H O reaction

The estimation based on the equations of the parabolic model indicates that a reaction of the type (ArO + H02 —> ArOH + 02) involving phenoxyl radicals also requires no activation energy (in this case, AH> A emin = 57kJ mol-1). However, the addition of the peroxyl radical to the aromatic ring of the phenoxyl radical occurs very rapidly. Hence, the rate constant for this reaction is determined by diffusion processes. The data on the Ee0 values are also consistent with this. For the ArO + HOOR reactions with the O H O reaction center and for Am + HOOR reactions with the N H O reaction center, these values are 45.3 and 39.8 kJ mol-1, respectively [23]. At the same time, the calculation of the preexponential factor in terms of the parabolic model indicates that the rate constant k 7 for the reaction of ROOH with the participation of the aminyl radical is several times higher than that for the reaction involving the phenoxyl radical, where the enthalpies of these reactions... 

If the products n, of this reaction are known, Eq. (1.11) can be solved for the flame temperature. For a reacting lean system whose product temperature is less than 1250 K, the products are the normal stable species C02, H20, N2, and 02, whose molar quantities can be determined from simple mass balances. However, most combustion systems reach temperatures appreciably greater than 1250K, and dissociation of the stable species occurs. Since the dissociation reactions are quite endothermic, a small percentage of dissociation can lower the flame temperature substantially. The stable products from a C—H—O reaction system can dissociate by any of the following reactions ... 

Consider for example reactants which are made up only of the atoms of C, H, and O. The stable products for a C - H - O reaction can dissociate by any one of the following reactions ... 

But besides an underproduction of CO by the calculations, HjO was also overproduced. This cannot only be caused by a simple overabundance of H O, since the model gives even lower production rates for H2O than those observed. Huebner and Giguere propose as an explanation the presence of an extended source of water vapor caused by icy grains, ejected from the nucleus, which reach considerable distances from the latter in the coma before they evaporate (Delsemme and Miller ). This would lower the -content in the inner coma and increase the CO" production, since CO" is lost by reacting with H O (reaction (2)). Moreover, the HjO abundance has been derived from the production rates of H and OH, and it cannot be excluded that H O is not the only parent of H and OH. 

Table 2.5. High-temperature, closed-loop chemical C-H-O reactions (Hanneman el ah, 1974 Harth et al, 1981).

Alkali metals. The principal hazards involved are l)explosions or fire resulting from contact with water, chlorinated hydrocarbons, or other reactive agents 2)fires resulting from exposure to air j 3)personneI injury from direct contact with hoc alkali metals and 4)caustic soda burns from the residue of a Na-H O reaction. Storage areas must be dry. Protective clothing must be worn (Ref 105)... 

FIGURE 11.11 Examination of mass transport limitation in the SdVl-H O reaction. Mass transport limitation is absent for points below and to the left of the indicated bounds (Schwartz, 1988). 

The results of this survey are presented in four sections. The first three provide critical reviews of rate constant data for 18 reactions in the N/O, N/H, and N/H/O systems. Arrhenius plots and tabulations of data from selected studies are included together, in most cases, with a recommended rate constant expression. In the fourth section, a more complete listing of N/H/O reactions and rate constants is given, primarily for the convenience of modelers and others interested in a more complete mechanism governing NO kinetics in N/H/O systems. These rate constants have been drawn from a variety of sources and include present evaluations, previous evaluations by the Leeds group, and a large number of estimates. The reader is warned to use such rate constant tabulations with caution in some cases the uncertainties may be quite large. 

It is in this spirit that we provide the following compilation of N/H/O reactions and rate constants (Tables 19-21). Although the list of reactions is not complete and the rate constants listed, and indeed even the products of the reactions in some cases are often not well known, we believe that such a compilation can be of value to combustion modelers and kineticists. The rate constants listed are intended for use only at high temperatures, e.g., T > 1000 K, and have been drawn from a variety of sources. They therefore should be used with considerable caution. In cases where the rate constant is based on a critical evaluation of experimental data, for example, the evaluations of Baulch et al (1973, 1976) or the 18 reactions evaluated in this study, the confidence level should be fairly high. In other cases, and these are in the majority, there is little or no experimental data and only estimates, often quite crude, are provided. These estimates were made in a variety of ways. Whenever possible, the specified rate constant was based on data, even if it was indirect or obtained at low temperatures. In the absence of such data, the expression was generally selected by comparison with results for one or more similar reactions. Obviously, considerable errors can be made by this... 

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