Displacement reactions as redox reactions
5 marks · HardIodine solution is added to potassium chloride solution. No reaction is observed. Consider the redox processes involved in displacement reactions between halogens and halide ions.
Explain, in terms of electron transfer, why iodine does not displace chloride ions from potassium chloride solution.
A reaction does occur when bromine water is added to potassium iodide solution. State which species is oxidised and which is reduced in this reaction.
Write the balanced ionic equation, including state symbols, for the reaction between bromine solution and iodide ions.
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) Iodine is a weaker oxidising agent than chlorine, so it cannot remove electrons from chloride ions. There is no transfer of electrons and therefore no reaction. · must show Reference to relative oxidising ability or tendency to gain electrons, Mention of electron transfer or lack thereof
(b) Iodide ions are oxidised; bromine is reduced. · must show Identifies iodide is oxidised and bromine is reduced
(c) 2I^−(aq) + Br_2(aq) → 2Br^−(aq) + I_2(aq) · must show Balanced ionic equation with state symbols
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Mentions that iodine is a weaker oxidising agent than chlorine/halogens above it or cannot remove electrons from chloride ions.
A1 (a) · 1 mark · States clearly that there is no electron transfer and so no reaction occurs.
B1 (b) · 1 mark · Identifies that iodide ions (I^−) are oxidised.
B2 (b) · 1 mark · Identifies that bromine (Br_2) is reduced.
C1 (c) · 1 mark · Writes correct balanced ionic equation for the reaction including state symbols.
Worked solution
a) Iodine cannot displace chloride ions as it is a weaker oxidising agent and cannot remove electrons from the chloride ions. Therefore, no electrons are transferred and no reaction occurs.
b) In the reaction of bromine water with potassium iodide, the iodide ions lose electrons (are oxidised) and bromine molecules gain electrons (are reduced).
c) The balanced ionic equation is: