Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Homogeneous formal Kinetics

Formal kinetic investigations (performed only with acidic ion exchange catalysts) revealed, in most cases, the first-order rate law with respect to the alkene oxide [285,310,312] or that reaction order was assumed [309,311]. Strong influence of mass transport (mainly internal diffusion in the polymer mass) was indicated in several cases [285,309, 310,312,314]. The first-order kinetics with respect to alkene oxide is in agreement with the mechanism proposed for the same reaction in homogeneous acidic medium [309,315—317], viz. [Pg.330]

Described in Section 2.1.1 the formal kinetic approach neglects the spatial fluctuations in reactant densities. However, in recent years, it was shown that even formal kinetic equations derived for the spatially extended systems could still be employed for the qualitative treatment of reactant density fluctuation effects under study in homogeneous media. The corresponding equations for fluctuational diffusion-controlled chemical reactions could be derived in the following way. As any macroscopic theory, the formal kinetics theory operates with physical quantities which are averaged over some physically infinitesimal volumes vq = Aq, neglecting their dispersion due to the atomistic structure of solids. Let us define the local particle concentrations... [Pg.67]

The forward and backward heterogeneous rate constants, kfh and kbh, are formal constants, which implies that activity coefficients are assumed to be unity. In normal homogeneous solution kinetics, for example,... [Pg.31]

The latter difficulty is easily overcome by extrapolation. Furthermore, it is clear that if the mean content of a component in a compound is equal to 40-60 %, while the range of homogeneity is 1-2 % or less, then possible deviations from the quasistationary concentration distribution can hardly be expected to have any noticeable effect on the results of analytical description of layer-growth kinetics. This is especially so in the case of reaction diffusion. The layer of any chemical compound grows mainly at the expense of stoichiometry of that compound and not at the expense of its range of homogeneity. Therefore, for formal kinetics it does not matter,... [Pg.58]

The results confirm that TGA experiments are not significantly affected by heat transport phenomena if low initial sample masses as well as the described TGA configuration and experimental procedures are applied. The temperature gradients inside the samples are sufficiently small to allow the fitting of formal kinetic models to the experimental mass loss curves assuming a homogeneous sample temperature. Cellulose samples with initial sample masses of around 5 mg and higher can only be submitted to kinetic analyses under consideration of the enthalpy balance. [Pg.1082]

Formal kinetic treatments of the homogeneous bimolecular combination of radicals and of the homogeneous scavenging of radicals by the matrix or any additive in the solid phase has been given by Waite [216] and by Lebedev [217]. These recombinations usually occur in crystalline and glassy substances at temperatures close to the melting point or close to the glass or other phase-transition temperature. The recombination can then be described by the same kinetic expression (usually a second-order equation) until total decay is observed. [Pg.239]

Simple homogeneous liquid-phase reactions can be described with the help of formal kinetic rate laws in which the reaction rate r depends on the concentrations of the reactants, on the temperature and possibly on the homogeneous catalyst only. Examples of such formal kinetic rate laws are presented in Table 4-1. [Pg.75]

The calculation principle on which the assessment of design for such reactors is based is a substitution of the multi-phase reaction system by a quasi-single-phase model. In two-phase systems both reactants have to get into contact at a certain place. Consequently a reaction and a transport phase are distinguished. If the mass transfer rate from the transport to the reaction phase is veiy fast compared to the actual reaction rate, the process in total is dominated by the reaction kinetics. In order to discriminate this situation from one taking the mass transfer into account, it is referred to as micro-kinetically dominated In this ease all formal kinetic laws presented for homogeneous systems may be applied directly. [Pg.80]

The formal kinetics of a heterogeneous reaction having been disentangled, the problem still presents itself why the route by way of the adsorbed condition should frequently prove more expeditious than that of a homogeneous reaction. There is no one single explanation, any more than there is one for the power of molecides to exert forces on one another in general. Numerous causes contribute. [Pg.405]

