Salt Effects on Other Reaction Orders

The methods developed in the preceding section, but not the explicit equations, are applicable for reactions that are not second-order. We start with an example of the reaction between Fe(IIl) and Sn(II), as studied in solutions of HCIO4, HC1, and LiC104. With these components both [H+] and [Cr ] could be varied at constant p. We consider conditions in which the major species are Fe3 and Sn2q+. The rate law is 

We will first explore what salt effect is expected for k and k2, and then examine the general situation. It is convenient to proceed by defining the net activation process.15 This is the chemical equation for the hypothetical process in which the transition state is formed from the predominant forms of the reagents, and not from the reactive entities. For the two pathways implicit in Eq. (9-47), these are the net activation processes  

These representations offer the advantage that one need not argue which of the reagents carries OH or Cl into the transition state. Since that is usually not known, this notation sidesteps the issue. From the Brpnsted-Debye-Huckel equation, we recognize that the concentration of each transition state (and therefore the reaction rate) will vary with ionic strength in proportion to the values of K for the given equation. For the first term we have 

We see that k is largely independent of ionic strength. How can that be, given that several ions are involved The net activation process is 

For this reaction (A 2) is zero, and no ionic strength effect is expected. 

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