The reactions which are not truly of first order but become reactions of first order under certain conditions are called pseudo first order reactlons.
For example, hydrolysis of ester.
Rate \(=\mathrm{k}^{\prime}\left[\mathrm{CH}_{3} \mathrm{COOC}_{2} \mathrm{H}_{5}\right]\left[\mathrm{H}_{2} \mathrm{O}\right]\)
Since water is in excess and \(\left[\mathrm{H}_{2} \mathrm{O}\right]\) is constant, so
Rate \(=\mathrm{k}\left[\mathrm{CH}_{3} \mathrm{COOC}_{2} \mathrm{H}_{5}\right]\), where \(\mathrm{k}=\mathrm{k}^{\prime}\left[\mathrm{H}_{2} \mathrm{O}\right]\)
