Science (086) — AI-generated practice question

(i) Ethyne is an unsaturated hydrocarbon with a high carbon-to-hydrogen ratio. When burnt in air, the oxygen supply is insufficient for complete combustion, so it burns with a yellow, sooty flame and produces less heat. Pure oxygen ensures complete combustion, giving a much hotter, cleaner flame.

(ii) Ethyne (C₂H₂) has a higher proportion of carbon relative to hydrogen compared to ethane (C₂H₆), so it requires more oxygen per carbon atom for complete combustion.

Balanced equations:

Ethane: $2\text{C}_2\text{H}_6 + 7\text{O}_2 \rightarrow 4\text{CO}_2 + 6\text{H}_2\text{O}$

Ethyne: $2\text{C}_2\text{H}_2 + 5\text{O}_2 \rightarrow 4\text{CO}_2 + 2\text{H}_2\text{O}$

Ethyne needs 2.5 mol O₂ per carbon, while ethane needs 1.75 mol O₂ per carbon, confirming unsaturated hydrocarbons demand more oxygen.

Source: Chapter 4, Section 4.3.1 Combustion; Section 4.2.1 Saturated and Unsaturated Carbon Compounds

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• Part (i) is directly from the textbook: unsaturated hydrocarbons burn with a sooty, yellow flame due to insufficient oxygen; this is exactly why pure oxygen (not air) is used in oxy-acetylene welding.
• Part (ii): examiners expect both balanced equations AND a comparison of O₂ required per carbon atom. Show the ratio explicitly — this earns the justification mark.
• Common mistake: forgetting to balance equations properly or not relating the result back to the question's claim.