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Topic 3.3.2: Alkanes - Exam Practice

AQA A-Level Chemistry • Organic Chemistry • Exam-Style Questions

Exam Practice

Topic 3.3.2: Alkanes Exam Practice

Test your understanding of alkane properties, fractional distillation, cracking types, combustion, and free radical substitution with exam-style questions.

Alkanes › Exam Practice

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📋 Structured Questions

Complete each question on paper, then check your answers against the mark scheme.

Question 1: Physical Properties of Alkanes

4 marks

(a) Explain the trend in boiling points of the straight-chain alkanes: hexane, heptane, and octane. [2]

(b) Compare the boiling points of hexane and 2,2-dimethylbutane. Explain the difference in terms of structure and intermolecular forces. [2]

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(a)

  • Boiling point increases as chain length increases. [1]
  • This is because molecular mass and the number of electrons increase, leading to stronger van der Waals forces between molecules, which require more energy to break. [1]

(b)

  • Hexane has a higher boiling point than 2,2-dimethylbutane. [1]
  • 2,2-dimethylbutane is highly branched, which reduces the surface area of contact between molecules and prevents them from packing closely together. This results in weaker van der Waals forces that require less energy to overcome. [1]
Examiner tip: Always refer to "van der Waals forces" and specify that they are *between molecules* (intermolecular). Do not imply that covalent C-C or C-H bonds are broken during boiling.

Question 2: Catalytic Converters and Air Pollution

4 marks

(a) Explain how nitrogen monoxide (NO) is formed in the internal combustion engine of a car, even though nitrogen is normally unreactive. [2]

(b) Write a balanced equation showing how nitrogen monoxide and carbon monoxide are removed in a catalytic converter. Identify the metal catalysts used. [2]

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(a)

  • Inside the car engine, temperatures are very high (spark ignition provides energy). [1]
  • This provides the high activation energy required for nitrogen gas and oxygen gas from the air to react together (N2 + O2 -> 2NO). [1]

(b)

  • Equation: 2NO + 2CO -> N2 + 2CO2 [1]
  • Catalysts: Platinum (Pt), palladium (Pd), or rhodium (Rh). [1]
Examiner tip: Ensure your equation in part (b) is fully balanced. The catalytic metals must be transition metals: Pt, Pd, or Rh.

Question 3: Free Radical Substitution Mechanism

5 marks

(a) Write the initiation, propagation, and termination steps for the reaction of chloromethane (CH3Cl) with chlorine to form dichloromethane (CH2Cl2). [4]

(b) Explain why this reaction is not a suitable industrial method for preparing pure dichloromethane. [1]

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(a)

  • Initiation: Cl2 -> 2Cl. [1]
  • Propagation 1: CH3Cl + Cl. -> .CH2Cl + HCl [1]
  • Propagation 2: .CH2Cl + Cl2 -> CH2Cl2 + Cl. [1]
  • Termination (any one):
    .CH2Cl + Cl. -> CH2Cl2
    2.CH2Cl -> CH2ClCH2Cl (1,2-dichloroethane) [1]

(b)

  • Further substitution can occur, leading to a mixture of products (trichloromethane, tetrachloromethane) which must be separated by fractional distillation. [1]
Examiner tip: Ensure the radical dot is placed clearly on the correct atom (the Carbon atom in .CH2Cl). Do not write Cl. + CH3Cl -> CH2Cl2 + H., as hydrogen radicals are never formed in these conditions.

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