How the physical properties of hydrocarbons change with molecule size
3 marks · NormalThe boiling points of fractions from crude oil are related to the number of carbon atoms in their molecules.
| Fraction | Typical Number of Carbon Atoms | Boiling Point (°C) |
|---|---|---|
| Gasoline (petrol) | 8 | 110 |
| Diesel | 15 | 260 |
Calculate the increase in boiling point per additional carbon atom between these two fractions.
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Approximately 21.43 °C per carbon atom °C per atom (±0.1)
Also accept: 21.4; 21.43
Common mistakes: Dividing the boiling points, not the difference
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Marking points
M1 (a) · 1 mark · Finds the difference in boiling points (260-110 = 150 °C).
M2 (a) · 1 mark · Finds the difference in number of carbon atoms (15-8 = 7).
A1 (a) · 1 mark · Divides correctly to give 150/7 = 21.43 °C per carbon atom.
Worked solution
Difference in boiling point = 260 - 110 = 150 °C
Difference in number of carbon atoms = 15 - 8 = 7