Chemistry 9701 · AS & A Level · 6.1

6.1 — practice question

Question 3 deals with catalysis, reaction kinetics and rate equations.
(a)[1]

Solid manganese(IV) oxide, $\text{MnO}_2$, catalyses the decomposition of hydrogen peroxide. $2\text{H}_2\text{O}_2(\text{aq}) \rightarrow 2\text{H}_2\text{O}(\ell) + \text{O}_2(\text{g})$. State the type of catalysis shown by this reaction. Explain your answer.

(b(i))[4]

Hydrogen peroxide reacts with iodide ions in acidic conditions according to: $\text{H}_2\text{O}_2 + 2\text{I}^- + 2\text{H}^+ \rightarrow 2\text{H}_2\text{O} + \text{I}_2$. Use the information in Table 3.1 to deduce the rate equation for this reaction. Explain your reasoning.

(b(ii))[2]

Use your rate equation from (b)(i) together with the Experiment 1 data to calculate the rate constant, $k$, for this reaction. State the units of $k$.

(c(i))[2]

The thermal decomposition of azomethane, $\text{CH}_3\text{N}=\text{NCH}_3$, is examined. $\text{CH}_3\text{N}=\text{NCH}_3 \rightarrow \text{N}_2 + \text{C}_2\text{H}_6$. Fig. 3.1 shows the results obtained. The reaction is first order with respect to $\text{CH}_3\text{N}=\text{NCH}_3$. Use Fig. 3.1 to calculate two half-lives, $t_{\frac{1}{2}}$, to show that the reaction is first order.

(c(ii))[1]

Use your result in (c)(i) to calculate the rate constant, $k$, for the decomposition of azomethane.

(d)[1]

Describe the effect that increasing temperature has on the rate constant and on the rate of a reaction.

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