Chemical kinetics definition

Ammonia decomposition over Fe, Cu, Ag, Au, and Pt Hydrolysis of starch to glucose catalyzed by acids Mixture of coal gas and air makes a platinum wire white hot Measurements on the rate of H2O2 decomposition Selective oxidation of ethanol to acetic acid over platinum Comprehensive paper on the H2 + O2 reaction on platinum foils, including reaction rates, deactivation, reactivation, and poisoning Definition of catalysis, catalyst, and catalytic force First quantitative analysis of reaction rates Systematic studies on the concentration dependence of reaction rates First concise monograph on chemical kinetics Definition of order of reaction Arrhenius equation k = u exp (-Ea/RT)... 

The key to experimental gas-phase kinetics arises from the measurement of time, concentration, and temperature. Chemical kinetics is closely linked to time-dependent observation of concentration or amount of substance. Temperature is the most important single statistical parameter influencing the rates of chemical reactions (see chapter A3.4 for definitions and fiindamentals). 

Figure 10 shows that Tj is a unique function of the Thiele modulus. When the modulus ( ) is small (- SdSl), the effectiveness factor is unity, which means that there is no effect of mass transport on the rate of the catalytic reaction. When ( ) is greater than about 1, the effectiveness factor is less than unity and the reaction rate is influenced by mass transport in the pores. When the modulus is large (- 10), the effectiveness factor is inversely proportional to the modulus, and the reaction rate (eq. 19) is proportional to k ( ), which, from the definition of ( ), implies that the rate and the observed reaction rate constant are proportional to (1 /R)(f9This result shows that both the rate constant, ie, a measure of the intrinsic activity of the catalyst, and the effective diffusion coefficient, ie, a measure of the resistance to transport of the reactant offered by the pore stmcture, influence the rate. It is not appropriate to say that the reaction is diffusion controlled it depends on both the diffusion and the chemical kinetics. In contrast, as shown by equation 3, a reaction in solution can be diffusion controlled, depending on D but not on k. 

Chemical engineering inherited the definition for the reaction rate from chemical kinetics. The definition is for closed systems, like batch reactors, in which most of the classical kinetic studies were done. Inside a batch reactor little else besides chemical reaction can change the concentration of reactant A. In a closed system, for the reaction of... 

A recent definition of catalysis that is based on dier-modynamics was advanced by the Subcommittee on Chemical Kinetics, Physical Chemistry Division, lUPA ... 

This chapter treats the descriptions of the molecular events that lead to the kinetic phenomena that one observes in the laboratory. These events are referred to as the mechanism of the reaction. The chapter begins with definitions of the various terms that are basic to the concept of reaction mechanisms, indicates how elementary events may be combined to yield a description that is consistent with observed macroscopic phenomena, and discusses some of the techniques that may be used to elucidate the mechanism of a reaction. Finally, two basic molecular theories of chemical kinetics are discussed—the kinetic theory of gases and the transition state theory. The determination of a reaction mechanism is a much more complex problem than that of obtaining an accurate rate expression, and the well-educated chemical engineer should have a knowledge of and an appreciation for some of the techniques used in such studies. 

The main goal of this chapter is to review the most widely used modeling techniques to analyze sorption/desorption data generated for environmental systems. Since the definition of sorption/desorption (i.e., a mass-transfer mechanism) process requires the determination of the rate at which equilibrium is approached, some important aspects of chemical kinetics and modeling of sorption/desorption mechanisms for solid phase systems are discussed. In addition, the background theory and experimental techniques for the different sorption/ desorption processes are considered. Estimations of transport parameters for organic pollutants from laboratory studies are also presented and evaluated. 

The main reasons for investigating the rates of solid phase sorption/desorption processes are to (1) determine how rapidly reactions attain equilibrium, and (2) infer information on sorption/desorption reaction mechanisms. One of the important aspects of chemical kinetics is the establishment of a rate law. By definition, a rate law is a differential equation [108] as shown in Eq. (32) ... 

Although a laminar flame speed. S L is a physicochemical and chemical kinetic property of the unbumed gas mixture that can be assigned, a turbulent flame speed. S T is, in reality, a mass consumption rate per unit area divided by the unbumed gas mixture density. Thus,. S r must depend on the properties of the turbulent field in which it exists and the method by which the flame is stabilized. Of course, difficulty arises with this definition of. S T because the time-averaged turbulent flame is bushy (thick) and there is a large difference between the area on the unbumed gas side of the flame and that on the burned gas side. Nevertheless, many experimental data points are reported as. S T. 

