Displacement reactions involving the halogens
4 marks · NormalA student investigates the reactivity of three halogens: chlorine, bromine and iodine, using solutions of their salts.
Explain, with an equation, what happens when bromine solution is added to potassium iodide solution.
The student adds iodine solution to potassium chloride solution. State and explain the observation.
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) Bromine displaces iodine to form iodine, which makes the solution brown. Equation: 2KI(aq) + Br2(aq) -> 2KBr(aq) + I2(aq)
(b) No change; no reaction takes place because iodine is less reactive than chlorine.
Also accept: Equations may omit state symbols but must be correct
Common mistakes: Reversing products and reactants; Saying a reaction occurs in part b
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Identifies that bromine displaces iodine, or describes formation of brown solution.
A1 (a) · 1 mark · Correct equation: 2KI(aq) + Br2(aq) -> 2KBr(aq) + I2(aq).
M2 (b) · 1 mark · States no reaction or no change is observed.
A2 (b) · 1 mark · Explanation: iodine is less reactive than chlorine so cannot displace it.
Worked solution
(a) When bromine solution is added to potassium iodide solution, bromine displaces iodine. The solution turns brown because iodine forms.
(b) When iodine solution is added to potassium chloride solution, no reaction occurs and no colour change is seen because iodine is less reactive than chlorine.