A saturated organic compound 'A' belongs to the homologous series of alcohols. On heating 'A' with concentrated sulphuric acid at 443 K, it forms an unsaturated compound 'B' with molecular mass 28 u. The compound 'B' on addition of one mole of hydrogen in the presence of Nickel, changes to a saturated hydrocarbon 'C'.
Generated by claude-sonnet-4-6 · 2026-06-15 06:51 · grounding rag
Model Answer
(i) Identification:
- A = Ethanol (C₂H₅OH) — a saturated alcohol
- B = Ethene (C₂H₄) — molecular mass = 2(12) + 4(1) = 28 u ✓ (unsaturated)
- C = Ethane (C₂H₆) — saturated hydrocarbon
(ii) Conversion of A into B (dehydration):
$$\text{C}_2\text{H}_5\text{OH} \xrightarrow{\text{conc. H}_2\text{SO}_4,\ 443\text{ K}} \text{CH}_2{=}\text{CH}_2 + \text{H}_2\text{O}$$
(iii) Combustion of C (Ethane):
Ethane burns in air/oxygen to form carbon dioxide and water, releasing heat and light:
$$2\text{C}_2\text{H}_6 + 7\text{O}_2 \rightarrow 4\text{CO}_2 + 6\text{H}_2\text{O} + \text{heat and light}$$
(iv) Industrial application of hydrogenation:
Hydrogenation is used to convert vegetable oils (unsaturated) into vanaspati ghee (solid fat) using a nickel catalyst.
(v) Reaction of Ethanol with Sodium:
When ethanol reacts with sodium, sodium ethoxide and hydrogen gas are produced:
$$2\text{C}_2\text{H}_5\text{OH} + 2\text{Na} \rightarrow 2\text{C}_2\text{H}_5\text{ONa} + \text{H}_2\uparrow$$
Products: Sodium ethoxide and hydrogen gas.
Source: Chapter 4, Sections 4.3.3, 4.2.4
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Explanation
- Key logic for A: Molecular mass of B = 28 u → C₂H₄ (ethene). Since B forms by dehydration of an alcohol A, A must be ethanol (C₂H₅OH).
- Examiners expect balanced equations with conditions (conc. H₂SO₄, 443 K) for part (ii) — don't skip these.
- For combustion (iii), write a balanced equation; saying "CO₂ and H₂O are formed" without the equation loses marks.
- The hydrogenation application (iv) — vegetable oil → vanaspati — is the standard textbook answer; always mention the Ni catalyst.
- For part (v), name both products clearly: sodium ethoxide + hydrogen gas.