A student has two unlabelled bottles, each containing a different carbonyl compound. One bottle contains pentan-2-one and the other contains pentanal. The student carries out two chemical tests to identify which bottle contains which compound. Describe how the student could use Fehling's solution and acidified potassium dichromate(VI) solution to distinguish between pentan-2-one and pentanal, and explain the observations expected for each test with each compound.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
1. Fehling’s solution (Cu²⁺ in alkaline solution) is added to each bottle.
• Pentanal (an aldehyde) is oxidised and reduces Cu²⁺ to Cu⁺, giving a brick‑red/orange precipitate of Cu₂O.
• Pentan‑2‑one (a ketone) is not oxidised under these conditions and no colour change or precipitate occurs.
2. Acidified potassium dichromate(VI) solution is added to each bottle.
• Pentanal is oxidised, reducing the orange dichromate to green Cr²⁺; the solution turns green.
• Pentan‑2‑one is not oxidised; the orange colour remains unchanged.
Thus the bottle that gives a brick‑red precipitate with Fehling’s and turns green with dichromate contains pentanal; the other bottle contains pentan‑2‑one.
• Pentanal (an aldehyde) is oxidised and reduces Cu²⁺ to Cu⁺, giving a brick‑red/orange precipitate of Cu₂O.
• Pentan‑2‑one (a ketone) is not oxidised under these conditions and no colour change or precipitate occurs.
2. Acidified potassium dichromate(VI) solution is added to each bottle.
• Pentanal is oxidised, reducing the orange dichromate to green Cr²⁺; the solution turns green.
• Pentan‑2‑one is not oxidised; the orange colour remains unchanged.
Thus the bottle that gives a brick‑red precipitate with Fehling’s and turns green with dichromate contains pentanal; the other bottle contains pentan‑2‑one.
Examiner tips
- Use the command word ‘describe’ – give the sequence of steps and the expected observation for each compound.
- Mention that aldehydes are reducing agents and ketones are not, linking to the colour changes.
- Show the colour change for each reagent clearly for each compound.
Common mistakes
- Confusing the colour change of Fehling’s (brick‑red) with that of dichromate (green).
- Forgetting that ketones do not react with either reagent under these conditions.
Mark scheme (5 marks)
- Fehling's solution turns from blue to brick-red/orange precipitate with pentanal only
- Aldehydes (pentanal) are oxidised / act as reducing agents, so reduce Cu²⁺ to Cu⁺ (copper(I) oxide), which is the brick-red precipitate
- Ketones (pentan-2-one) do not react with Fehling's solution because ketones cannot be oxidised under mild conditions / are not reducing agents
- Acidified potassium dichromate(VI) changes colour from orange to green with pentanal
- Acidified potassium dichromate(VI) remains orange with pentan-2-one because ketones are not oxidised by this reagent under these conditions
Key terms in this question
carbonyl compound · Fehling's solution · acidified potassium dichromate(VI)
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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