When one reactant is in large excess, its concentration remains approximately constant during the reaction. The reaction appears first order with respect to the other reactant. Example: acid hydrolysis of ethyl acetate: CH3COOC2H5 + H2O -> CH3COOH + C2H5OH. True rate = k[ester][H2O]. Since water is solvent ([H2O] ≈ 55.5 M, essentially constant): rate = k'[ester] where k' = k[H2O]. This k' is the pseudo-first order rate constant. Other examples: hydrolysis of sucrose (C12H22O11 + H2O in dilute acid), inversion of cane sugar. Pseudo-first order is a practical simplification — the reaction is still mechanistically second order but kinetically first order.