Chlorination of alkanesAQA A-Level Chemistry: Subtopic test

1. 1
   Methane reacts with chlorine in the presence of ultraviolet light to form chloromethane, CH₃Cl, and hydrogen chloride. The reaction takes place by a free-radical substitution mechanism.

   (a)

   Which equation represents the initiation step?

   [1 mark]

   • ACl₂ → 2Cl•
   • BCH₄ → •CH₃ + H•
   • CCl• + CH₄ → •CH₃ + HCl
   • D•CH₃ + Cl₂ → CH₃Cl + Cl•

   (b)

   Which statement describes a radical?

   [1 mark]

   • AA species with a positive charge
   • BA species with a lone pair of electrons
   • CA molecule containing a C=C bond
   • DA species with an unpaired electron

   (c)

   Explain why ultraviolet light is needed for the initiation step.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   When methane reacts with chlorine in ultraviolet light, the product mixture also contains small amounts of ethane, C₂H₆, and dichloromethane, CH₂Cl₂.

   (a)

   Which equation represents a termination step that forms ethane?

   [1 mark]

   • ACl• + Cl• → Cl₂
   • B•CH₃ + Cl• → CH₃Cl
   • C•CH₃ + •CH₃ → C₂H₆
   • D•CH₃ + Cl₂ → CH₃Cl + Cl•

   (b)

   Which statement about the propagation steps is correct?

   [1 mark]

   • ARadicals are used up and not regenerated
   • BA radical reacts and another radical is formed, so the chain continues
   • CTwo radicals combine to form a stable molecule
   • DA covalent bond breaks to form ions

   (c)

   Write equations for the two propagation steps that convert chloromethane, CH₃Cl, into dichloromethane, CH₂Cl₂.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Ethane, CH₃CH₃, reacts with chlorine in the presence of ultraviolet light to form chloroethane, CH₃CH₂Cl, by the same type of mechanism as the reaction of methane with chlorine.

   (a)

   Write equations for the initiation step and the two propagation steps in this reaction.

   [3 marks]

   (b)

   Butane, C₄H₁₀, is formed as a by-product. Write an equation for the termination step that forms butane and explain why termination steps end the chain reaction. Suggest how the formation of dichloroethane can be reduced, giving a reason.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student is studying the reaction of methane with chlorine in ultraviolet light, which gives chloromethane and hydrogen chloride as the main products, together with several other products.

   (a)

   Describe the mechanism of the reaction of methane with chlorine to form chloromethane. Name each stage, give the equation for each step and explain how the chain reaction continues.

   [6 marks]

   (b)

   Explain why the reaction produces a mixture of products, including ethane and tetrachloromethane, and suggest how the yield of chloromethane can be maximised.

   [6 marks]

   Total for question 4: 12 marks

End of questions