Displacement reactions involving the halogens
5 marks · HardA student adds solutions of bromine and iodine to separate samples of potassium chloride and potassium iodide. The results are summarised below:
| Added halogen solution | Potassium chloride solution | Potassium iodide solution |
|---|---|---|
| Bromine | No change | Brown colour appears |
| Iodine | No change | No change |
Use the information and your knowledge of halogen reactivity to answer the following.
Write a balanced chemical equation for the reaction that occurs when bromine solution is added to potassium iodide solution.
Explain why there is no reaction when bromine solution is added to potassium chloride solution.
Describe and explain the difference in the observations when bromine and iodine solutions are added to potassium iodide solution.
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.
Show answer
(a) 2KI(aq) + Br2(aq) → 2KBr(aq) + I2(aq)
(b) Bromine is less reactive than chlorine, so cannot displace chlorine from potassium chloride. No reaction occurs.
(c) Bromine turns potassium iodide solution brown because iodine is formed, showing a displacement reaction. Iodine does not cause a colour change because it is not reactive enough to displace iodide ions.
Common mistakes: Wrong products or unbalanced equation (part a); Bromine can displace chloride (incorrect, part b); Incorrect observation colours (part c)
Show mark scheme and worked solution
Marking points
A1 (a) · 1 mark · Correct balanced equation for bromine and potassium iodide: 2KI(aq) + Br2(aq) → 2KBr(aq) + I2(aq)
B1 (b) · 1 mark · States bromine is less reactive than chlorine OR cannot displace chlorine.
B2 (b) · 1 mark · Links to no reaction occurring (no colour change or no displacement).
B3 (c) · 1 mark · Describes brown colour appears with bromine but not with iodine in potassium iodide solution.
B4 (c) · 1 mark · Explains the difference as bromine displaces iodine from iodide, but iodine is not reactive enough to do so.
Worked solution
a) The balanced equation: 2KI(aq) + Br2(aq) → 2KBr(aq) + I2(aq)
b) Bromine is below chlorine in Group 7 and is less reactive, so it cannot displace chlorine from potassium chloride. No reaction occurs.
c) When bromine is added to potassium iodide, the solution turns brown because iodine is formed as bromine displaces iodine from the iodide ions. When iodine is added to potassium iodide, no colour change occurs because iodine cannot displace itself, and is not reactive enough to displace iodide ions from the solution.