The overall effect of the preceding chemical reaction on the voltammetric response of a reversible electrode reaction is determined by the thermodynamic parameter K and the dimensionless kinetic parameter . The equilibrium constant K controls mainly the amonnt of the electroactive reactant R produced prior to the voltammetric experiment. K also controls the prodnction of R during the experiment when the preceding chemical reaction is sufficiently fast to permit the chemical equilibrium to be achieved on a time scale of the potential pulses. The dimensionless kinetic parameter is a measure for the production of R in the course of the voltammetric experiment. The dimensionless chemical kinetic parameter can be also understood as a quantitative measure for the rate of reestablishing the chemical equilibrium (2.29) that is misbalanced by proceeding of the electrode reaction. From the definition of follows that the kinetic affect of the preceding chemical reaction depends on the rate of the chemical reaction and duration of the potential pulses. 

The sensitivity of explosives has been defined by Koenen et al (Ref 10) as the minimum amount of energy that must be imparted to the explosive, within limited time and space, to initiate explosive decomposition. This definition is, accdg to Ma ek (Ref 13, p 60) meaningful and can serve as a basis of quantitative fundamental treatments provided the imparted energy is thermal and provided its initial distribution in time and space is known. The accuracy of treatments of thermal explosion described in Section IIA of Mafcek s paper is then limited mainly by the accuracy of chemical kinetic data... 

While Gray Yang are not denying the usefulness of such ideas, they consider that too literal an application on the distinction can lead to difficulties. For this reason they tried to unify both theories and this problem is discussed in their paper. They also examined the effect of fuel consumption on thermal explosions, definition of critical conditions and the effects of vessel shapes. Finally, the relationship between thermal explosion criteria and flame theoty described by Belles (Ref 2), as well as detonability limits were pointed out. Comments on the paper of Gray Yang of Profs R.R. Baldwin R. Ben-Aim are given on p 1061 of Ref 3 Refs 1) N.N. Semenov, "Chemical Kinetics... 

Charge transfer resistance, 1056 Charge transfer overpotential, 1231 Charge transfer, partial. 922. 954 Charges in solution, 882 chemical interactions, 830 Charging current. 1056 Charging time, 1120 Chemical catalysis, 1252 Chemical and electrochemical reactions, differences, 937 Chemical equilibrium, 1459 Chemical kinetics, 1122 Chemical potential, 937, 1058 definition, 830 determination, 832 of ideal gas, 936 interactions, 835 of organic adsorption. 975 and work function, 835... 

We adopt this kinetic definition of reaction order without reference to the actual number of molecules involved in each act of chemical transformation. In homogeneous reactions the kinetically determined order is equal to the number of molecules participating in the actual change of which the rate is being measured. In heterogeneous reactions this equality is not necessarily preserved. It will be convenient to call the order inferred from the effect of pressure on the time of half-change the apparent order, and to refer to the number of molecules involved as the true order of the reaction. We have now to consider the relation of the true and the apparent order in various cases. ... 

The foregoing chapters mark a long and not yet finished journey through the special field of the chemical kinetics of solids. It differs from the more common textbooks on kinetics not only because of the immense variety of crystalline phases, but even more in view of the ambiguity in the definition of the correct number of independent thermodynamic state variables. This is the source of many difficulties and particularly with solids containing one or more immobile components or multiphase systems composed of coherent or semicoherent crystals. In coping with this inherent complexity in the foregoing chapters, we chose to restrict ourselves mainly to the fundamental aspects rather than to present many uncorrelated details. 

Many years ago Polya [20] formulated the key problem of random walks on lattices does a particle always return to the starting point after long enough time If not, how its probability to leave for infinity depends on a particular kind of lattice His answer was a particle returns for sure, if it walks in one or two dimensions non-zero survival probability arises only for the f/iree-dimensional case. Similar result is coming from the Smoluchowski theory particle A will be definitely trapped by B, irrespectively on their mutual distance, if A walks on lattices with d = 1 or d = 2 but it survives for d = 3 (that is, in three dimensions there exist some regions which are never visited by Brownian particles). This illustrates importance in chemical kinetics of a new parameter d which role will be discussed below in detail. 

The phenomenon of catalysis is a subject of chemical kinetics, as follows from Ostwald s definition of catalysis. Therefore, the accumulation of data on the kinetics of concrete catalytic reactions favors the progress of the theory of catalysis. 

The application of chemical kinetics to even homogeneous solutions is often arduous. When kinetic theories are applied to heterogeneous soil constituents, the problems and difficulties are magnified. With the latter in mind, one must give definitions immediately for two terms—kinetics and... 

Definition and Verification. The use of mechanistic rate laws to study soil chemical reactions assumes that only chemical kinetics phenomena are... 

For chemical kinetics to be operational and thus Eqs. (2.5) and (2.6) to be valid, Eq. (2.4) must be an elementary reaction. To definitively determine this, one must prove experimentally that Eq. (2.4) and the rate law are valid. 

Methods such as nuclear magnetic resonance (NMR), electron spectroscopy for chemical analysis (ESCA), electron spin resonance (ESR), infrared (IR), and laser raman spectroscopy could be used in conjunction with rate studies to define mechanisms. Another alternative would be to use fast kinetic techniques such as pressure-jump relaxation, electric field pulse, or stopped flow (Chapter 4), where chemical kinetics are measured and mechanisms can be definitively established. 

