ABSTRACT
The experimental determination of the rate law is considerably simplified by the method of isolation in which all the reactants except one are present in large excess. To a good approximation, the concentrations of the excess reactants remain constant during the reaction which enables the order of the reaction with respect to the isolated reactant to be determined directly from observation of the kinetics of just the isolated species. For example, if the true rate law for a reaction is:
rate=k[A][B]2
and reactant B is in excess, then the concentration of B throughout the reaction can be approximated by its initial value [B]0 and the rate law becomes:
rate=k′[A]
where k′=k [B]02 is still a constant. Since the original third order reaction has been converted into a first order form the latter rate law is classified as pseudo-first order to indicate that the rate law disguises intrinsic higher order and only applies under particular conditions of reactant relative concentrations. k′ is called the pseudo-first order rate constant.
