Salt Effects and Reaction Mechanisms

We shall demonstrate that the magnitude of the salt effect is an attribute of the rate law, not of the reaction mechanism. To do so, let us consider two mechanisms by which the second term of the rate law in Eq. (9-49) might proceed. It is easy to envisage that 

The experimental constant ki is either K k or K k. Now each of the equilibrium constants has a predictable ionic strength effect. If the Debye-Hiickel equation is applied to them, we have 

Both logl0( iki) and logl0(FTn A n) both give a coefficient of 2A in Eq. (9-53), regardless of which scheme is applicable. 

The reader can show that a third scheme also gives the same answer. In it the two cations first associate (however unlikely), and this dinuclear complex reacts with Cl-. To summarize any reaction scheme consistent with the rate law is characterized by the same ionic strength effects. In other words, it is useless to study salt effects in the hopes of resolving one kinetically indistinguishable mechanism from another. 

The precept just set forth so rigidly is violated in only one special case. In every instance that constitutes an exception, the rate law does not give the composition of the transition state. Most of the exceptions are therefore chain reactions. 

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