Kinetics

Sample questions

Original practice questions written in the style of the exam.

1. Question 14 marks

   The initial rate of the reaction between P and Q was measured at constant temperature.

   experiment	$[\text{P}]$ / $\text{mol dm}^{-3}$	$[\text{Q}]$ / $\text{mol dm}^{-3}$	initial rate / $\text{mol dm}^{-3}\,\text{s}^{-1}$
   1	0.10	0.10	$2.0 \times 10^{-4}$
   2	0.20	0.10	$4.0 \times 10^{-4}$
   3	0.10	0.30

   Deduce the order with respect to each reactant, write the rate equation, and calculate the rate constant with its units.

   Mark scheme

   • First order with respect to P: doubling [P] (expts 1 and 2) doubles the rate (1)
   • Second order with respect to Q: tripling [Q] (expts 1 and 3) increases the rate nine times (1)
   • $\text{rate} = k[\text{P}][\text{Q}]^2$ (1)
   • $k={\displaystyle \frac{2.0\times {10}^{-4}}{0.10\times {0.10}^2}}=0.20{\text{dm}}^6{\text{mol}}^{}$ (1)
2. Question 23 marks

   The hydrolysis of 2-bromo-2-methylpropane by aqueous hydroxide ions has the rate equation:

   $\text{rate} = k[(\text{CH}_3)_3\text{CBr}]$

   Explain what this rate equation shows about the mechanism of the reaction.
3. Question 32 marks

   A student measured the rate constant $k$ of a reaction at several temperatures and plotted $\ln k$ against $1/T$. The line of best fit had a gradient of $-6250\,\text{K}$.

   Calculate the activation energy of the reaction in $$.