Displacement reactions as redox reactions
2 marks · EasyBromine water is added to a solution of sodium iodide. Explain, in terms of electron transfer, why bromine acts as an oxidising agent in this reaction.
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Bromine acts as an oxidising agent because it gains electrons from the iodide ions. Bromine (Br₂) takes electrons from I⁻ ions, so Br₂ is reduced to Br⁻ ions, while I⁻ ions lose electrons and are oxidised to I₂.
Also accept: Bromine gains electrons from iodide ions and is itself reduced; iodide ions lose electrons and are oxidised.; Br₂ gains electrons (is reduced); I⁻ loses electrons (is oxidised); bromine is the oxidising agent because it removes electrons from iodide.
Common mistakes: Incorrectly stating bromine loses electrons.; Mixing up oxidation and reduction definitions.
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Marking points
M1 · 1 mark · States or explains that bromine gains electrons from (or takes electrons from) iodide ions, or is reduced.
A1 · 1 mark · States that iodide ions lose electrons/are oxidised or correctly describes the electron transfer.
Worked solution
In this reaction, bromine acts as an oxidising agent because it takes electrons from the iodide ions.
Bromine (Br₂) molecules gain electrons (are reduced) to form bromide ions (Br⁻).
The iodide ions (I⁻) lose electrons (are oxidised) to form iodine (I₂).
Therefore, bromine is reduced and iodide is oxidised. Bromine is the oxidising agent because it gains electrons from iodide ions.