CAPF AC 2025 Q98: Science & Technology – Chemistry
- (a) 1, 2 and 3
- (b) 1, 3 and 4
- (c) 2 and 3 only
- (d) 4 only
Answer: (c) 2 and 3 only
Explanation
Oxidising an alcohol
How to think: NCERT names two reagents that oxidise ethanol to ethanoic acid: alkaline potassium permanganate and acidified potassium dichromate.
Hot concentrated sulphuric acid does a different job: it dehydrates ethanol to ethene.
| Reagent | What it does to ethanol |
|---|---|
| 1. Hot concentrated H₂SO₄ | Dehydrates it to ethene; not an oxidation |
| 2. Alkaline KMnO₄ | Oxidises it to ethanoic acid |
| 3. Acidified K₂Cr₂O₇ | Oxidises it to ethanoic acid |
| 4. Acidified KMnO₄ | Not named in the textbook as an alcohol oxidant |
So the answer built from the textbook is items 2 and 3.
Note: in a laboratory acidified KMnO₄ also oxidises alcohols, but no option contains 2, 3 and 4 together; (c) is the only option with the two textbook reagents and without the dehydrating acid.
Answer: (c) 2 and 3 only.
Why each option is right or wrong
- (a) Incorrect. Hot concentrated H₂SO₄ dehydrates alcohols; it is not the oxidising reagent here.
- (b) Incorrect. It includes hot concentrated H₂SO₄ and leaves out alkaline KMnO₄.
- (c) CORRECT. Alkaline KMnO₄ and acidified K₂Cr₂O₇ oxidise alcohols, as NCERT states.
- (d) Incorrect. It leaves out K₂Cr₂O₇ and alkaline KMnO₄, the textbook reagents.
Key point
Oxidising agents for alcohols (NCERT): alkaline KMnO₄ or acidified K₂Cr₂O₇ give ethanoic acid. Conc. H₂SO₄ at 443 K: dehydration of ethanol to ethene.
Remember
Alkaline KMnO₄ and acidified K₂Cr₂O₇ oxidise alcohols; hot conc. H₂SO₄ dehydrates them.
Common mistakes
- Treating the dehydrating action of hot conc. H₂SO₄ as oxidation.
- Forgetting that potassium dichromate must be acidified.
- Mixing oxidation (reagent adds oxygen) with dehydration (reagent removes water).
Exam tip
Educated guess: 'alkaline KMnO₄ and acidified K₂Cr₂O₇' is the stock phrase for alcohol oxidation. Only (c) contains exactly those two.
Sources
- NCERT Class X Science, Chapter 4 (Carbon and its Compounds: oxidation of ethanol and dehydration)
- NCERT Class XII Chemistry, Part II, Chapter 11 (Alcohols, Phenols and Ethers: oxidation of alcohols)