Pre-exponential factor for reactions

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. 

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. 

An estimate for the relative rates of radiation and quenching may be made 2md the rate constant of (IS)calculated. It was shown that the pre-exponential factors for reaction (15) and for the non-radiative process... 

The pre-exponential factor for reaction rate constants, activation energies and heats of reactions are given in Table 3.15. And... 

This expression predicts abnormally low pre-exponential factors for reactions between ions of like sign, and abnormally high pre-exponential factors for reactions between ions of unlike sign. It is seen that the influence of the electrostatic interaction between the reactants—unfavorable for ions of like sign, fovorable for ions of unlike sign—makes itself felt exclusively in the pre-exponential factor, the apparent energy of activation being influenced in the opposite direction. 

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]). 

Ao pre-exponential factor of reaction rate constant per attacked atom among bonds with equireactivity same units as for A... 

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. 

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. 

The pre-exponential factor for this reaction should be about 108 0 M"1 sec. 1. 

The pre-exponential factor for the reverse of Reaction 18b was determined by Hanst and Calvert (16) to be 10ir>4 sec."1 for R = CH3. Berces and Trotman-Dickenson (5) estimate the standard entropy of CH3OOCH3 and CH30 to be about 73 and 54.7 cal. per mole-°K., respectively. With these values, kisu can be computed to be 108 8 M"1 sec."1... 

Although an entropy of activation of about —15 cal. mole 1 °K."1 is involved in forming the transition state ring in reactions such as 4, the over-all entropy change accompanying intramolecular hydrogen abstraction is small, and the pre-exponential factor for a unimolecular reaction involving a cyclic transition state has been estimated (10) as 1011... 

Because of the necessity for the interaction of potential curves, the pre-exponential factor for 03 with SO producing the lB state of S02, is much less than that for the reaction producing the ground state. In the 03+N0 reaction these preexponential factors differ only by a factor of 2. 

---