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

Exam questions on Rate-determining step and reaction mechanisms

1. Nitrogen dioxide reacts with carbon monoxide: NO₂(g) + CO(g) → NO(g) + CO₂(g). At 500 K the rate equation is rate = k[NO₂]². A proposed mechanism has two steps, and the first step is the slower.
   Explain why CO does not appear in the rate equation.2 marks
2. The hydrolysis of two bromoalkanes by aqueous sodium hydroxide was studied at constant temperature. For 2-bromo-2-methylpropane, doubling the concentration of the bromoalkane doubled the initial rate, but doubling the concentration of hydroxide ions had no effect on the initial rate. For 1-bromobutane, doubling the concentration of either the bromoalkane or the hydroxide ions doubled the initial rate.
   Explain why the data show that 2-bromo-2-methylpropane is hydrolysed by an SN1 mechanism.2 marks
3. The acid-catalysed reaction of propanone with iodine is CH₃COCH₃(aq) + I₂(aq) → CH₃COCH₂I(aq) + HI(aq). Initial rates were measured at constant temperature. Experiment 1: [CH₃COCH₃] = 1.00 mol dm⁻³, [I₂] = 0.0100 mol dm⁻³, [H⁺] = 0.50 mol dm⁻³, initial rate = 2.4 × 10⁻⁶ mol dm⁻³ s⁻¹. Experiment 2: [CH₃COCH₃] = 2.00 mol dm⁻³, [I₂] = 0.0100 mol dm⁻³, [H⁺] = 0.50 mol dm⁻³, initial rate = 4.8 × 10⁻⁶ mol dm⁻³ s⁻¹. Experiment 3: [CH₃COCH₃] = 1.00 mol dm⁻³, [I₂] = 0.0200 mol dm⁻³, [H⁺] = 0.50 mol dm⁻³, initial rate = 2.4 × 10⁻⁶ mol dm⁻³ s⁻¹. Experiment 4: [CH₃COCH₃] = 1.00 mol dm⁻³, [I₂] = 0.0100 mol dm⁻³, [H⁺] = 1.00 mol dm⁻³, initial rate = 4.8 × 10⁻⁶ mol dm⁻³ s⁻¹.
   Deduce the order of reaction with respect to each of propanone, iodine and hydrogen ions, and write the rate equation.3 marks