Pseudo-First Order

The approximation is an experimental device as much as an algebraic one. A reaction genuinely second order overall, first order in each of two reactants, becomes first order in appearance when one reactant is supplied in tenfold or greater excess, since its concentration changes negligibly over the course of the run and can be folded into the rate constant. The observed pseudo-first-order constant is then the true second-order constant multiplied by the concentration held in excess, and repeating the experiment at several excess concentrations recovers the true constant from the slope of a linear plot. The practical advantage is that first-order kinetics yield a straight line from a logarithmic plot and a half-life independent of concentration. Hydrolysis in aqueous solution is the standard case, water being present in vast excess.