Physical properties
4 marks · NormalThe table below gives information about three alkenes.
| Alkene | Number of carbon atoms | Boiling point (°C) |
|---|---|---|
| Ethene | 2 | -104 |
| Propene | 3 | -47 |
| Butene | 4 | -6 |
Describe the trend in boiling points shown in the table.
Explain why the boiling point increases as the number of carbon atoms increases.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) — · must show Boiling point increases as number of carbon atoms increases, Boiling point becomes less negative/more positive
(b) — · must show Larger molecules = stronger intermolecular forces/van der Waals, More energy needed to separate larger alkenes
Common mistakes: Reversing the trend; Not relating to forces
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Correctly states boiling point increases with number of carbons
B2 (a) · 1 mark · Mentions less negative/more positive values as chain length increases
B3 (b) · 1 mark · Explains larger molecules = stronger intermolecular forces
B4 (b) · 1 mark · States more energy/heat is needed to separate/boil bigger alkenes
Worked solution
(a) The boiling point increases as the number of carbon atoms increases. The boiling point values become less negative.
(b) As the number of carbons increases, the molecule is larger. Larger molecules have stronger intermolecular (van der Waals) forces, so more energy is needed to boil them.