A carbon compound 'A' on heating with excess conc. H₂SO₄ forms a compound 'B', which on addition of one mole of hydrogen gas in the presence of nickel catalyst forms a compound 'C'. 'C' on combustion in air forms 2 moles of carbon dioxide and 3 moles of water. Identify 'A', 'B' and 'C' and write their structures. Give chemical equations of the reactions involved. Also state the role of concentrated sulphuric acid in the formation of 'B' from 'A'.
Generated by claude-sonnet-4-6 · 2026-06-15 06:49 · grounding rag
Model Answer
Identification:
Combustion of C gives 2CO₂ and 3H₂O → C has 2 carbon and 6 hydrogen atoms → C = Ethane (C₂H₆).
C + H₂ → B, so B = Ethene (C₂H₄) and A = Ethanol (C₂H₅OH).
Structures:
- A (Ethanol): CH₃–CH₂–OH
- B (Ethene): CH₂=CH₂
- C (Ethane): CH₃–CH₃
Chemical Equations:
- A → B (Dehydration):
$$\text{CH}_3\text{CH}_2\text{OH} \xrightarrow{\text{conc. H}_2\text{SO}_4, \Delta} \text{CH}_2{=}\text{CH}_2 + \text{H}_2\text{O}$$
- B → C (Hydrogenation):
$$\text{CH}_2{=}\text{CH}_2 + \text{H}_2 \xrightarrow{\text{Ni}} \text{CH}_3\text{CH}_3$$
- Combustion of C:
$$2\text{CH}_3\text{CH}_3 + 7\text{O}_2 \rightarrow 4\text{CO}_2 + 6\text{H}_2\text{O}$$
(1 mole gives 2CO₂ + 3H₂O ✓)
Role of conc. H₂SO₄: It acts as a dehydrating agent, removing water from ethanol to form ethene. It also acts as a catalyst in the reaction.
Source: Chapter 4, Section 4.3
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Explanation
- Work backwards from combustion data: 2CO₂ → 2C atoms; 3H₂O → 6H atoms → C is C₂H₆ (ethane).
- C₂H₆ + H₂ (addition) → C is obtained from B, so B = C₂H₄ (ethene, unsaturated).
- Ethene is formed by dehydration of ethanol → A = ethanol (C₂H₅OH).
- Examiners specifically look for: correct identification with justification, all three structures, all three balanced equations, and the role of H₂SO₄ as a dehydrating agent — each carries marks.