The Halogens
5 marks · EasyA student adds bromine water to a solution of potassium iodide.
Write the ionic equation for the reaction that takes place.
State and explain what you would observe when bromine water is added to potassium iodide solution.
Predict whether a similar reaction would occur if the student added iodine solution to potassium bromide solution. Explain your answer in terms of halogen reactivity.
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) 2I^−(aq) + Br_2(aq) → 2Br^−(aq) + I_2(aq) · must show Both halide and halogen correct on both sides, No spectator ions
(b) The solution turns brown because iodine is formed. An explanation that iodide ions are oxidised to iodine by bromine. · must show Brown colour observed due to iodine, Explanation linked to oxidation of iodide ions by bromine
(c) No reaction would occur. Iodine is less reactive than bromine and cannot displace bromide ions from solution. · must show Prediction: no reaction, Explanation refers to trend in halogen reactivity
Common mistakes: Including spectator ions in the ionic equation; Assuming colourless solution instead of brown for iodine
Show mark scheme and worked solution
Marking points
A1 (a) · 1 mark · Correct complete ionic equation with correct state symbols, showing only halide and halogen ions.
B1 (b) · 1 mark · Correct observation: solution turns brown (or similar wording for appearance of iodine).
B2 (b) · 1 mark · Explanation: iodide ions are oxidised by bromine, or reference to bromine being a stronger oxidising agent.
C1 (c) · 1 mark · Prediction: no reaction occurs between iodine and potassium bromide.
C2 (c) · 1 mark · Explanation: iodine is less reactive than bromine and cannot displace bromide ions.
Worked solution
a. The ionic equation for the reaction is:
b. When bromine water is added to potassium iodide solution, the solution turns brown. This is because the bromine reacts with iodide ions, oxidising them to iodine, which is brown in solution.
c. No reaction would occur if iodine solution was added to potassium bromide solution, because iodine is less reactive than bromine and cannot displace bromide ions from solution.