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2021 O Level Chem P1 Q33 — Alkanes / Combustion Solution

Alkanes / Combustion 2021 O Level Pure Chemistry · Paper 1 · Q33

Question

Which equations are correct for alkanes burning in air? 1 C₃H₈ + 5O₂ → 3CO₂ + 4H₂O. 2 C₄H₈ + 5O₂ → 4CO₂ + 4H₂O. 3 C₂H₆ + 3O₂ → 2CO₂ + 3H₂O.

A1 and 2
B1 only
C2 and 3
D3 only

Correct answer: B

Video Solution

Explanation

Answer: B (1 only). Check two things for each equation: that the formula really is an alkane (general formula CₙH₂ₙ₊₂) and that the equation is balanced.

Equation 1, C₃H₈ + 5O₂ → 3CO₂ + 4H₂O: propane is an alkane (n = 3, 2n+2 = 8) and the equation balances (C 3=3, H 8=8, O 10=10) ✓.

Equation 2 uses C₄H₈, which is an alkene (CₙH₂ₙ), not an alkane, and it is also unbalanced ✗. Equation 3 uses the alkane C₂H₆ but is not balanced (oxygen is 6 on the left, 7 on the right; the balanced version is 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O) ✗.

Only equation 1 is correct.

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