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

1. 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].

   (a)

   Which statement about the rate-determining step is correct?

   [1 mark]

   • AIt involves (CH₃)₃CBr colliding with OH⁻
   • BIt involves (CH₃)₃CBr only
   • CIt involves OH⁻ only
   • DIt is the last step in the mechanism

   (b)

   Which mechanism is consistent with this rate equation?

   [1 mark]

   • ASN2
   • BElectrophilic addition
   • CFree-radical substitution
   • DSN1

   (c)

   Explain why doubling the concentration of OH⁻ has no effect on the rate of this reaction.

   [2 marks]

   Total for question 1: 4 marks

2. 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.

   (a)

   Which rate equation fits these data?

   [1 mark]

   • Arate = k[CH₃CH₂CH₂CH₂Br]
   • Brate = k[OH⁻]
   • Crate = k[CH₃CH₂CH₂CH₂Br][OH⁻]
   • Drate = k[CH₃CH₂CH₂CH₂Br][OH⁻]²

   (b)

   Why is it unlikely that 1-bromobutane reacts by an SN1 mechanism?

   [1 mark]

   • AThe primary carbocation that would form is unstable
   • BButane contains no polar bonds
   • CBromide is a very poor leaving group
   • DThe carbon bonded to bromine is not δ+

   (c)

   Explain how the rate data support an SN2 mechanism for 1-bromobutane and describe the transition state.

   [2 marks]

   Total for question 2: 4 marks

3. 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.

   (a)

   Show that this mechanism is consistent with both the overall equation and the experimental rate equation.

   [3 marks]

   (b)

   A second student proposes a different mechanism. Step 1 (slow): 2ICl → I₂ + Cl₂. Step 2 (fast): Cl₂ + H₂ → 2HCl. Show that this gives the correct overall equation, deduce the rate equation it predicts, and decide whether it is consistent with the experimental data.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   In the acid-catalysed iodination of propanone, CH₃COCH₃ + I₂ → CH₃COCH₂I + HI, the following initial-rate data were obtained at constant temperature. Experiment 1: [CH₃COCH₃] = 1.00 mol dm⁻³, [I₂] = 0.0050 mol dm⁻³, [H⁺] = 0.20 mol dm⁻³, rate = 2.4 × 10⁻⁶ mol dm⁻³ s⁻¹. Experiment 2: [CH₃COCH₃] = 2.00 mol dm⁻³, [I₂] = 0.0050 mol dm⁻³, [H⁺] = 0.20 mol dm⁻³, rate = 4.8 × 10⁻⁶ mol dm⁻³ s⁻¹. Experiment 3: [CH₃COCH₃] = 1.00 mol dm⁻³, [I₂] = 0.0100 mol dm⁻³, [H⁺] = 0.20 mol dm⁻³, rate = 2.4 × 10⁻⁶ mol dm⁻³ s⁻¹. Experiment 4: [CH₃COCH₃] = 1.00 mol dm⁻³, [I₂] = 0.0050 mol dm⁻³, [H⁺] = 0.40 mol dm⁻³, rate = 4.8 × 10⁻⁶ mol dm⁻³ s⁻¹.

   (a)

   Deduce the order of reaction with respect to each of propanone, iodine and hydrogen ions, write the rate equation, and calculate the rate constant with its units. State what the rate equation shows about the species in the rate-determining step.

   [6 marks]

   (b)

   The rate equation for the reaction is rate = k[CH₃COCH₃][H⁺]. Suggest a possible mechanism in words, showing which step is rate-determining and explaining how your mechanism accounts for the rate equation, the role of H⁺ and the overall equation.

   [6 marks]

   Total for question 4: 12 marks

End of questions