May/June 2026 Paper 42

Ethanal, CH${}_3$CHO, reacts with methanol, CH${}_3$OH, under acidic conditions to form compound B, as shown in reaction 1. B H${}^{+}$ reaction 1 CH${}_3$CHO + 2CH${}_3$OH CH${}_3$CH(OCH${}_3$)${}_2$ + H${}_2$O The initial rate of reaction is investigated using different starting concentrations of CH${}_3$CHO, CH${}_3$OH and H${}^{+}$. The results obtained are shown in Table 3.1. Table 3.1

Use the information in Table 3.1 to deduce the rate equation for this reaction. Explain your reasoning. State clearly which experiments you use in your explanation.

State the units of the rate constant, $k$, for this reaction.

Another experiment to investigate reaction 1 is carried out as shown in Table 3.2.

Calculate the initial rate of experiment 5.

The reaction of propanone, CH${}_3$COCH${}_3$, with bromine, Br${}_2$, in the presence of an acid catalyst is shown in reaction 2.

The rate of reaction is first order with respect to [CH${}_3$COCH${}_3$] and first order with respect to [H${}^{+}$] under certain conditions. A three-step mechanism for reaction 2 is suggested.

Complete Table 3.3 to suggest steps 1 and 3 of the mechanism.

Catalysts can be described as homogeneous or heterogeneous. Describe the mode of action of a heterogeneous catalyst.