Main Concepts of Chemical Kinetics

Chemical kinetics together with chemical thermodynamics forms two comer stones of the chemical knowledge. However, classic thermodynamic approach to the description of the chemical systems is based on the consideration of the unconfigured and finite states of the system exceptionally, with the absolute abstracting from any assumptions about the methods (ways) of the transition of the system from the unconfigured to the finite state. Thermodynamics can define whether the system is in equilibrium state. If it is not, than thermodynamics states that the system would certainly pass into the equilibrium state, because the factors for such transitimi exist. StiU, it is impossible to predict, what the dynamics of such transitimi would be, that is in what time the equilibrium state will come, in terms of classic thermodynamic approaches. Such problems are not in interest of thermodynamics, and the time coordinate is absolutely extraneous to the thermodynamics approach. This is the distinction of kind between thermodynamic and kinetic methods of the description of the chemical systems. 

The mainframe notion of the chemical kinetics is the rate of the reaction. Reaction rate is defined as the change in the quantity of the reagent in time unit, referred to the unit of the reactionary space. The concept of reactionary space differs depending on the nature of the reaction. In the homogeneous system the 

L Korobov and VT. Ochkov, Chemical Kinetics with Mathcad and Maple, DOI 10.1007/978-3-7091-0531-3 l, Springer-Verlag/Wien 2011 

Derivative s side here formally shows if the current substance is accumulated or consumed during the reaction. If the volume of the system in the homogeneous reaction is constant (closed system), then dnIV = dC, and therefore, the rate is interrelated to the change of the molar concentration of the reagent in time  

The change of the concentrations of the substances is different due to the different stoichiometry of the interaction between them therefore more exact expression for the rate is as following  

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It is easy to notice that the usual concepts and laws of the kinetics of homogeneous chemical reactions can hardly be used in analysing the examined heterogeneous process. Indeed, difficulties already arise when employing the main concepts of chemical kinetics, namely, the concentration of a reactant or a product in the system and the rate of a chemical reaction. 

This representation of the relationship between the energy and the structural changes occuring in the course of a reaction is the most frequently used. It clearly shows direct connection between the PES characteristics and the main theoretical concepts of chemical kinetics. 

In order to estimate the region of this approximation applicability, it is necessary to examine macrokinetics of a polymeranalogous reaction with explicit allowance for the diffusion of a reagent Z into a globule. In this case, the profile of its constituent monomeric units will be fuzzy rather than stepwise (see Fig. 1). This brings up two questions. The first one is how this profile depends on kinetic and diffusion parameters of a reaction system. The second question is concerned with the effect of the profile shape on the statistical characteristics of the chemical structure of the products of a polymeranalogous reaction. A rigorous theory has been developed [22,23] which enables us to answer these questions. The main concepts of this theory are outlined in the subsequent Sections. 

The rather complex issue of chemical kinetics has been discussed in a quantitative way, in order to stress out two main ideas, namely, the necessity of resorting to simplified kinetic models and the need of adequate methods of data analysis to estimate the kinetic parameters. These results introduce Chap. 3, in which basic concepts and up-to-date methods of identification of kinetic parameters are presented. 

In this chapter, the main concepts and relations of chemical kinetics that are used in reactor design were discussed. We covered the following topics ... 

Other ideas about reaction rate were quoted in the literature in the context of research into students views of chemical equilibrium. For instance, in a review of the literature on students alternative conceptions in chemistry, Garnett, Garnett Hackling (1995) identified, as the main conceptions of 17-19 years old students concerning chemical kinetics ... 

There are two principal chemical concepts we will cover that are important for studying the natural environment. The first is thermodynamics, which describes whether a system is at equilibrium or if it can spontaneously change by undergoing chemical reaction. We review the main first principles and extend the discussion to electrochemistry. The second main concept is how fast chemical reactions take place if they start. This study of the rate of chemical change is called chemical kinetics. We examine selected natural systems in which the rate of change helps determine the state of the system. Finally, we briefly go over some natural examples where both thermodynamic and kinetic factors are important. This brief chapter cannot provide the depth of treatment found in a textbook fully devoted to these physical chemical subjects. Those who wish a more detailed discussion of these concepts might turn to one of the following texts Atkins (1994), Levine (1995), Alberty and Silbey (1997). 

Next, we review findings of educational research about the main areas of physical chemistry. Most of the work done was in the areas of basic thermodynamics and electrochemistry, and some work on quantum chemistry. Other areas, such as chemical kinetics, statistical thermodynamics, and spectroscopy, have not so far received attention (although the statistical interpretation of entropy is treated in studies on the concepts of thermodynamics). Because many of the basics of physical chemistry are included in first-year general and inorganic courses (and some even in senior high school), many of the investigations have been carried out at these levels. 

In 1937, dost presented in his book on diffusion and chemical reactions in solids [W. lost (1937)] the first overview and quantitative discussion of solid state reaction kinetics based on the Frenkel-Wagner-Sehottky point defect thermodynamics and linear transport theory. Although metallic systems were included in the discussion, the main body of this monograph was concerned with ionic crystals. There was good reason for this preferential elaboration on kinetic concepts with ionic crystals. Firstly, one can exert, forces on the structure elements of ionic crystals by the application of an electrical field. Secondly, a current of 1 mA over a duration of 1 s (= 1 mC, easy to measure, at that time) corresponds to only 1(K8 moles of transported matter in the form of ions. Seen in retrospect, it is amazing how fast the understanding of diffusion and of chemical reactions in the solid state took place after the fundamental and appropriate concepts were established at about 1930, especially in metallurgy, ceramics, and related areas. 

Hydrolysis of metal alkoxides is the basis for the sol-gel method of preparation of oxide materials therefore, reactions of metal alkoxides with water in various solvents, and primarily in alcohols, may be considered as their most important chemical properties. For many years the sol-gel method was mosdy associated with hydrolysis of Si(OR)4, discussed in numerous original papers and reviews [242, 1793,243]. Hydrolysis of M(OR) , in contrast to hydrolysis of Si(OR)4, is an extremely quick process therefore, the main concepts well developed for Si(OR)4 cannot be applied to hydrolysis of alcoholic derivatives of metals. Moreover, it proved impossible to apply classical kinetic approaches successfully used for the hydrolysis of Si(OR)4 to the study of the hydrolysis of metal alkoxides. A higher coordination number of metals in their alcoholic derivatives in comparison with Si(OR)4 leads to the high tendency to oligomerization of metal alkoxides in their solutions prior to hydrolysis step as well as to the continuation of this process of oligomerization and polymerization after first steps of substitution of alkoxide groups by hydroxides in the course of their reactions with water molecules. This results in extremely complicated oligomeric and polymeric structures of the metal alkoxides hydrolysis products. 

Chemical kinetics had originated in the classical studies by Van t Hoff and Arrhenius in the 1880s. Then the physical sense of reaction orders was interpreted and the concept of activation energy was suggested. The main ideas in Van t Hoff s book [4] are still appropriate. 

In spite of their seeming variety, theoretical approaches of different authors to the consideration of solid-state heterogeneous kinetics can be divided into two distinct groups. The first group takes account of both the step of diffusional transport of reacting particles (atoms, ions or, in exceptional cases if at all, radicals) across the bulk of a growing layer to the reaction site (a phase interface) and the step of subsequent chemical transformations with the participation of these diffusing particles and the surface atoms (ions) of the other component (or molecules of the other chemical compound of a binary multiphase system). This is the physicochemical approach, the main concepts and consequences of which were presented in the most consistent form in the works by V.I. Arkharov.1,46,47... 

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