Transition state theory of chemical

Isotope effects on rates (so-called kinetic isotope effects, KIE s) of specific reactions will be discussed in detail in a later chapter. The most frequently employed formalism used to discuss KIE s is based on the activated complex (transition state) theory of chemical kinetics and is analogous to the theory of isotope effects on thermodynamic equilibria discussed in this chapter. It is thus appropriate to discuss this theory here. 

Temperature Effect Determination of Activation Energy. From the transition state theory of chemical reactions, an expression for the variation of the rate constant, k, with temperature known as the Arrhenius equation can be written... 

The rate constant can be expressed by using molecular, statistical mechanical, and thermodynamical quantities, functions, and formulations. For instance, in the transition state theory of chemical reactions for an elementary reaction... 

The quantities A %H A t/, A S and A G are used in the transition state theory of chemical reaction. They are normally used only in connection with elementary reactions. The relation between the rate constant k and these quantities is... 

Now, according to the transition-state theory of chemical reaction rates, the pre-exponential factors are related to the entropy of activation, A5 , of the particular reaction [A = kT ere k and h are the Boltzmann and Planck constants, respectively, and An is the change in the number of molecules when the transition state complex is formed.] Entropies of polymerization are usually negative, since there is a net decrease in disorder when the discrete radical and monomer combine. The range of values for vinyl monomers of major interest in connection with free radical copolymerization is not large (about —100 to —150 JK mol ) and it is not unreasonable to suppose, therefore, that the A values in Eq. (7-73) will be approximately equal. It follows then that... 

In general, both and k will decrease with increasing solvent polarity. The transition-state theory of chemical reactions suggests that this is because the initial state (monomer plus ion or ion pair) is more polar than the activated complex in which the monomer is associated with the cation, and the charge is dispersed over a larger volume. More polar solvents will tend, then, to stabilize the initial slate at the expense of the transition complex and reduce k and kp. 

The second point is that the new phase-space representation permits the definition of a true dividing surface in phase space which truly separates the reactant and product sides of a reaction. Traditional transition state theory of chemical reactions, based simply on coordinate-space definitions of the degrees of freedom, required an empirical correction factor, the transmission... 

More importantly, a molecular species A can exist in many quantum states in fact the very nature of the required activation energy implies that several excited nuclear states participate. It is intuitively expected that individual vibrational states of the reactant will correspond to different reaction rates, so the appearance of a single macroscopic rate coefficient is not obvious. If such a constant rate is observed experimentally, it may mean that the process is dominated by just one nuclear state, or, more likely, that the observed macroscopic rate coefficient is an average over many microscopic rates. In the latter case k = Piki, where ki are rates associated with individual states and Pi are the corresponding probabilities to be in these states. The rate coefficient k is therefore time-independent provided that the probabilities Pi remain constant during the process. The situation in which the relative populations of individual molecular states remains constant even if the overall population declines is sometimes referred to as a quasi steady state. This can happen when the relaxation process that maintains thermal equilibrium between molecular states is fast relative to the chemical process studied. In this case Pi remain thermal (Boltzmann) probabilities at all times. We have made such assumptions in earlier chapters see Sections 10.3.2 and 12.4.2. We will see below that this is one of the conditions for the validity of the so-called transition state theory of chemical rates. We also show below that this can sometime happen also under conditions where the time-independent probabilities Pi do not correspond to a Boltzmann distribution. 

In the case of one independent variable, a local maximum could be distinguished from a local minimum or an inflection point by determining the sign of the second derivative. With more than one variable, the situation is more complicated. In addition to inflection points, we can have points corresponding to a maximum with respect to one variable and a minimum with respect to another. Such a point is called a saddle point, and at such a point, the surface representing the function resembles a mountain pass or the surface of a saddle. Such points are important in the transition-state theory of chemical reaction rates. 

A useful way to do this is to use the transition state theory of chemical reaction rates (e.g., see Glasstone, Laidler, and Eyring [55] also, for a current review, sec Laidler [56]). This is based on the hypothesis that all elementary reactions proceed through an activated complex ... 

The activation enthalpy, in (2.23) plays the role of activation energy, Fa, in the Arrhenius equations (2.21) and (2.22). In a number of textbooks, dealing with the transition-state theory of chemical reaction kinetics, we can find the formula... 

For many years the Tafel equation was viewed as an empirical equation. A theoretical interpretation was proposed only after Eyring, Polanyi and Horiuti developed the transition-state theory for chemical kinetics, in the early 1930s. Since the Tafel equation is one of the most important fundamental equations of electrode kinetics, we shall derive it first for a single-step process and then extend the treatment for multiple consecutive steps. Before we do that, however, we shall review very briefly the derivation of the equations of the transition-state theory of chemical kinetics. 

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