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2019 O Level Chem P1 Q32 — Petroleum / Fractional Distillation Solution

Petroleum / Fractional Distillation 2019 O Level Pure Chemistry · Paper 1 · Q32

Question

Petroleum can be separated into fractions using fractional distillation. Which statements are correct? 1 Alkanes used in polishes and waxes have a higher boiling point than those used as diesel fuel. 2 Any of the fractions could be used as fuels because their enthalpy changes of combustion are negative. 3 The fraction used for petrol (gasoline) is extracted from higher up the fractionating column than the fraction used for paraffin (kerosene). 4 The fraction obtained at a particular point in the fractionating column always contains the same compounds in the same ratio.

A1, 2 and 3
B1 and 4
C2 only
D3 and 4

Correct answer: A

Video Solution

Explanation

Answer: A (statements 1, 2 and 3). Check each statement about fractional distillation of petroleum.

Statement 1: correct. Alkanes in polishes and waxes are very long-chain molecules (C20 and above) with high boiling points, higher than the diesel fraction (around C12–C20).

Statement 2: correct. Every hydrocarbon fraction burns exothermically, so each has a negative enthalpy change of combustion and could in principle be used as a fuel.

Statement 3: correct. Petrol (gasoline, around C5–C10) is more volatile and condenses higher up the cooler top of the fractionating column, while kerosene (paraffin, around C10–C16) condenses lower down where it is hotter. So petrol is collected higher than kerosene.

Statement 4 is false: the exact composition of a fraction depends on the crude oil source, so it is not always the same compounds in the same ratio. Statements 1, 2 and 3 are correct, giving A.

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