Chemistry 9701 · AS & A Level · Reaction kinetics

Reaction kinetics — practice question

Chlorine dioxide reacts in aqueous solution as follows: $\text{2ClO}_2 + 2\text{OH}^- \rightarrow \text{ClO}_2^- + \text{ClO}_3^- + \text{H}_2\text{O}$. The starting rate of this reaction was determined for several different initial concentrations of $\text{ClO}_2$ and $\text{OH}^-$. The results are listed in the table.
(a(i))[3]

Use the table data to work out the rate equation, including the order for each reactant. Show how you reach your answer.

(a(ii))[2]

Calculate the value of the rate constant, $k$, from the data in experiment 2. Give its units.

(b(i))[1]

Explain the difference between heterogeneous and homogeneous catalysts.

(b(ii))[2]

Complete the table using ticks ($\checkmark$) to show whether the catalyst used in each reaction is heterogeneous or homogeneous: manufacture of ammonia in the Haber process; removal of nitrogen oxides from car exhausts; oxidation of sulfur dioxide in the atmosphere.

(c(i))[2]

Some reactions are catalysed by one of the products formed. This is known as autocatalysis. One example is the reaction between acidified manganate(VII) ions, $\text{MnO}_4^-$, and ethanedioic acid, $(\text{CO}_2\text{H})_2$. $\text{Mn}^{2+}$ ions act as the catalyst for this reaction. Without a catalyst, the reaction is slow. Balance the equation for this reaction: $\ldots\text{MnO}_4^- + \ldots\text{H}^+ + \ldots(\text{CO}_2\text{H})_2 \rightarrow \ldots\text{Mn}^{2+} + \ldots\text{CO}_2 + \ldots\text{H}_2\text{O}$.

(c(ii))[2]

The graph shown is a concentration-time graph for a typical reaction. On the axes below, sketch the curve you would expect for the autocatalysed reaction in (i).

(d(i))[3]

Describe, with the aid of a reaction pathway diagram, the effect of a catalyst on a reversible reaction. Suggest why catalysts are used in industrial processes.

(d(ii))[2]

The reaction used in the Haber process to make ammonia is shown here: $\text{N}_2(g) + 3\text{H}_2(g) \rightleftharpoons 2\text{NH}_3(g)$, $\Delta H^{\circ} = -92\,\text{kJ mol}^{-1}$. At $500\,^{\circ}\text{C}$, when pressure is expressed in atmospheres, the numerical value of $K_p$ for this equilibrium is $1.45 \times 10^{-5}$. Write the expression for $K_p$ for this equilibrium. Calculate the partial pressure of $\text{NH}_3$ at equilibrium at $500\,^{\circ}\text{C}$, when the partial pressure of $\text{N}_2$ is $20\,\text{atm}$ and that of $\text{H}_2$ is $60\,\text{atm}$.

Worked solution & mark scheme

This 17-mark question has a full step-by-step worked solution and mark scheme. One marking point: Doubling $[\mathrm{ClO_2}]$ makes the rate four times greater so second order

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