Pseudo First Order

The hydrolysis of an ester is inherently second order, depending on both the ester and water concentrations through rate = k[ester][water]. However, when water is present in very large excess, the amount consumed is negligible compared with the total, so the water concentration remains effectively constant throughout the reaction. The term k[water] then becomes a constant pseudo-rate-constant k', and the rate simplifies to rate = k'[ester], which is mathematically first order. Such a reaction is termed pseudo first order because it behaves as first order only because one reactant's concentration is held nearly fixed. The option that water concentration changes significantly contradicts the excess condition. The claim that the ester acts as a catalyst is wrong; the acid is the catalyst and the ester is a reactant. The option that it is truly first order ignores the hidden dependence on water. This is the classic NCERT example of pseudo order. Carefully relating the data to the governing principle ensures the reasoning remains valid even when the numbers or species in the question are changed. Plausibility check: removing the excess water would restore the observed second order behaviour, confirming the pseudo nature.