Kinetic simple

Warning if a chemical process comprises several reaction steps, only the progress of the slowest step can be followed kinetically. These graphical methods of determining k are only useful for obtaining the rate-determining step (RDS) of such reactions. Although the reaction may appear kinetically simple, it is wisest to assume otherwise. 

The rate-law (2) can usually be established without too much difficulty by appropriate kinetic experiments, but it must be remembered that the same system may follow different rate-laws under different conditions, and it is evident that such kinetically complicated systems are generally unsuitable for attempts to determine the fundamental rate constants. However, a sufficient number of kinetically simple systems is now known which are much more useful for such studies. 

In this review there is for the first time a comparative discussion of the three propagating species the unpaired cation, the paired cation and the ester formed from the monomer and an acidic initiator. The relative kinetic importance of these three under different conditions of temperature and of solvent polarity are discussed qualitatively and by means of a three-term rate-equation. From these considerations are derived the optimum conditions for achieving a monoeidic system with the aim of obtaining kinetically simple reactions. It is also emphasised that an initiation reaction that is fast compared to the propagation, and the chemistry of which is known and simple, is essential for the unambiguous determination of propagation rate constants. 

Our studies have confirmed that the oligomerisation of styrene by perchloric acid is both chemically and kinetically simple. However, the reactions which follow completion of the polymerisation, and during which carbonium ions are formed and destroyed, are complicated. They can be rationalised in terms of equilibria involving ions, acid, double bonds, and esters cyclisation of olefinic oligomers and formation of polyenes by way of allylic ions add further complications. We have thus shown in some detail just why such systems must be treated with the greatest circumspection if they are to yield valid information. 

The simplest example of a flame-supporting medium is a pure chemical compound which decomposes exothermically. The widespread interest in such flames is due to their possibilities as monopropellants. Many studies are motivated by purely fundamental considerations, since a decomposition flame can be a kinetically simple flame. The most widely used and studied combustion reactions are those between hydrocarbons or hydrocarbon derivatives and air or oxygen. However, many other chemical substances may be substituted for the common fuels and/or oxidizers. Flames of uncommon fuels and oxidizers are most important because of their possibility of surpassing ordinary hydrocarbon oxidation as a source of energy. Some unusual flames are discussed in reference (PI). 

Uncomplicated model reactions should be chosen in which the rate-determining step is an acid-catalyzed transformation. Reactants should therefore be selected that do not undergo extensive side reactions and that undergo kinetically simple reactions such as unimolecular cracking or isomerization. 

Reactions that take place in a single step, that is, with a single transition state and no intermediates, are known as elementary reactions. They may represent an entire, kinetically simple reaction or a step in a more complex mechanism. In either case, the rate law for an elementary reaction is derived from its stoichiometry. This stands in contrast with more complex reactions, where the relationship between the overall stoichiometry and the rate law cannot be predicted, and both must be established experimentally. 

In a kinetically simple medium, the overall rate constant of product formation, kas, can be defined by the relations... 

An early study" " of the decomposition of sodium perborate in aqueous solution indicated that the process is not kinetically simple. Although approximately first-order, the reaction is said to be subject to retardation (possibly due to reversibility) by the reaction products and to slight acceleration by polymeric borate species. A number of catalytic species were found and others have been reported since, e.g. Wilson" and Prokopeikas et The reaction with iodide has been used as the basis for a kinetic method of analysis for molybdate". ... 

Arsenite appears to react in a kinetically simple manner with periodate in acetic acid-acetate buffers. The reaction is reported to correspond to the process... 

Direct photolysis often is kinetically simple and easily modeled, especially if the absorption spectrum of the compound and its quantum yield of disappearance are known. The average photoreaction rate in direct photolysis can be expressed as... 

These thermal and photoreactions clearly involve an initiation step in which radicals are formed, followed by a variety of propagation steps. It is reasonable to assume that the radicals finally disappear by reaction in pairs. In any case, only the initiation step is different in the two reactions, propagation and termination being identical. The overall energy of activation, Eq, in a kinetically simple radical reaction of this kind, which is strictly analogous to radical polymerization, is given by... 

In the presence of halogen-carriers , such as iodine or pyridine, further kinetic complexities result. Buckles and Yuk found that the bromination of crotonic acid was a kinetically simple reaction only in the presence of tetra-methylammonium tribromide, the overall kinetic form in ethylene dichloride being... 

Where the reaction does not proceed by a kinetically simple route, or where the reacting species are not immediately evident (e.g. in the acid-catalysed condensation of benzaldehyde with acetophenone), the kinetic equation is described in a footnote. 

The effect of oxidation temperature on the rate constants in the L-P regime is shown in Fig.6(18). It is seen that there is considerable curvature in the Arrhenius plots. This curvature indicates that neither of the steps yielding the fluxes, F and F2, represent kinetically simple rate steps. This finding should not really be surprising, as the L-P model is quite simplistic. 

D P. Bentz, Influence of water-to-cement ratio on hydration kinetics simple models based on spatial considerations, Cement and Concrete Research, Vol 36, N°2 (2006) p. 238... 

Most reactions are not elementary. The hydrogen-iodine reaction looks like an elementary bimolecular reaction it is first order in H2, first order in I2, and second order overall. However, this reaction is not elementary [425]. Reactions that behave kinetically just like a single-step, elementary reaction are called kinetically simple. The order of each reactant in such a reaction is equal to its stoichiometric coefficients. Such reactions are also said to obey the law of mass action or to have mass-action kinetics. 

WWER neutron kinetics, simple models of coolant mixing test facility... 

From the kinetic point of view, a BR is often presented as an attractive alternative. For the majority of various kinds of reaction kinetics— simple reactions with approximately elementary reaction kinetics, consecutive reactions, and mixed reactions—the BR gives a higher yield as well as a higher amount of desired intermediate products than a continuous stirred tank reactor (CSTR), and this is why the BR competes with a tube reactor in efficiency. 

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