Science (086) — CBSE Class 10 board question

(a) Molecular formula of ethyne: C₂H₂

Electron dot structure of ethyne:

$$\text{H} : \text{C} :: : \text{C} : \text{H}$$

(Each carbon shares 3 pairs of electrons with the other carbon — a triple bond — and 1 pair with hydrogen.)

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(b) Reactions of ethanol:

(i) With Sodium metal:
$$2\text{Na} + 2\text{CH}_3\text{CH}_2\text{OH} \rightarrow 2\text{CH}_3\text{CH}_2\text{O}^-\text{Na}^+ + \text{H}_2\uparrow$$
Product: Sodium ethoxide and hydrogen gas.

(ii) With Ethanoic acid (acid as catalyst) — Esterification:
$$\text{CH}_3\text{CH}_2\text{OH} + \text{CH}_3\text{COOH} \underset{\Delta}{\overset{\text{H}^+}{\rightleftharpoons}} \text{CH}_3\text{COOC}_2\text{H}_5 + \text{H}_2\text{O}$$
Product: Ethyl ethanoate (an ester) and water.

(iii) With Acidified Potassium Dichromate:
$$\text{CH}_3\text{CH}_2\text{OH} \xrightarrow{\text{Acidified K}_2\text{Cr}_2\text{O}_7} \text{CH}_3\text{COOH}$$
Product: Ethanoic acid (acetic acid). This is an oxidation reaction.

Source: Chapter 4, Section 4.4.1 (Reactions of Ethanol); Section 4.3.2 (Oxidation)

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• (a): Ethyne (C₂H₂) has a triple bond between the two carbons. In the electron dot structure, show 3 pairs of shared electrons (6 dots) between C atoms and 1 pair between each C and H. Examiners specifically check for the triple bond representation.
• (b)(i): The equation is directly from the textbook — memorise it exactly, including sodium ethoxide as the named product.
• (b)(ii): Esterification is the reaction between an alcohol and a carboxylic acid in the presence of an acid catalyst (conc. H₂SO₄), producing an ester. The product name (ethyl ethanoate) must be written.
• (b)(iii): Acidified K₂Cr₂O₇ acts as an oxidising agent, converting ethanol → ethanoic acid. The textbook confirms this is an oxidation reaction. Writing "oxidising agent" earns an extra mark.