$3\text{ mL}$ of ethanol is taken in a test tube and warmed gently in a water bath. A $5\%$ solution of alkaline potassium permanganate is added first drop by drop to this solution, then in excess.

(i) How is $5\%$ solution of $\mathrm{KMnO_4}$ prepared?

(ii) State the role of alkaline potassium permanganate in this reaction. What happens on adding it in excess?

(iii) Write chemical equation of this reaction.

Carbon and its Compounds

Chemical Properties of Carbon Compounds

Official Answer

(i) A $5\%$ solution of potassium permanganate is prepared by dissolving $5\text{ g}$ of solid potassium permanganate ( $\mathrm{KMnO_4}$ ) in water to make a total volume of $100\text{ mL}$ of solution.

(ii) Role: Alkaline potassium permanganate acts as an oxidising agent because it provides oxygen to convert ethanol into ethanoic acid.

Observation in excess: When added drop by drop, the purple color of $\mathrm{KMnO_4}$ disappears as it reacts with ethanol. However, when added in excess, the purple color of the solution persists because all the ethanol has been completely oxidised to ethanoic acid, leaving no more ethanol to react with the additional $\mathrm{KMnO_4}$ .

(iii) The chemical equation for the reaction is:

$\mathrm{CH_3CH_2OH \xrightarrow{\text{Alkaline } KMnO_4 + \text{Heat}} CH_3COOH}$

5 g in 100 mLoxidising agentpurple color persistsethanolethanoic acidCH3CH2OHCH3COOHAlkaline KMnO4

Marking Scheme

1Preparation of $5\%$ solution ( $5\text{ g}$ of $\mathrm{KMnO_4}$ in water to make $100\text{ mL}$ solution) (1 mark)
2Stating the role as an oxidising agent and explaining that the purple color persists when added in excess (1 mark)
3Writing the correct balanced chemical equation with reaction conditions (Alkaline $\mathrm{KMnO_4}$ + Heat) (1 mark)

Hint

Think about how mass/volume percentage solutions are prepared, the definition of substances that add oxygen to other compounds, and how a reactant behaves when it is present in excess after the reaction is complete.

Quick Oral Answer

Alkaline potassium permanganate acts as an oxidising agent. It oxidises ethanol to ethanoic acid. When added in excess, its purple color persists because all the ethanol has already been oxidised, leaving no reactant to reduce the incoming permanganate ions.

Analysis & Explanation

In this reaction, ethanol undergoes complete oxidation. Alkaline potassium permanganate ( $\mathrm{KMnO_4}$ ) is a powerful oxidising agent. It easily transfers oxygen atoms to the ethanol molecule, converting the $-\mathrm{CH_2OH}$ group into a $-\mathrm{COOH}$ group.

Initially, the purple permanganate ions ( $\mathrm{MnO_4^-}$ ) are reduced to a colorless form (or brown manganese dioxide precipitate under certain conditions), which explains why the purple color disappears. Once the stoichiometric amount of ethanol is fully converted to ethanoic acid, any further addition of $\mathrm{KMnO_4}$ has no ethanol left to react with, causing the purple color of the excess $\mathrm{MnO_4^-}$ ions to persist.

Common Mistakes

1Writing the formula of ethanoic acid incorrectly as $\mathrm{CH_3CHO}$ (acetaldehyde) instead of $\mathrm{CH_3COOH}$ .
2Forgetting to mention 'Heat' or 'Warm' over the reaction arrow.
3Stating that $\mathrm{KMnO_4}$ is a reducing agent.
4Claiming that the purple color disappears even when added in excess.

Interesting Facts

Alkaline potassium permanganate is a very strong self-indicator because its deep purple color changes to colorless upon reduction, making it easy to see when the reaction is complete.

This reaction is a key method used in laboratories to synthesize carboxylic acids from primary alcohols.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2020?

This question carries 3 marks in the CBSE Class 10 Science 2020 examination.

Which chapter does this question come from in Science?

This question is from the chapter "Carbon and its Compounds" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Chemical Properties of Carbon Compounds" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2020 paper?

This is a Short Answer question from Section B in the CBSE Class 10 Science 2020 paper.