At the present time, detection of chemical induction in complex reactions is a little-developed area in the field of conjugated reactions. Usually, difficulties occur in the interpretation of the induced reaction mechanism and its kinetic description. In fact, textbooks on chemical kinetics [1 1] either have no methodological notes on these aspects or consider only a narrow, definite type of conjugated processes. 

The term chemical interference defines the interaction between synchronously proceeding complex chemical reactions. To understand this definition, one has only to reject the analogies with optics, on the one hand, and understand that chemical interference is based on ideas of chemical kinetics and the mechanism of complex chemical processes. Thus, I believe that there will be no argument about the term. 

This chapter gives an introduction to the subject of chemical reaction engineering. The first part introduces basic definitions and concepts of chemical reaction engineering and chemical kinetics and the importance of mass and heat transfer to the overall chemical reaction rate. In the second part, the basic concepts of chemical reactor design are covered, including steady-state models and their use in the development... 

Finding the independent chemical reactions provides consistency and proper specification for both material balance and chemical kinetics. By definition, a set of... 

The kinetic method has been the object of an extended research in our laboratory starting in 1934 and although definite results are still to be achieved a discussion of the methods serves to illustrate several important points in chemical kinetics. 

Refs. [i] (1972) Definition, terminology and symbols in colloid and surface chemistry, Part I. Pure Appl Chem 51 77 [ii] Horanyi G (2002) Specific adsorption. State of art Present knowledge and understanding. In Bard AJ, Stratmann M, Gileadi M, Urbakh M (eds) Thermodynamics and electrified interfaces. Encyclopedia of electrochemistry, vol. 1. Wiley-VCH, Weinheim, pp 349-382 [Hi] Calvo EJ (1986) Fundamentals. The basics of electrode reactions. In Bamford CH, Compton RG (eds) Comprehensive chemical kinetics, vol. 26. Elsevier, Amsterdam, pp 1-78 [iv] Baltruschat H (1999) Differential electrochemical mass spectrometry as a tool for interfacial studies. In Wieckowski A (ed) Interfacial electrochemistry, theory, experiment, and applications. Marcel Dekker, New York, pp 577-597... 

One of the most useful static methods of microscopic chemical kinetics is based on the definition of the reaction path as introduced by Fukui. This method offers information on reactions in terms of the intrinsic reaction coordinate (62,144). A theoretical analysis of the minimum energy path was given in Section III,B. Fukui s definition is equivalent to Eq. (34). 

The chemical kinetics approach provides us with more insight with respect to the range of validity. We see from Table 4 that Fick s law indeed results without approximations (l.h.s.). For the pure electrical conduction (r.h.s.) we have to linearize the exponentials (Eqs. 98 and 99), i.e., to assume Ifa I RT which is definitely a good approximation for transport in usual samples it fails at boundaries or for ultrathin samples. Hence the application of Eq. (103) has to presuppose sufficiently thick samples and not too high fields. Table 4 also reveals the connection between Dk, uk and the transport rate constant kk and hence their microscopic meaning ... 

Hie selection of a solid catalyst for a given reaction is to a large extent still empirical and based on prior experience or analogy. However, there are now many aspects of this complex situation that are quite well understood. For example we know how the true chemical kinetics, which are an intrinsic property of the catalyst, and all the many aspects of transport of material and heat around the catalytic particles, interact. In other words, the physical characteristics around the catalyst system and their effects on catalyst performance are well known today. The chemist searching for new and better catalysts should always consider these physical factors, for they can be brought under control, and often in this way definite gains can usually be made both in activity and in selectivity. Further, this knowledge enables us to avoid... 

The concept of the rate limiting step in a sequence of chemical transfer mations is conventionally associated with a bottleneck. It is hardly pos sible to provide a consistent mathematical definition of a bottleneck in terms of classical chemical kinetics, but the concept of the rate controlling step can be defined easily. 

It is quite simple to say that this article deals with Chemical Dynamics. Unfortunately, the simplicity ends here. Indeed, although everybody feels that Chemical Dynamics lies somewhere between Chemical Kinetics and Molecular Dynamics, defining the boundaries between these different fields is generally based more on sur-misal than on knowledge. The main difference between Chemical Kinetics and Chemical Dynamics is that the former is more empirical and the latter essentially mechanical. For this reason, in the present article we do not deal with the details of kinetic theories. These are reviewed excellently elsewhere " The only basic idea which we retain is the reaction rate. Thus the purpose of Chemical Dynamics is to go beyond the definition of the reaction rate of Arrhenius (activation energy and frequency factor) for interpreting it in purely mechanical terms. 

It is true that in chemical kinetics one can disprove, but never definitely prove. A theory or model is conclusively disproved by experimental evidence to the contrary, but compatibility with such evidence is no proof because alternative explanations are always possible and new experiments might support those. ... 

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