Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

130 EXPERIMENTAL METHODS FOR FAST REACTIONSand for small differences (Xi - xi)-when squared terms in xi and Xi can be neglectedNow, k,,CBZC = klzZA is the condition for equilibrium, hencewhich is the general equation (2) with (l/z) = [klz+kz1(CB+EC)].This illustrates an unusual feature about relaxation times: their expressions alwayscontain the sum of a contribution from the forward and the backward reaction.This has the convenient result that z is independent of the sign of the perturbation,i.e., it is immaterial for a determination of T whether the disturbance causesthe equilibrium to move to the left or to the right.TABLE 1RELAXATION TIMES IN TERMS OF EQUILIBRIUM CONCENTRATIONSAND RATE COEFFICIENTS FOR SINGLE-STEP REACTIONS (cf. Ref. 33)The expressions for T for a few single-step reactions are shown in Table 1. It isinteresting that in the case of a catalyzed reaction (5) (where the catalyst concentrationappears on both sides of the equilibrium) or a buffered system (6)(where theconcentration of the buffered species appears on one side of the equilibrium only)the only concentration term to appear in the relevant part of the expression for t-lis that of the catalyst (G) or the buffered reactant (EB). This is because the changesin concentration of these species (xc, and xB, respectively) are effectively zero,hence the terms in the relaxation expression which contain xc and xB must also bezero. The only terms which do not contain xc and xB are those in Cc and CB, respectively+.So far, only reactions which involve a single step have been considered. The na-t An important proviso for this is that the buffer equilibrium is established rapidly comparedwith the equilibrium under consideration.