The secondary equations (9) and (10) are conventionally called the reactor kinetics equations, rather than the primary equations (1) and (2). They are of the same/orm as the bare homogeneous reactor kinetic equations. The definition of reactivity in (6) and the reactor kinetics equations (9) and (10), or more general formulations of the same concepts, are often used in reactor physics whether or not physical significance can be associated with the formalism. They do however represent generalizations in a mathematical sense. Thus, in this paper, (6) has been termed the generalized reactivity and (9) and (10) the generalized kinetic equations. [Pg.259]

To minimize diffusion effects, established kinetic practice requires that samples should be as small as possible (thin layers spread on multistory crucible [574,689]). However, the smaller the sample, the greater is the ratio of its surface to its bulk and this may overemphasize surface reactions and make correlation with large-scale processes poorer. Experience shows, however, that even very small samples (less than 1 mg) are far from being small enough to be free of diffusion inhibition. To justify the obvious errors, the adjectives "apparent", "formal" and "procedural" are used in conjunction with the otherwise strictly-defined terms of activation energy and reaction order as established in homogeneous chemical kinetics. [Pg.395]

In this chapter we consider some problems of the formal kinetics of homogeneous catalytic and enzymatic reactions. Since many public editions are devoted to the kinetics of chemical and enzymatic reactions, here we revise briefly only general problems. A great attention is given to sections directly associated with the specific problems of catalysis. The theory of elementary acts of chemical and enzymatic processes is presented in Giapters 4 and 6. [Pg.406]

Thus the kinetic equation may be derived for operator (7.21), though it does not exist for an average dipole moment. Formally, the equation is quite identical to the homogeneous differential equation of the impact theory with the collisional operator (7.27). It is of importance that this equation holds for collisions of arbitrary strength, i.e. at any angle of the field reorientation. From Eq. (7.10) and Eq. (7.20) it is clear that the shape of the IR spectrum... [Pg.234]

Kinetics based on the idea of spreading is formally based on the model of development of an infectious disease among human population [59,60]. The formalism of chemical kinetics, however, should be treated with a care as a similar equation can be derived from the homogeneous model assuming bimolecular decomposition of hydroperoxides as an initiating event. [Pg.482]

Spatial homogeneity of a system (needed for making use of the formal chemical kinetics) is secured, first of all, by complete particle mixing. On... [Pg.66]

The product elimination step proceeds with cleavage of the catalyst-substrate bonds. This may occur by dissociation, solvolysis, or a coupling of substrate moieties to form the product. The last of these involves covalent bond formation within the product, and corresponds to the microscopic reverse of oxidative addition. Upon reductive elimination both the coordination number and formal oxidation state of the metal complex decrease. In most homogeneous catalytic processes, the product elimination step, while essential, is usually not rate determining. The larger kinetic barriers are more frequently encountered in substrate activation and/or transformation. [Pg.83]

These properties are likely to have an important influence on the behavior of intact biochemical systems, e.g., within the living cell, enzymes do not function in dilute homogeneous conditions isolated from one another. The postulates of the Michaelis-Menten formalism are violated in these processes and other formalisms must be considered for the analysis of kinetics in situ. The intracellular environment is very heterogeneous indeed. Many enzymes are now known to be localized within 2-dimensional membranes or quasi 1-dimensional channels, and studies of enzyme organization in situ [26] have shown that essentially all enzymes are found in highly organized states. The mechanisms are more complex, but they are still composed of elementary steps governed by fractal kinetics. [Pg.39]


See other pages where Homogeneous formal Kinetics is mentioned: [Pg.39]    [Pg.273]    [Pg.421]    [Pg.419]    [Pg.256]    [Pg.21]    [Pg.21]    [Pg.22]    [Pg.23]    [Pg.24]    [Pg.27]    [Pg.38]    [Pg.248]    [Pg.154]    [Pg.372]    [Pg.301]    [Pg.316]    [Pg.327]    [Pg.53]    [Pg.472]    [Pg.16]    [Pg.260]    [Pg.51]    [Pg.54]    [Pg.247]    [Pg.253]    [Pg.54]   
See also in sourсe #XX -- [ Pg.26 ]




SEARCH



Formal Kinetics of Homogenous Reactions

Formal Kinetics of Multiple Homogenous Reactions

Formal Kinetics of Single Homogenous Reactions

Homogeneous kinetics

Kinetic homogeneity

Kinetic homogenity

© 2024 chempedia.info