Conversion of Pseudo to Real Rate Constants

Having obtained the exponent a in (1.15) by monitoring the concentration of A in deficiency we may now separately vary the concentration of the other reactants, say B and C, still keeping them however in excess of the concentration of A. The variation of the pseudo rate-constant k with [B] and [C] will give the order of reaction b and c with respect to these species, leading to the expression 

We can use this approach also to examine the effects on the rate, of reactants that may not be directly involved in the stoichiometry (for example, H+) or even of products. It is the most popular method for determining the rate law, and only rarely cannot be used. 

Considering just one other reactant B, we generally find a limited number of observed variations of A (= k7[A] ) with [B]. These are shown as (a) to (c) in Fig. 1.4. Nonlinear plots of k vs [B] signify complex multistep behavior (Sec. 1.6.3). 

In the oxidation of Fe(II)P + (5) by excess O2 there is a second-order loss of Fe(II). The second-order rate constant Ar is linearly dependent on [Oj] with a zero intercept for the [O2] plot. An overall third-order rate law therefore holds. 

When a = b = 1 in (1.42) the overall reaction is second-order. Even a quite small excess of one reagent (here B) can be used and pseudo first-order conditions will still pertain. As the reaction proceeds, the ratio of concentration of the excess to that of the deficient reagent progressively increases so that towards the end of the reaction, pseudo first-order conditions certainly hold. Even if [B] is maintained in only a two-fold excess over [A], the error in the computed second-order rate constant is 2% for 60% conversion.  

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