of halogens and their displacement reactions.
Write a balanced chemical equation for the reaction between bromine and potassium iodide solution.
Explain why a reaction occurs between bromine and potassium iodide solution. Refer to the reactivity of the halogens in your answer.
Describe what you would see during this reaction.
Name the solution you would use to confirm the presence of iodine formed in this reaction. State the colour observed.
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) Br2(aq) + 2KI(aq) → 2KBr(aq) + I2(aq)
(b) Bromine is more reactive than iodine, so it displaces iodine from potassium iodide.
(c) A brown colour appears (due to iodine forming).
(d) Starch solution; blue-black colour.
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Marking points
A1 (a) · 1 mark · Correct balanced equation for the displacement: Br2 + 2KI → 2KBr + I2 (state symbols optional but accepted).
E1 (b) · 1 mark · States that bromine is more reactive than iodine / reference to Group 7 reactivity.
E2 (b) · 1 mark · Bromine displaces iodine from potassium iodide / explanation of displacement due to relative reactivity.
O1 (c) · 1 mark · States brown colour or brown solution appears (due to iodine forming).
T1 (d) · 1 mark · Correct test and observation (starch solution gives blue-black colour with iodine).
Worked solution
a) Balanced equation:
b) Explanation: Bromine is more reactive than iodine because it is higher up Group 7. Therefore, bromine can displace iodine from potassium iodide solution.
c) During the reaction, the solution turns brown due to the formation of iodine.
d) To confirm the presence of iodine, add starch solution; a blue-black colour will be observed.