Forces between moleculesAQA A-Level Chemistry: Subtopic test

1. 1
   Propane, CH₃CH₂CH₃, and ethanal, CH₃CHO, have almost the same relative molecular mass (44). Propane boils at −42 °C, whereas ethanal boils at +20 °C.

   (a)

   Which type of intermolecular force is the main attraction between propane molecules?

   [1 mark]

   • AInduced dipole-dipole forces
   • BPermanent dipole-dipole forces
   • CHydrogen bonding
   • DIonic attraction

   (b)

   Which statement explains why ethanal has a higher boiling point than propane?

   [1 mark]

   • AHydrogen bonds form between ethanal molecules
   • BThe covalent bonds in ethanal are stronger
   • CEthanal has permanent dipole-dipole forces in addition to induced dipole-dipole forces
   • DEthanal molecules are held together by ionic bonds

   (c)

   Explain why ethanal has a higher boiling point than propane.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   The boiling points of the hydrogen halides are: hydrogen fluoride +20 °C, hydrogen chloride −85 °C, hydrogen bromide −67 °C and hydrogen iodide −35 °C.

   (a)

   Which statement explains why hydrogen fluoride has a much higher boiling point than hydrogen chloride?

   [1 mark]

   • AThe H–F covalent bond is stronger than the H–Cl bond, so it is harder to boil
   • BHydrogen bonds form between hydrogen fluoride molecules
   • CHydrogen fluoride has more electrons than hydrogen chloride
   • DHydrogen fluoride molecules are larger than hydrogen chloride molecules

   (b)

   Which of these compounds can form hydrogen bonds between its own molecules?

   [1 mark]

   • ACH₃CHO
   • BCH₃OCH₃
   • CCH₃Cl
   • DCH₃OH

   (c)

   Explain why the boiling point increases from hydrogen chloride to hydrogen iodide.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Ice floats on liquid water, and water has an unusually high boiling point of 100 °C compared with hydrogen sulfide, H₂S, which boils at −60 °C even though H₂S has a larger relative molecular mass.

   (a)

   Explain why water has a much higher boiling point than hydrogen sulfide.

   [3 marks]

   (b)

   Explain, with reference to hydrogen bonding, why ice is less dense than liquid water.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   The boiling points of the straight-chain alkanes are: methane −162 °C, ethane −89 °C, propane −42 °C, butane −1 °C and pentane 36 °C. The branched isomer of pentane, 2,2-dimethylpropane, boils at 10 °C. Ethanol, CH₃CH₂OH, methoxymethane, CH₃OCH₃, and propane have similar relative molecular masses (46, 46 and 44) but boil at 78 °C, −24 °C and −42 °C respectively.

   (a)

   Explain the trend in boiling points from methane to pentane, and why 2,2-dimethylpropane boils at a lower temperature than pentane.

   [6 marks]

   (b)

   Explain why ethanol, methoxymethane and propane have different boiling points even though their relative molecular masses are similar.

   [6 marks]

   Total for question 4: 12 marks

End of questions