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Rate-determining step and reaction mechanismsEdexcel International A Level Chemistry: Flashcards

What these 12 flashcards ask

  • What is the rate-determining step?
  • What is an intermediate?
  • A reactant is zero order. Where does it react in the mechanism?
  • How do you predict the rate equation from a mechanism?
  • What two conditions must a proposed mechanism satisfy?
  • What is the rate equation for the acid-catalysed iodination of propanone?
  • What does zero order with respect to iodine show in the iodination of propanone?
  • What is the organic intermediate in the iodination of propanone?
  • What rate equation is typical of a tertiary halogenoalkane hydrolysis?
  • Describe the SN1 mechanism.
  • Describe the SN2 mechanism.
  • Why do tertiary halogenoalkanes favour SN1?

Exam questions on Rate-determining step and reaction mechanisms

  1. 2-Bromo-2-methylpropane, (CH₃)₃CBr, is hydrolysed by aqueous sodium hydroxide: (CH₃)₃CBr + OH⁻ → (CH₃)₃COH + Br⁻. Experiments show that the rate equation is rate = k[(CH₃)₃CBr].
    Explain why doubling the concentration of OH⁻ has no effect on the rate of this reaction.2 marks
  2. 1-Bromobutane, CH₃CH₂CH₂CH₂Br, reacts with aqueous sodium hydroxide to form butan-1-ol. When the concentration of hydroxide ions is doubled with the concentration of 1-bromobutane unchanged, the rate doubles. When the concentration of 1-bromobutane is doubled with the concentration of hydroxide ions unchanged, the rate also doubles.
    Explain how the rate data support an SN2 mechanism for 1-bromobutane and describe the transition state.2 marks
  3. The overall equation for a gas-phase reaction is H₂ + 2ICl → I₂ + 2HCl. Experiments show that the reaction is first order with respect to H₂ and first order with respect to ICl. A student proposes a mechanism with two steps. Step 1 (slow): H₂ + ICl → HI + HCl. Step 2 (fast): HI + ICl → I₂ + HCl.
    Show that this mechanism is consistent with both the overall equation and the experimental rate equation.3 marks
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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).