Reactions of alkanes and free radical substitutionEdexcel International A Level Chemistry: Subtopic test

1. 1
   A student mixes methane and chlorine gases in a flask and exposes the flask to ultraviolet light. A reaction occurs and misty fumes of hydrogen chloride form.

   (a)

   Which species is a free radical?

   [1 mark]

   • ACH₃Cl
   • BCl⁻
   • CCl•
   • DHCl

   (b)

   Which equation represents a propagation step in this reaction?

   [1 mark]

   • A•CH₃ + Cl₂ → CH₃Cl + Cl•
   • BCl• + Cl• → Cl₂
   • CCl₂ → 2Cl•
   • D•CH₃ + Cl• → CH₃Cl

   (c)

   Write the equation for the initiation step, and name the type of bond fission involved.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A chemist reacts ethane with bromine under ultraviolet light, hoping to make bromoethane. The product mixture contains bromoethane, some dibromoethanes and a small amount of butane.

   (a)

   Which equation is the termination step that explains the formation of butane?

   [1 mark]

   • AC₂H₅• + Br₂ → C₂H₅Br + Br•
   • BC₂H₆ + Br• → C₂H₅• + HBr
   • CBr• + Br• → Br₂
   • D2C₂H₅• → C₄H₁₀

   (b)

   Why is free-radical substitution of alkanes of limited use for making a single halogenoalkane?

   [1 mark]

   • AAlkanes cannot react with halogens in any conditions
   • BFurther substitution gives a mixture of products with more than one halogen atom
   • CThe initiation step needs an expensive catalyst
   • DFree radicals are too unstable to form any products

   (c)

   Write the equations for the two propagation steps that form bromoethane from ethane.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Methane reacts with an excess of chlorine in ultraviolet light, forming chloromethane, CH₃Cl, together with further substituted products. Relative atomic masses: H = 1.0, C = 12.0, Cl = 35.5.

   (a)

   Write the two propagation steps that convert chloromethane into dichloromethane, and explain why the reaction gives a mixture of chlorinated products.

   [3 marks]

   (b)

   In an experiment, 3.20 g of methane gave 4.04 g of chloromethane. Calculate the percentage yield of chloromethane and suggest why it is less than 100%.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A chemical company wants to prepare halogenoalkanes by reacting alkanes with halogens in ultraviolet light. A chemist is asked to explain the mechanism and the limits of the method.

   (a)

   Describe the mechanism of the reaction of methane with chlorine in ultraviolet light, giving the equation for each step.

   [6 marks]

   (b)

   A chemist wants to make 1-bromopropane from propane and bromine using ultraviolet light. Evaluate how suitable this method is.

   [6 marks]

   Total for question 4: 12 marks

End of questions