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

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