FUNDAMENTALS OF REACTION RATES

In the preceding chapters, we are primarily concerned with an empirical macroscopic description of reaction rates, as summarized by rate laws. This is without regard for any description of reactions at the molecular or microscopic level. In this chapter and the next, we focus on the fundamental basis of rate laws in terms of theories of reaction rates and reaction mechanisms.  

We first introduce the idea of a reaction mechanism in terms of elementary reaction steps, together with some examples of the latter. We then consider various aspects of molecular energy, particularly in relation to energy requirements in reaction. This is 

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Irreversible statistical mechanics could in principle be used to develop a fundamental theory of reaction rates by treating chemical reaction as the response to chemical potential fluctuations against a reaction barrier, A. 

This chapter presents the underlying fundamentals of the rates of elementary chemical reaction steps. In doing so, we outline the essential concepts and results from physical chemistry necessary to provide a basic understanding of how reactions occur. These concepts are then used to generate expressions for the rates of elementary reaction steps. The following chapters use these building blocks to develop intrinsic rate laws for a variety of chemical systems. Rather complicated, nonseparable rate laws for the overall reaction can result, or simple ones as in equation 6.1-1 or -2. 

Lewis wrote to Paul Ehrenfest that he thought he had "hit upon something pretty fundamental." But Lewis had difficulty publishing his paper, "On the Theory of Reaction Rate," which the editor and referees of the Journal of the American Chemical Society worried was too speculative and insufficiently oriented toward experimental verification.96 Lewis ended by rejecting the radiation hypothesis, even issuing a press release to announce that a decision now had been made between the two rival theories of violent collision and radiation absorption in favor of the former. 97... 

This is fundamentally different from the dependence of reaction rate on temperature that you learned about in Unit 3. The reaction between Hg( ), 02(g), and HgO(s) does not just change its rate with a change in temperature, it changes its direction. 

From a practical point of view, which effect is responsible for the performance of an electrode material is in principle uninteresting in that both converge to improve the electrode behavior. However, from a fundamental point of view, such a distinction is essential to be able to improve and optimize the experimental situation. In terms of reaction rate (current density), only knowledge of the real surface area can allow one to separate experimentally the two factors. If a plot of j against S (real surface area) at constant potential gives a straight line, the effects are more likely to be geometric only. On the other hand, if the correlation deviates from linearity, electronic factors are most likely to operate. This approach assumes that the... 

In the relevant literature, many definitions of reaction rates can be found, especially in the case of catalytic systems. Depending on the approach followed, a catalytic reaction rate can be based on catalyst volume, surface, or mass. Moreover, in practical applications, rates are often expressed per volume of reactor. Each definition leads to different manipulations and special attention is required when switching from one expression to another, hi the following, the various forms of catalytic reaction rates and their connection is going to be presented. Stalling from the fundamental rate defined per active site, the reader is taken step -by step to the rate based on the volume of the reactor and the concept of the overall rate in two- and three-phase systems. 

After in the foregoing chapter thermodynamic properties at high pressure were considered, in this chapter other fundamental problems, namely the influence of pressure on the kinetic of chemical reactions and on transport properties, is discussed. For this purpose first the molecular theory of the reaction rate constant is considered. The key parameter is the activation volume Av which describes the influence of the pressure on the rate constant. The evaluation of Av from measurement of reaction rates is therefor outlined in detail together with theoretical prediction. Typical value of the activation volume of different single reactions, like unimolecular dissociation, Diels-Alder-, rearrangement-, polymerization- and Menshutkin-reactions but also on complex homogeneous and heterogeneous catalytic reactions are presented and discussed. 

While studies of reactions in supercritical fluids abound, only a few researchers have addressed the fundamental molecular effects that the supercritical fluid solvent has on the reactants and products that can enhance or depress reaction rates. A few measurements of reaction rate constants as a function of pressure do exist. For instance, Paulaitis and Alexander (1987) studied the Diels Alder cycloaddition reaction between maleic anhydride and isoprene in SCF CO2. They observed bimolecular rate constants that increased with increasing pressure above the critical point and finally at high pressures approached the rates observed in high pressure liquid solutions. Johnston and Haynes (1987) found the same trends in the... 

Fundamental deactivation data are more difficult to obtain than fundamental catalytic reaction rate data because the latter must be known before the nature of the deactivation function can be determined. This is largely due to the kinds of reactors that are used to study deactivation. Many of the usual difficulties experienced in trying to get fundamental deactivation data can be obviated by using a reactor system in which the conversion and hence the compositions of the major components remain constant both in time and in space within the reactor. A description of an apparatus of this type and its utilization to study the deactivation of a real catalytic reaction are presented in this paper. The problem of determining the initial activity in a rapidly deactivating system is also discussed. 

Colorimetric detection relies on the enzymatic production or consumption of a molecule with a distinctive absorption spectrum. Measurement of reaction rate or extent is made via absorption measurements at or near the maximal absorption wavelength. The technique has several fundamental limitations that restrict its use in highly sensitive assays. However, it is undeniably the simplest method of assessing an assay endpoint, and suitable instruments are available in most laboratories. 

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