What is psuedo first order reaction? Give an example.

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Solution

Reactions that have higher order true rate law but are found to behave as first order are called psuedo first order reactions.
Hydrolysis of methyl acetate is psuedo first order reaction.
$C H_{3} - C O O C H_{3} + H_{2} O \to C H_{3} C O O H + C H_{3} - O H$
The true rate law is rate $= k^{'} [C H_{3} - C O O C H_{3}] [H_{2} O]$ . It represents second order reaction. But water (solvent) is present in large excess and its concentration is constant.
$k^{'} [H_{2} O] = k =$ constant
Hence, the rate law becomes, rate $= k [C H_{3} - C O O C H_{3}]$
It represents first order reaction.

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