Read the following and answer the questions that follow:
Preeti carried out an experiment in the school laboratory. She took some ethanol in a test tube and warmed it gently in a water bath. She then added alkaline potassium permanganate solution drop by drop. She noted that the purple colour disappeared initially. When she added excess potassium permanganate, the colour persisted. Her teacher told her that the product formed has a sharp smell and turns blue litmus red. The teacher further demonstrated that when this product is added to a solution of sodium carbonate, brisk effervescence is observed.
(i) Name the product formed when ethanol is oxidised by alkaline potassium permanganate. (1 mark)
(ii) Why does the purple colour of potassium permanganate disappear when added initially, but persist when added in excess? (1 mark)
(iii) Write the balanced chemical equation for the reaction of this product with sodium carbonate solution. (1 mark)
(iv) Name the class of organic compound to which the product belongs and write its functional group. (1 mark)
Generated by claude-sonnet-4-6 · 2026-06-26 01:08 · grounding rag
Model Answer
(i) The product formed is ethanoic acid (CH₃COOH). It has a sharp smell and turns blue litmus red.
(ii) Initially, the KMnO₄ is consumed in oxidising ethanol to ethanoic acid, so the purple colour disappears. When ethanol is completely used up, excess KMnO₄ has nothing left to oxidise, so the purple colour persists.
(iii) Balanced equation:
$$2CH_3COOH + Na_2CO_3 \rightarrow 2CH_3COONa + H_2O + CO_2$$
(Brisk effervescence is due to CO₂ gas evolved.)
(iv) The product belongs to the class carboxylic acids. Its functional group is –COOH (carboxyl group).
Source: Chapter 4, Section 4.3.2 – Oxidation
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Explanation
- (i) Ethanol + alkaline KMnO₄ → ethanoic acid is the standard oxidation reaction; examiners expect the IUPAC name and formula.
- (ii) The key logic: KMnO₄ acts as the oxidising agent and gets reduced (loses purple colour) until all ethanol is oxidised; excess KMnO₄ remains unreacted.
- (iii) Write the balanced equation carefully — common error is forgetting to balance the 2 moles of acid vs. 1 mole of Na₂CO₃.
- (iv) "Carboxylic acid" as the class and "–COOH" as the functional group must both be stated for full credit.