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Question

What is pseudo first order reaction? Give one example of it.


Solution

Pseudo first order reactions: The reactions that have higher order true rate law but are found to behave as first order are called pseudo first order reactions.
Example: Consider the acid hydrolysis of methyl acetate
$$CH_3 COOCH_{3(aq)} + H_2O_{(l)} \rightarrow CH_3 COOH_{(aq)} + CH_3OH_{(aq)}$$
Since the solvent water is also participating in the reaction, the true rate law of the reaction shoul be,
$$rate = k' [CH_3 COOCH_3] [H_2O]$$          .......... (i)
According to the rate law equation, the reaction is expected to have second order kinetics. However, experimentally the reaction is found to follow first order kinetics. The reason is that water as solvent is present in such a large excess that its concentration remains constant. Thus,
$$[H_2O] = constant = k^n$$                     ......... (ii)
Substitution of equation (ii) in (i) gives
$$rate = k' [CH_3 COOCH_3] \times [H_2O]$$
$$= k' [CH_3COOCH_3] \times k"$$
$$\therefore$$ Actual rate laws, $$rate = k'k" [CH_3 COOCH_3]$$
$$\therefore$$   $$rate = k [CH_3 COOCH_3]$$            ......... (iii)
where, $$k = k'k"= constant$$ 
This indicates that the second order true rate law (represented by equation (i) is forced into first order rate law (represented by equation (iii). Hence, this bimolecular reaction which appears of second order is actually pseudo first order reaction.

Chemistry

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