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Question
Explain pseudo first order reaction with a suitable example.
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Solution
- Certain reactions which are expected to be of higher order follow the first-order kinetics. Such reactions are said to be pseudo-first-order reactions.
- Example: Consider hydrolysis of methyl acetate.
\[\ce{CH3COOCH_{3(aq)} + H2O_{(l)} -> CH3COOH_{(aq)} + CH3OH_{(aq)}}\]
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.
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