Pre-exponential factor, for

Table 3.6. Experimental activation energies and pre-exponential factors for CO and NO desorbing from a range of clean and well-defined single crystals. All data were obtained in the low coverage regime. [From V.P. Zhdanov, J. Pavlicek and Z. Knor, Catal. Rev.-Sci.

Sketch the transition state and give an order of magnitude for the pre-exponential factor for the following reactions ... 

The pre-exponential factor for the H -i- H2 reaction has been determined to be approximately 2.3 x lO " mol cm s . Taking the molecular radii for H2 and H to be 0.27 and 0.20 nm, respectively, calculate the value of the probability factor P necessary for agreement between the observed rate constant and that calculated from collision theory at 300 K. 

The products from the reaction of CH3 + Q monolayers on the promoted Cu3Si surface are the same as those for pure Cu3Si, but both the absolute rates and the selectivities are significantly different. In experiments analogous to those described in section 3.1, methylchlorosilanes are evolved from the promoted CusSi surface between 300 and 450 K. This temperature is 200 K lower than that from the pure Cu3Si surface. This 200 K difference in reaction temperature corresponds to a difference of six orders of magnitude in rate (if the rates are extrapolated to a common reaction temperature of 500 K assuming standard and equivalent pre exponential factors for the reactions on these two surfaces [10]). 

The procedure of limiting the RHR and finding a pre—exponential factor for each test series proved to be very robust... 

Following the conceptual idea introduced by Milliken [68], Takahashi and Glassman [53] have shown, with appropriate assumptions, that, at a fixed temperature, i/c could correlate with the number of C—C bonds in the fuel and that a plot of the log ipc versus number of C—C bonds should give a straight line. This parameter, number of C—C bonds, serves as a measure of both the size of the fuel molecule and the C/H ratio. In pyrolysis, since the activation energies of hydrocarbon fuels vary only slightly, molecular size increases the radical pool size. This increase can be regarded as an increase in the Arrhenius pre-exponential factor for the overall rate coefficient and hence in the pyrolysis and precursor formation rates so that the C/H ratio determines the OH concentration [12]. The 4>c versus C—C bond plot is shown in Fig. 8.14. When these... 

A similar transition complex can be written for the decomposition of the 1,1-diethylcyclopropane. However, since the pre-exponential factor for 6... 

Table 13 Arrhenius activation energies and pre-exponential factors for thermal isomerization of tetraphenylethenes [42] and the standard enthalpy differences in benzene solution."...

Rate constants for the self-reactions of a number of tertiary and secondary peroxy radicals have been determined by electron spin resonance spectroscopy. The pre-exponential factors for these reactions are in the normal range for bi-molecular radical-radical reactions (109 to 1011 M"1 sec 1). Differences in the rate constants for different peroxy radicals arise primarily from differences in the activation energies of their self reactions. These activation energies can be large for some tertiary peroxy radicals (—10 kcal. per mole). The significance of these results as they relate to the mechanism of the self reactions of tertiary and secondary peroxy radicals is discussed. Rate constants for chain termination in oxidizing hydrocarbons are summarized. 

A Pre-exponential factor for surface reaction B Pre-exponential factor for gas phase reaction D Diffusion coefficient E Activation energy... 

For the fresh and the specifically aged catalyst materials, the dependence of the normalized NOx storage capacity on temperature could be kept the same (Giithenke et al, 2007b). This minimized the number of parameters to be re-adapted for two catalysts with different ageing level. Thus, only the maximum NOx storage capacity and the pre-exponential factors for the reactions R1-R22 had to be re-evaluated, cf. Table III and Eq. (36). 

Regular Relation between Activation Energies and of Pre-Exponential Factors For Elementary Reactions... 

It may be seen from Figure 5 that pre-exponential factors for oxygen atom reactions are lower than those for OH reactions by a power of ten. 

Pre-exponential factors for reactions of CO with HO or O are lower by a power of ten than those corresponding to the curve. This might appear to be a rule. 

More explicit knowledge of absolute rate constants, activation energies and pre-exponential factors for elementary reactions involving other active particles, as in the interaction between radicals with a longer carbon chain and various molecules, is necessary for obtaining a better insight into the nature of these regularities. 

Activation energies and pre-exponential factors for the first-order rate coefficients for anthracene oxidation... 

The importance of Reaction 7 can be estimated from the following considerations. When R is C2H5, then k8/k9 is greater than 103 at room temperature (29). Since k8 = 108 8 M 1 sec."1 and the pre-exponential factor for Reaction 9 must be about 109 M 1 sec."1, the activation energy Eo for Reaction 9 must exceed 4.4 kcal. per mole. On the other hand, it cannot be much larger than this because Reaction 9 is important at moderate temperatures. Bensons calculations (3) yield about 4 kcal. per mole. A value of 5 kcal. per mole is adopted here. If the intermediate to product formation is the same in Reactions 7 and 9, then by straightforward manipulation of thermodynamic data, k7 is k8k8/k8 1014 0 exp (—33,000/RT) sec. 1. 

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