Q68 · Practice questionComplex familiar5 marks
QUESTION 68 (5 marks)
Butan-1-ol and butan-2-one have similar molar masses but boiling points of approximately 118 °C and 80 °C respectively.
a) Explain the boiling-point difference using intermolecular forces. [3 marks]
b) Explain why both are more soluble in water than butane. [2 marks]
WORKED SOLUTION
5 marksPractice marking scheme
Butan-1-ol forms hydrogen bonds between its own molecules; butan-2-one does not. Both can interact with water through their oxygen atoms.
Both have dispersion and dipole attractions. The alcohol additionally donates and accepts hydrogen bonds, giving stronger intermolecular attractions and a higher boiling point. The ketone can accept hydrogen bonds from water, and the alcohol can form them with water; butane is non-polar and cannot hydrogen-bond.
- Identifies forces shared by both compounds. [1 marks]
- Identifies hydrogen bonding between alcohol molecules but not ketone molecules. [1 marks]
- Links stronger attractions to higher boiling point. [1 marks]
- Explains oxygen groups hydrogen-bond with water. [1 marks]
- Contrasts with non-polar butane/no hydrogen bonding. [1 marks]
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
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