Q60 · Practice questionSimple familiar4 marks
QUESTION 60 (4 marks)
The graph gives approximate normal boiling points of the first five straight-chain primary alcohols.
a) Explain the upward trend. [2 marks]
b) Explain why ethanol has a much higher boiling point than ethane despite having only one additional oxygen atom. [2 marks]
WORKED SOLUTION
4 marksPractice marking scheme
a) Increasing chain size strengthens dispersion attractions. b) Ethanol forms intermolecular hydrogen bonds, unlike ethane.
Larger electron clouds are more polarisable and longer molecules offer greater contact area, strengthening dispersion forces. Alcohols also hydrogen-bond through O–H groups. Ethane has only dispersion attractions.
- Connects increasing chain length to larger electron cloud/contact area. [1 marks]
- Connects stronger dispersion forces to higher boiling point. [1 marks]
- Identifies ethanol hydrogen bonding. [1 marks]
- Contrasts with only dispersion attractions in ethane. [1 marks]
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