Chemistry 9701 · AS & A Level · 6.1

6.1 — practice question

Solid manganese(IV) oxide, $\text{MnO}_2$, speeds up the decomposition of hydrogen peroxide. Hydrogen peroxide reacts with iodide ions under acidic conditions. The thermal decomposition rate of azomethane, $\text{CH}_3\text{N}=\text{NCH}_3$, is being studied.
(a)[1]

$2\text{H}_2\text{O}_2(\text{aq}) \rightarrow 2\text{H}_2\text{O}(\ell) + \text{O}_2(\text{g})$ State the type of catalysis for this reaction. Explain your response.

(b(i))[4]

$\text{H}_2\text{O}_2 + 2\text{I}^{-} + 2\text{H}^{+} \rightarrow 2\text{H}_2\text{O} + \text{I}_2$ The initial rate for this reaction is studied using different concentrations of $\text{H}_2\text{O}_2$, $\text{I}^{-}$ and $\text{H}^{+}$. Use the information in Table 3.1 to deduce the rate equation for this reaction. Explain your reasoning.

(b(ii))[2]

Using your rate equation from (b)(i) and the data from Experiment 1, calculate the rate constant, $k$, for this reaction. Include the units of $k$.

(c(i))[2]

$\text{CH}_3\text{N}=\text{NCH}_3 \rightarrow \text{N}_2 + \text{C}_2\text{H}_6$ 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, $t_{1/2}$, to show that the reaction is first order.

(c(ii))[1]

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

(d)[1]

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

Worked solution & mark scheme

This 11-mark question has a full step-by-step worked solution and mark scheme. One marking point: heterogeneous AND MnO2 is in a different state / phase from the reactants / H2O2

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