Chemistry 9701 · AS & A Level · Reaction kinetics

Reaction kinetics — practice question

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

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

(a(ii))

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

(a(iii))

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

(a(iv))[6]

Calculate the relative rate for a mixture whose starting concentrations of all three reactants are $0.20\;\text{mol dm}^{-3}$.

(b(i))

Complete the second row of the table in terms of $x$, the concentration of acetal A at equilibrium. You may find it useful to look at the chemical equation opposite.

(b(ii))

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

(b(iii))

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

(b(iv))[9]

Use the values in the third row of the table to work out $K_c$.

Worked solution & mark scheme

This 15-mark question has a full step-by-step worked solution and mark scheme. One marking point: Order for $[CH_3CHO] = 1$

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