The Halogens
5 marks · EasyChlorine, bromine and iodine are Group 7 elements. In the laboratory, a student mixes chlorine water with potassium iodide solution.
Write the ionic equation for the reaction that occurs between chlorine water and potassium iodide solution.
State what you would observe during this reaction.
Explain why chlorine is able to displace iodine from potassium iodide solution, but bromine cannot.
Predict what you would see if bromine water was added to potassium chloride 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) Cl2(aq) + 2I^-(aq) -> 2Cl^-(aq) + I2(aq) · must show balanced ionic equation, chlorine and iodide ions as reactants, chloride ions and iodine as products
(b) The solution turns brown (due to iodine formation). · must show brown colour, reference to iodine forming
(c) Chlorine is more reactive than iodine because chlorine atoms are smaller, so gained electrons are attracted more strongly to the nucleus. Therefore, chlorine can displace iodine from its compounds, but bromine is less reactive than chlorine and cannot displace it. · must show chlorine more reactive than iodine, reference to stronger attraction for electrons or position in group, explanation relating to size or electron attraction
(d) No reaction would occur: There is no colour change (the solution stays the same). Explanation: bromine cannot displace chlorine because chlorine is more reactive. · must show no reaction/no colour change, bromine less reactive than chlorine
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Marking points
A1 (a) · 1 mark · Correct, fully balanced ionic equation written.
B1 (b) · 1 mark · Brown colour or reference to iodine formation observed.
M1 (c) · 1 mark · Recognises chlorine more reactive than iodine/bromine OR references group trend.
E1 (c) · 1 mark · Explains the reason for the group reactivity difference using electron attraction or atomic size.
B2 (d) · 1 mark · States no reaction/no colour change and connects this to bromine being less reactive than chlorine.
Worked solution
a) The ionic equation is:
b) Observation: The solution turns brown, showing iodine has formed.
c) Chlorine is more reactive than iodine because its atoms are smaller and gained electrons are more strongly attracted to the nucleus. This greater attraction means chlorine can displace iodine from compounds, but bromine cannot because it is less reactive.
d) There will be no reaction and no colour change. Bromine is less reactive than chlorine so cannot displace chlorine from chloride ions.