Chemistry 9701 · AS & A Level · Reaction kinetics

Reaction kinetics — practice question

Propanone, $\text{CH}_3\text{COCH}_3$, is a familiar organic solvent and reagent. Under acidic conditions, propanone reacts with methanol, $\text{CH}_3\text{OH}$, to produce compound A, as shown in reaction 1: $\text{CH}_3\text{COCH}_3 + 2\text{CH}_3\text{OH} \xrightarrow{\text{H}^+} \text{A} + \text{H}_2\text{O}$ The overall order of reaction 1 can be determined from experimental observations. Table 2.1 shows the way the initial rate changes when $[\text{CH}_3\text{OH}]$ and $[\text{H}^+]$ are altered. In every experiment, a large excess of $\text{CH}_3\text{COCH}_3$ is employed.
(a(i))[1]

Explain why each experiment uses a large excess of $\text{CH}_3\text{COCH}_3$.

(a(ii))[2]

Use the data in Table 2.1 to determine the order of reaction 1 with respect to $\text{CH}_3\text{OH}$ and to $\text{H}^+$ ions. Explain your answers.

(a(iii))[1]

In a different experiment, a solution containing a known concentration of $\text{CH}_3\text{COCH}_3$ has a large excess of $\text{CH}_3\text{OH}$ and $\text{H}^+$ ions added to it. Fig. 2.2 shows how $[\text{CH}_3\text{COCH}_3]$ changes with time. Use Fig. 2.2 to show how, under these conditions, reaction 1 is first order with respect to $\text{CH}_3\text{COCH}_3$.

(c(iii))[1]

Use Fig. 2.2 to show that, under these conditions, reaction 1 is first order with respect to $\text{CH}_3\text{COCH}_3$.

(b)[3]

Propanone also reacts with acidified cyanide ions to form the hydroxynitrile compound B, as shown by reaction 2. Suggest which proposed mechanism matches the rate equation for reaction 2. Explain your answer.

(c(i))[1]

Draw the structure of carboxylic acid $ ext{C}$, $ ext{C}_4 ext{H}_8 ext{O}_3$, which is produced when B undergoes hydrolysis under hot acidic conditions.

(c(ii))[2]

The $\text{p}K_\text{a}$ of $\text{C}$ is $3.95$. Calculate the pH of a $0.500\,\text{mol dm}^{-3}$ solution of $\text{C}$. Show your working.

(d(i))[2]

Define the term buffer solution.

(d(ii))[3]

Buffer solution D is prepared by adding $20.0\,\text{cm}^3$ of $1.00\,\text{mol dm}^{-3}$ $\text{NaOH(aq)}$ to $100\,\text{cm}^3$ of a $0.500\,\text{mol dm}^{-3}$ solution of $\text{C}$. The $\text{p}K_\text{a}$ of $\text{C}$ is $3.95$. Calculate the pH of buffer solution D. Show your working.

Worked solution & mark scheme

This 16-mark question has a full step-by-step worked solution and mark scheme. One marking point: $[\text{CH}_3\text{COCH}_3]$ remains (almost) unchanged / has no influence on the rate

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