How the physical properties of hydrocarbons change with molecule size
5 marks · HardCrude oil is separated into fractions by fractional distillation. Two fractions, A and B, are collected at different heights in the fractionating column.
Fraction A consists of large hydrocarbon molecules, while fraction B consists of small hydrocarbon molecules. State which fraction is collected higher up the column. Give a reason for your answer.
Describe how the physical properties (boiling point and viscosity) of fraction A compare to those of fraction B.
Suggest one use for fraction B, based on its physical properties.
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(a) Fraction B (small molecules) is collected higher up the column because it has a lower boiling point than fraction A.
(b) Fraction A has a higher boiling point and is more viscous than fraction B.
(c) Fraction B can be used as a fuel for cars (petrol/gasoline) or for heating (gas fuel).
Common mistakes: Mix up properties of small and large fractions
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Correct fraction named: Fraction B (small molecules) is collected higher up the column.
M1 (a) · 1 mark · Correct reason: small molecules have lower boiling points / collected at top because they condense last / are gases at lowest temperatures.
B2 (b) · 1 mark · Boiling point: Fraction A has a higher boiling point than B.
B3 (b) · 1 mark · Viscosity: Fraction A is more viscous than B (or B is more runny).
B4 (c) · 1 mark · Suitable use for fraction B linked to property: e.g. fuel for cars, domestic gas, or bottled gas.
Worked solution
a) Fraction B is collected higher up the column because it has smaller molecules and a lower boiling point. b) Fraction A has a higher boiling point and is more viscous (flows less easily) than fraction B. c) Fraction B can be used as a fuel for cars, bottled gas, or heating.