Ethanal and propanone are both carbonyl compounds. Describe how Tollens' reagent can be used to distinguish between ethanal and propanone, and explain the chemistry that accounts for the different results observed.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Tollens' reagent is an ammoniacal solution of silver nitrate, containing Ag⁺ ions.
1. When ethanal is added, the aldehyde is oxidised to ethanoic acid while the Ag⁺ ions are reduced to metallic silver.
2. The silver metal deposits as a silver mirror on the inner wall of the test tube.
3. Propanone, being a ketone, is not oxidised under these conditions; the Ag⁺ ions remain unchanged.
4. Consequently no silver mirror forms and the solution stays colourless.
5. Thus the presence or absence of a silver mirror distinguishes ethanal from propanone.
1. When ethanal is added, the aldehyde is oxidised to ethanoic acid while the Ag⁺ ions are reduced to metallic silver.
2. The silver metal deposits as a silver mirror on the inner wall of the test tube.
3. Propanone, being a ketone, is not oxidised under these conditions; the Ag⁺ ions remain unchanged.
4. Consequently no silver mirror forms and the solution stays colourless.
5. Thus the presence or absence of a silver mirror distinguishes ethanal from propanone.
Examiner tips
- Mention that Tollens' reagent is ammoniacal AgNO₃ and contains Ag⁺. Explain the redox: aldehyde → carboxylic acid, Ag⁺ → Ag. State the result: silver mirror for ethanal, none for propanone. Keep answer to 5 points, use correct terminology.
- common_mistakes
- :
- Confusing the reaction of ketones with aldehydes; claiming a mirror forms for propanone. Forgetting to specify that the silver is reduced to metallic silver. Using vague terms like "oxidised" without linking to the formation of ethanoic acid.
Mark scheme (5 marks)
- Tollens' reagent contains silver ions (Ag⁺) / is an ammoniacal silver nitrate solution
- Ethanal (the aldehyde) produces a silver mirror on the test tube / a grey/silver precipitate forms with ethanal
- Propanone (the ketone) gives no reaction / no silver mirror / solution remains colourless
- Ethanal is oxidised (by the Tollens' reagent) / aldehydes are reducing agents that are oxidised to carboxylic acids (ethanoic acid)
- The silver ions (Ag⁺) are reduced to silver metal (Ag), forming the silver mirror / ketones cannot be oxidised under these conditions so no reduction of silver ions occurs
Key terms in this question
carbonyl compound · Tollens' reagent
Related
- All Eduqas A-Level Chemistry revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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