Reaction mechanismsAQA A-Level Chemistry: Subtopic test

1. 1
   Methane reacts with chlorine in the presence of ultraviolet light to form chloromethane and hydrogen chloride. The reaction does not occur in the dark. It is a free-radical reaction.

   (a)

   What is a free radical?

   [1 mark]

   • AA species with a lone pair of electrons
   • BAn ion with a positive charge
   • CA species with an unpaired electron
   • DA molecule in which all electrons are paired

   (b)

   Which equation represents the initiation step of this reaction?

   [1 mark]

   • ACl₂ → 2Cl•
   • BCH₄ → CH₃• + H•
   • CCl₂ → Cl⁺ + Cl⁻
   • DCl• + CH₄ → CH₃• + HCl

   (c)

   Write equations for the two propagation steps of this reaction.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   An organic chemist is drawing the mechanism for the reaction of bromoethane with hydroxide ions: CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻. The C–Br bond is polar, with the carbon atom δ+.

   (a)

   Where does a curly arrow start when it shows the formation of a covalent bond?

   [1 mark]

   • AOn the nucleus of the attacking atom
   • BOn a lone pair of electrons or on a covalent bond
   • COn a positive charge
   • DOn the atom that receives the electrons

   (b)

   Which curly arrow shows the breaking of the C–Br bond?

   [1 mark]

   • AFrom the carbon atom to the bromine atom
   • BFrom a lone pair on bromine to the carbon atom
   • CFrom a lone pair on the oxygen of OH⁻ to the bromine atom
   • DFrom the middle of the C–Br bond to the bromine atom

   (c)

   Describe the two curly arrows needed to show this mechanism.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In the stratosphere, ultraviolet radiation breaks a C–Cl bond in the CFC dichlorodifluoromethane, CCl₂F₂, to form chlorine atoms. These chlorine atoms then take part in reactions that destroy ozone, O₃.

   (a)

   Write an equation for this bond-breaking step, name the type of bond fission involved and state what happens to the pair of electrons in the C–Cl bond.

   [3 marks]

   (b)

   Chlorine atoms react with ozone in a chain. Write equations for the two propagation steps, using Cl• and O₃, and the overall equation. Explain why Cl• acts as a catalyst.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Methane reacts with chlorine in sunlight to form chloromethane, and a small amount of ethane is also found in the product mixture. In a separate study, ethene reacts with hydrogen bromide to form bromoethane: CH₂=CH₂ + HBr → CH₃CH₂Br.

   (a)

   Describe the free-radical mechanism for the reaction of methane with chlorine, giving an equation for each step. Explain how ethane can be formed.

   [6 marks]

   (b)

   Outline the mechanism for the reaction of ethene with hydrogen bromide, describing each curly arrow.

   [6 marks]

   Total for question 4: 12 marks

End of questions