Rate of reaction in solution

If rates in solution and in gas phase are to be equal, the activity coefficient factor, i.e./A/B//x must be equal to unity. For unimolecular reactions, where the reactant and activated complex have similar structures and/A and /x do not differ widely, the rate of reaction in solution will be quite similar to that in the gas phase. 

In Sect. 2, a few experimental results were mentioned which strongly indicate that some molecular reactions are limited by the rate of approach of reactants to form an encounter pair. There have been many hundreds of studies of the rates of reaction in solution. Some studies are discussed in books and reviews by Grunwald et al. [19], Pilling [35], Sutin [15], Rice and Pilling [39], Hart and Anbar [17], Birks [6] and Lorand [22]. As it is not the purpose of this article to consider all these studies but to select some of the more recent, detailed and interesting studies to compare theory and experiment, the reader is encouraged to consult these articles as well as this review. 

Rates of reactions in solution and unimolecular reactions in the gas phase are dependent on pressure. 

Change of solvent, permittivity, viscosity and ionic strength can all affect the rates of reactions in solution. 

In both phases, reactants must first diffuse together. In the gas phase the rate of diffusion is so fast that it has no effect on the rate of reaction. In solution, either diffusion or reaction can be rate determining. Normally reaction is the slower process, but in some reactions, such as very fast ionic reactions, diffusion can be rate determining. 

Perhaps a less obvious external variable for investigation of rates of reactions in solution is hydrostatic pressure. Application of the pressure variable is much less common in all branches of chemistry relative to those experimental variables reported thus far. Yet in principle it will be shown that, although not without limitations when reactions have complex mechanisms, the information provided from measuring reaction rates as a function of pressure is of potentially greater value than might be contemplated. Pressure is the empirical parameter with respect to equilibrium and kinetic properties that form the pivotal focus of this contribution, and in subsequent... 

Since the activity coeflficients of the reactants vary with the ionic strength I of the solution, the value of which is determined by the molar concentrations c of the ions i of charge Z[ according to / = 1/2 Ci z , the rate of reactions in solution must also... 

Numerous studies have appeared of the effect of added salts on the rates of reaction in solution, and the area has been reviewed (228). The key concept of the activity coefficient of the activated complex affecting the reaction rate was introduced by Brpnsted (34), which led to an expression of the type in Eq. (15), though variations exist. 

It may seem strange that the droplet growth law is so different in form from the transpon-limited law. After all. the gas-phase species mu.st be transponed to the droplets. Actually, both laws are obeyed. The explanation is that the reactive species are nearly in equilibrium in the gas and droplet phases. Their small displacement from equilibrium differs, however. depending on droplet xfee. but not sufficiently to affect the rate of reaction in solution. 

Other important applications of TST to nonideal reactions have been made for ion/molecule reactions in solution, for reactions in thermodynamically nonideal solutions, and for the effect of pressure on the rates of reaction in solution. These applications are also discussed in detail by Laidler. 

Reactions between species, where the interaction energy is large compared with thermal energies, is markedly different from those reactions where no such interaction occurs. The energetics of reaction of encounter pairs, the timescale for approach of reactants, and the relative importance of other factors are all changed. In principle, these modifications to reaction processes enable more information to be obtained about the whole range of factors complicating any analysis of diffusion-limited reaction rates. However, in practice, the more important factors (such as initial distribution of pair separations, hydrodynamic repulsion, and electric field-dependent mobility) are of themselves unable to explain all the differences between experimental results and theoretical predictions. There is a clear need for further work. Finally, it can be remarked that when interactions between reactants are specifically included in an analysis of these rates of reaction in solution, the chosen theoretical techniques has been almost exclusively the Debye—Smoluchowski equation... 

When we turn from dilute gases to solutions the situation is much more complicated. The rates of reactions in solution, especially those involving ions, are no longer accurately proportional to the volume concentrations, or to their simple powers. In other words, if the experimental results are constrained into the form of an expression such as equation (15 6), the velocity constants turn out to be no longer quite constant, but vary somewhat with the concentrations and they depend, in particular, on the ionic strength. 

The rates of reactions In solution are discussed in detail in the Oxford Chemistry Primer entitled Modern Uqwd Phase Kinetics written by B. G. Cox (OCP 21). 

A similar result has been obtained by Tait et al. (28), who have found that the /-values for various substituted phenoxy radicals also correlate well with gaseous ionization even for reactions in solution and makes less serious the neglect of relative solvation energies so common in interpretations of the effect of structure on relative rates of reactions in solution. 

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