Question

Answer the following in brief.

Give one example and explain why it is pseudo-first-order.

Answer 1

Example: Consider the hydrolysis of methyl acetate.

$$\mathrm{CH}{\phantom{A}}_3\mathrm{COOCH}{\phantom{A}}_3{\phantom{A}}_{(\mathrm{aq})}+\mathrm{H}{\phantom{A}}_2\mathrm{O}{\phantom{A}}_{(\mathrm{l})}⟶\mathrm{CH}{\phantom{A}}_3\mathrm{COOH}{\phantom{A}}_{(\mathrm{aq})}+\mathrm{CH}{\phantom{A}}_3\mathrm{OH}{\phantom{A}}_{(\mathrm{aq})}$$

The rate law is rate = k' [CH₃COOCH₃][H₂O]

Explanation: The reaction was expected to follow the second-order kinetics, however, obeys the first order. The reason is that solvent water is present in such large excess that the change in its concentration is negligible compared to the initial one or its concentration remains constant. Thus [H₂O] = constant = k''. The rate law becomes rate = k' [CH₃COOCH₃]k'' = k [CH₃COOCH₃]

where, k = k'k''

The reaction is thus of first order.