How the physical properties of hydrocarbons change with molecule size
4 marks · NormalCrude oil is separated into fractions by fractional distillation. Each fraction contains hydrocarbons of different chain lengths.
Name one fraction obtained from crude oil and state a common use for this fraction.
Describe two ways in which the physical properties of the hydrocarbons in this fraction differ from those in the fraction with the largest molecules.
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(a) E.g. gasoline (petrol) — used as fuel for cars; kerosene — used as jet fuel; diesel — used as diesel fuel, etc.
(b) Small hydrocarbon fraction: lower boiling point, less viscous, more volatile, lighter colour, burns more easily, compared to large hydrocarbon fraction.
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Marking points
B1 (a) · 1 mark · Names a correct fraction of crude oil.
B2 (a) · 1 mark · States a suitable use for that fraction.
B3 (b) · 1 mark · Gives one correct difference in physical property (e.g., lower boiling point, less viscous, more volatile, burns more easily).
B4 (b) · 1 mark · Gives a second correct difference in physical property.
Worked solution
a) Petrol (gasoline) is a fraction of crude oil. It is commonly used as a fuel in cars.
b) The hydrocarbons in petrol have a lower boiling point and are less viscous (more runny) than those in bitumen (the fraction with the largest molecules). They are also more volatile and burn more easily.