Reaction kinetics (A Level only): Practice Questions

In terms of collision theory, why does an increase in the concentration of a reactant in the aqueous phase usually increase the rate of reaction?

In the industrial manufacture of ammonia by the Haber process, iron serves as a heterogeneous catalyst. Which of the following describes the correct sequence of steps for the mechanism on the catalyst surface?

The reaction between \(P\) and \(Q\) is represented by the equation: \(2P + Q \rightarrow R + S\).
A proposed mechanism for this reaction is:
Step 1: \(P + P \rightarrow P_2\) (slow)
Step 2: \(P_2 + Q \rightarrow R + S\) (fast)
What is the rate equation consistent with this mechanism?

Which statement best explains how a homogeneous catalyst functions in a chemical reaction?

The initial rate of a reaction between substances \(P\) and \(Q\) was measured in three separate experiments at constant temperature:

• Experiment 1: \([P] = 0.10 \text{ mol dm}^{-3}\), \([Q] = 0.10 \text{ mol dm}^{-3}\), Initial rate = \(2.0 \times 10^{-4} \text{ mol dm}^{-3} \text{ s}^{-1}\)
• Experiment 2: \([P] = 0.20 \text{ mol dm}^{-3}\), \([Q] = 0.10 \text{ mol dm}^{-3}\), Initial rate = \(8.0 \times 10^{-4} \text{ mol dm}^{-3} \text{ s}^{-1}\)
• Experiment 3: \([P] = 0.10 \text{ mol dm}^{-3}\), \([Q] = 0.30 \text{ mol dm}^{-3}\), Initial rate = \(6.0 \times 10^{-4} \text{ mol dm}^{-3} \text{ s}^{-1}\)

What is the overall order of the reaction?

Define the term activation energy, \(E_A\).

Explain how the Boltzmann distribution curve changes when the temperature of a reaction mixture is increased and how this affects the rate of reaction.

The decomposition of substance \(A\) follows first-order kinetics.
(a) State the relationship between the half-life of a first-order reaction and the initial concentration of the reactant.
(b) The rate constant for the decomposition of \(A\) is \(3.85 \times 10^{-4}\,s^{-1}\) at \(298\,K\). Calculate the half-life, \(t_{1/2}\), for this reaction.
(c) Explain, with reference to the Boltzmann distribution, why a small increase in temperature leads to a significant increase in the rate of reaction.
(d) State how a catalyst increases the rate of a reaction in terms of the activation energy, \(E_a\), and the reaction mechanism.<\/p>

The reaction between hydrogen peroxide and iodide ions in acidic solution is a common reaction studied in kinetics:

\(H_2O_2(aq) + 2I^-(aq) + 2H^+(aq) \rightarrow I_2(aq) + 2H_2O(l)\)

The following initial rate data were obtained at a constant temperature:

(a) Determine the order of reaction with respect to \(H_2O_2\), \(I^-\), and \(H^+\).

(b) Write the rate equation for this reaction.

(c) Calculate the rate constant, k, for this reaction, stating its units.

(d) Suggest a two-step mechanism for this reaction that is consistent with the derived rate equation. Clearly identify the rate-determining step.