Physical properties
5 marks · HardThe physical properties of alkenes with 2 to 4 carbon atoms show a trend.
State the physical state of ethene, propene and butene at room temperature.
Describe how the boiling points of these alkenes change as the number of carbon atoms increases from ethene to butene.
Explain why the trend you described in part (b) occurs.
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) All are gases at room temperature.
(b) The boiling point increases as the number of carbon atoms increases from ethene to butene.
(c) Because larger alkenes have stronger intermolecular forces (van der Waals forces), so more energy is needed to separate the molecules.
Common mistakes: Incorrectly identifying physical state; Reversing the boiling point trend
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Marking points
B1 (a) · 2 marks · 1 mark for correctly stating all three are gases at room temperature, 1 mark for listing all three names (ethene, propene, butene) with correct states.
B2 (b) · 2 marks · 1 mark for stating the boiling point increases, 1 mark for referencing the trend with increasing carbon number.
B3 (c) · 1 mark · 1 mark for correctly explaining the increase in boiling point is due to stronger intermolecular (van der Waals) forces with longer carbon chains.
Worked solution
a) Ethene, propene and butene are all gases at room temperature.
b) The boiling point increases as you go from ethene to propene to butene; larger alkenes have higher boiling points.
c) As the size of the alkene increases, the molecules have stronger intermolecular forces (van der Waals forces), so more energy is needed to change them into a gas.