Tamilnadu BoardEnglish MediumSTD 12ChemistryChemical Kinetics3 Marks
Question
Explain pseudo-first-order reaction with an example.
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Answer
A second order reaction can be altered to a first order reaction by taking one of the reactant in large excess, such reaction is called pseudo first order reaction. Let us consider the acid hydrolysis of an ester,
$CH_3 COOCH_{3(a q)}+H_2 O_{(l)} \xrightarrow{H^{+}} CH_3 COOH_{(a q)}+CH_3 OH_{(a q)}$
$\text { Rate }=k\left[CH_3 COOCH_3\right]\left[H_2 O\right]$
If the reaction is carried out with a large excess of water, there is no significant change in the concentration of water during hydrolysis. i.e., the concentration of water remains almost constant.
Now we can define $k\left[ H _2 O \right]= k ^{\prime}$
$\therefore$ The above rate equation becomes
$\text { Rate }=k^{\prime}\left[CH_3 COOCH_3\right]$
Thus it follows first order kinetics.
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