Intermolecular forces and physical propertiesEdexcel International A Level Chemistry: Subtopic test

1. 1
   Alkanes are used as fuels. The boiling temperatures of the first four straight-chain alkanes are: methane 112 K, ethane 184 K, propane 231 K and butane 273 K.

   (a)

   Which intermolecular forces act between molecules of butane?

   [1 mark]

   • ALondon forces and permanent dipole–permanent dipole attractions
   • BLondon forces and hydrogen bonds
   • CPermanent dipole–permanent dipole attractions only
   • DLondon forces only

   (b)

   Which statement best explains the increase in boiling temperature from methane to butane?

   [1 mark]

   • AThe covalent bonds between carbon atoms get stronger
   • BHydrogen bonds form between the longer molecules
   • CLarger molecules have more electrons, so the London forces between them are stronger
   • DThe molecules become polar as the chain gets longer

   (c)

   Predict whether the boiling temperature of pentane is higher or lower than that of butane. Give a reason.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Pentane, 2-methylbutane and 2,2-dimethylpropane are isomers with the molecular formula C₅H₁₂. Pentane has an unbranched chain, 2-methylbutane has one methyl branch and 2,2-dimethylpropane has two methyl branches on the same carbon atom. Their boiling temperatures are 309 K, 301 K and 283 K respectively.

   (a)

   Which statement about pentane, 2-methylbutane and 2,2-dimethylpropane is correct?

   [1 mark]

   • AThe molecules have different numbers of electrons
   • BEach molecule has the same number of electrons, so the differences in boiling temperature arise from shape
   • CThey have different molecular formulae
   • DThey contain different functional groups

   (b)

   Which statement best explains why 2,2-dimethylpropane has the lowest boiling temperature?

   [1 mark]

   • AIts compact, branched molecules have less surface contact with each other, so the London forces are weaker
   • BIts molecules are polar, which weakens the attractions
   • CIt has fewer electrons per molecule than pentane
   • DIts covalent bonds are weaker than those of pentane

   (c)

   Explain why pentane has a higher boiling temperature than 2-methylbutane.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Methanol, CH₃OH, and ethane, C₂H₆, each have 18 electrons per molecule. Methanol boils at 338 K and ethane boils at 184 K. Propan-1-ol, CH₃CH₂CH₂OH, and butane, C₄H₁₀, each have 34 electrons per molecule, and ethanol, C₂H₅OH, has 26 electrons per molecule.

   (a)

   Explain why methanol has a much higher boiling temperature than ethane.

   [3 marks]

   (b)

   (i) Explain why propan-1-ol is less volatile than butane. (ii) Predict, with a reason, whether propan-1-ol boils at a higher or lower temperature than ethanol.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A data book gives these boiling temperatures. Hydrogen halides: HF 293 K, HCl 188 K, HBr 206 K, HI 238 K. Alkanes: hexane 342 K, octane 399 K and 2,2-dimethylbutane 323 K. Hexane and 2,2-dimethylbutane are isomers with the formula C₆H₁₄.

   (a)

   Explain the trend in boiling temperatures of the hydrogen halides from HF to HI.

   [6 marks]

   (b)

   Explain the order of the boiling temperatures of octane, hexane and 2,2-dimethylbutane.

   [6 marks]

   Total for question 4: 12 marks

End of questions