Displacement reactions involving the halogens
4 marks · NormalThe results below show the observed colour changes when halogen solutions are mixed with different potassium halide solutions.
| Added halogen solution | Potassium halide solution | Observation |
|---|---|---|
| Chlorine (Cl2) | Potassium bromide (KBr) | orange |
| Bromine (Br2) | Potassium iodide (KI) | brown |
| Iodine (I2) | Potassium chloride (KCl) | no change |
Which halogen is the most reactive? Give evidence from the table.
Write a balanced chemical equation for the reaction between bromine solution and potassium iodide solution.
Predict the result if chlorine solution is added to potassium iodide solution. Explain your answer.
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) Chlorine
(b) 2KI(aq) + Br2(aq) -> 2KBr(aq) + I2(aq)
(c) Brown colour observed. Explanation: chlorine is more reactive than iodine and displaces it from KI solution forming iodine.
Also accept: Accept orange-brown or brown for iodine colour
Common mistakes: Incorrectly stating bromine is most reactive; Not balancing equation
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Marking points
B1 (a) · 1 mark · Correctly states chlorine is the most reactive; evidence: displaces both Br and I.
B2 (b) · 1 mark · Correct balanced equation: 2KI(aq) + Br2(aq) -> 2KBr(aq) + I2(aq).
B3 (c) · 1 mark · States correct observation: brown colour.
B4 (c) · 1 mark · Correct explanation: chlorine is more reactive than iodine, so displaces it.
Worked solution
(a) Chlorine is the most reactive; it can displace bromine and iodine as shown by the orange and brown colours.
(b) 2KI(aq) + Br2(aq) → 2KBr(aq) + I2(aq)
(c) Adding chlorine solution to potassium iodide gives a brown colour because chlorine displaces iodine from KI, forming iodine.