Rate-determining step and reaction mechanismsAQA A-Level Chemistry: Mind map

Exam questions on Rate-determining step and reaction mechanisms

1. The reaction NO₂(g) + CO(g) → NO(g) + CO₂(g) has the experimentally determined rate equation rate = k[NO₂]². A two-step mechanism is proposed. Step 1 (slow): 2NO₂ → NO₃ + NO. Step 2 (fast): NO₃ + CO → NO₂ + CO₂.
   Explain why CO does not appear in the rate equation even though it is a reactant.2 marks
2. In acidic solution, propanone reacts with iodine: CH₃COCH₃ + I₂ → CH₃COCH₂I + HI. The experimentally determined rate equation is rate = k[CH₃COCH₃][H⁺]. The reaction is zero order with respect to iodine. Hydrogen ions are not used up.
   Explain why H⁺ appears in the rate equation although it does not appear in the overall equation.2 marks
3. Two halogenoalkanes are hydrolysed by aqueous sodium hydroxide at constant temperature. For 2-bromo-2-methylpropane, (CH₃)₃CBr, the reaction is first order with respect to the halogenoalkane and zero order with respect to hydroxide ions. For bromoethane, CH₃CH₂Br, the reaction is first order with respect to the halogenoalkane and first order with respect to hydroxide ions.
   Use the orders of reaction for 2-bromo-2-methylpropane to explain what they show about its mechanism.3 marks