Redox Reactions: Question 10
Syllabus 6.4
A student tests three unknown colourless aqueous solutions, X, Y and Z, using the two standard test reagents for oxidising and reducing agents. The results are shown in the table.
| Solution | Acidified potassium manganate(VII) | Aqueous potassium iodide |
|---|---|---|
| X | Purple colour remains (no change) | Colourless to brown |
| Y | Purple colour turns colourless | No colour change |
| Z | Purple colour remains (no change) | No colour change |
(a) Identify which solution acts as an oxidising agent, and explain your answer using the results in the table. [2]
(b) Identify which solution acts as a reducing agent, and explain your answer using the results in the table. [2]
(c) State what the results for solution Z show about its behaviour with these two reagents. [1]
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Worked solution
Part (a): Identifying the oxidising agent
Aqueous potassium iodide turning from colourless to brown, as iodine forms, is the standard test for an oxidising agent. Only solution X produces this colour change with potassium iodide. X also does not decolourise the acidified potassium manganate(VII), so it is not behaving as a reducing agent. This confirms X is an oxidising agent.
Part (b): Identifying the reducing agent
Acidified potassium manganate(VII) turning from purple to colourless is the standard test for a reducing agent. Only solution Y produces this colour change. Y also produces no colour change with potassium iodide, so it is not behaving as an oxidising agent. This confirms Y is a reducing agent.
Part (c): Interpreting solution Z
Solution Z produces no colour change with either reagent: the potassium manganate(VII) stays purple, and the potassium iodide stays colourless. This means Z is behaving as neither an oxidising agent nor a reducing agent towards these two reagents.
Final answers
- (a) Solution X is the oxidising agent, it turns potassium iodide from colourless to brown.
- (b) Solution Y is the reducing agent, it turns acidified potassium manganate(VII) from purple to colourless.
- (c) Solution Z shows neither oxidising nor reducing behaviour with these reagents.