General Types of Solution Reactions

Important solution reactions are well known from the study of quantitative analysis in analytical chemistry. These include acid-base reactions, redox reactions, and complex formation reactions. Often the reaction is so fast that the system is considered to come instantaneously to equilibrium, for example, in the acid-base reaction involved in a titration. In fact, any of these reactions has a finite rate whose kinetics can be determined using modern experimental techniques. 

The above reactions may be described in a more fundamental way in terms of the elementary act involved in the reaction. For example, the reaction between an acid and a base involves proton transfer so that it is better described as a proton 

In a bimolecular solution reaction, the reactants A and B diffuse to a point close to one another at which reaction is possible. This process is called formation of the precursor complex. At this point, rearrangement of bond lengths and bond angles in the two reactants, and of the surrounding solvent molecules, can occur to form an activated complex or transition state between the reactants and products. As one would expect, the nature of this process depends on the specific reaction involved. It is the focus of the development of the theory of the elementary step in the reaction and the associated energy requirements. In some cases it has been studied experimentally using very fast laser spectroscopic techniques which provide time-resolved information about the elementary step in the femtosecond range. 

In the case of very fast bimolecular reactions, the contributions of both the diffusion process and the rate of decomposition of the precursor complex must be considered in analyzing the experimental observations. The overall reaction mechanism can be described as 

In Lhe case of slow reacLions for which k,. A di(Y, the overall rate constant Arnet is equal to the rate constant for transformation of the precursor complex, k. However, for very fast reactions, k is greater than A difr, so that the overall rate constant is close to Ardiff- Therefore, it is important to understand that there is an upper limit on reaction rates that can be measured experimentally for diffusion controlled processes. In section 7.5 it is shown that A difr for molecular reactants is given by 

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