The Halogens
7 marks · HardThis question is about the properties and reactions of halogens.
State the physical state and colour of bromine and iodine at room temperature (20 °C).
Aqueous chlorine is added to a solution containing potassium iodide. The solution changes colour.
Write the ionic equation, including state symbols, for the reaction that takes place.
Explain why chlorine can displace iodine from potassium iodide solution but not the other way around.
Identify the oxidising agent and the substance that is reduced in this reaction.
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) Bromine: brown liquid; Iodine: dark grey solid · must show Bromine = liquid, brown/red-brown/amber, Iodine = solid, dark grey/black
(b(i)) Cl2(aq) + 2I^-(aq) -> 2Cl^-(aq) + I2(aq) · must show Cl2(aq), 2I^-(aq), 2Cl^-(aq), I2(aq), all correct state symbols
(b(ii)) Chlorine is more reactive than iodine. Therefore, chlorine can displace iodine from its compounds, but iodine cannot displace chlorine. · must show correctly compares reactivity (chlorine > iodine), states that more reactive halogen displaces less reactive
(b(iii)) Oxidising agent: chlorine; substance reduced: chlorine/Cl2 (accept Cl2 or chlorine) · must show chlorine is oxidising agent, chlorine/Cl2 is reduced
Common mistakes: Giving symbol instead of colour; Leaving out state symbols; Confusing oxidising and reducing agent
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Bromine: states liquid and correct colour (brown, red-brown or orange)
B2 (a) · 1 mark · Iodine: states solid and correct colour (dark grey or black)
M1 (b(i)) · 1 mark · Correct chemical equation, balanced (allow correct species)
A1 (b(i)) · 1 mark · All correct state symbols (aq for ions and halogens, except I2 as aq or s allowed)
B3 (b(ii)) · 1 mark · States that chlorine is more reactive than iodine
B4 (b(ii)) · 1 mark · Refers that a more reactive halogen displaces a less reactive one (or equivalent)
B5 (b(iii)) · 1 mark · Identifies chlorine as oxidising agent and that it is reduced
Worked solution
a) Bromine is a brown liquid at room temperature; iodine is a dark grey solid at room temperature.
b)(i) The ionic equation for the displacement reaction is:
b)(ii) Chlorine is more reactive than iodine because it is higher up in Group 7. More reactive halogens can displace less reactive halogens from their compounds, so chlorine can displace iodine but not the other way around.
b)(iii) Chlorine is the oxidising agent (it causes iodide ions to lose electrons and is itself reduced to chloride ions). The substance reduced is chlorine (Cl2).