Chemistry 9701 · AS & A Level · Reaction kinetics

Reaction kinetics — practice question

Acetals are compounds produced when an aldehyde reacts with an alcohol in the presence of an acid catalyst. The reaction between ethanal and methanol was investigated in the inert solvent dioxan. $\text{CH}_3\text{CHO} + 2\text{CH}_3\text{OH} \xrightleftharpoons[ ]{H^+} \text{CH}_3\text{CH(OCH}_3)_2 + \text{H}_2\text{O}$ ethanal + methanol $\rightarrow$ acetal A + water
(a(i))

The initial rate for this reaction was measured for a range of starting concentrations of the three reactants, and the outcomes are listed. Use the table to work out the order with respect to each reactant: order with respect to $[\text{CH}_3\text{CHO}]$ order with respect to $[\text{CH}_3\text{OH}]$ order with respect to $[H^+]$

(a(ii))

Use the results from part (i) to state the rate equation for the reaction.

(a(iii))

State the units of the rate constant in the rate equation.

(a(iv))[6]

Calculate the relative rate of reaction for a mixture in which the starting concentrations of all three reactants are $0.20\ \text{mol dm}^{-3}$. relative rate $= \ldots\ldots\ldots\ldots\ldots$

(b(i))[2]

Complete the second row of the table in terms of $x$, the equilibrium concentration of acetal A. You may wish to refer to the chemical equation opposite.

(b(ii))[3]

Using the stated value of $[\text{acetal A}]$, $0.025\ \text{mol dm}^{-3}$, calculate the equilibrium concentrations of the other reactants and products and enter them in the third row of the table.

(b(iii))[2]

Write the equilibrium constant expression for this reaction, $K_c$, and state its units.

(b(iv))[2]

Use the values in the third row of the table to calculate the value of $K_c$.

Worked solution & mark scheme

This 15-mark question has a full step-by-step worked solution and mark scheme. One marking point: Order w.r.t. $[\mathrm{CH_3CHO}]$ is 1

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