Arrhenius pre-exponential factors

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... 

The observation of ratios of isotopic Arrhenius pre-exponential factors that are substantially distant from the semiclassical expectation of unity (Charts 1 and 3). The deviant values may be either substantially smaller than unity or substantially greater than unity. 

Arrhenius pre-exponential factor A, B reactants C molar heat capacity... 

Arrhenius pre-exponential factors (A) vary over a considerable range ... 

Solution According to the Arrhenius equation, k = AeEa/ RT Let ke and kn be the rate constants of the enzyme-catalysed and non-catalysed reactions, respectively. Assuming that the Arrhenius pre-exponential factor A is the same in both cases, we have... 

Comparing Eq. 2.54 with the Arrhenius equation k2=A crHa/RT, we find that the Arrhenius pre-exponential factor is given by... 

Class 1. Characteristics (i) The decomposition in seasoned vessels exhibits first-order kinetics and there are no apparent induction periods (i7) the rate is unaffected by packing the reaction vessel or by the addition of known radical-chain inhibitors such as propene iii) the Arrhenius pre-exponential factor is of the order of 10 sec L This behaviour is consistent with a unimolecular mechanism for the decomposition. Among reactions in this class are included the dehydrohalogena-tion of monochlorinated saturated hydrocarbons containing /S-hydrogen atoms e.g., chloropropane 2-chloropropane f-butyl chloride) and of most of the secondary and tertiary monobrominated saturated hydrocarbons. 

Isolated examples of studies of the decomposition of other nitrocompounds of specialized interest include the thermal decomposition of thin films of nitrocellulose examined by infrared spectroscopy and by a gravimetric technique s . The kinetics are best approximated by a first order curve with two or three branches. Typical values of the activation energy and Arrhenius pre-exponential factor are 45.0 kcal.mole S and 3.54 x 10 sec". ... 

Table I summarizes the KIE s on Arrhenius pre-exponential factor (Ah/Aj and Ad/Ax) and their standard experimental error calculated from a nonlinear least root mean square fit (see data analysis under Methods) for the different GO glycoforms. Additionally, the ratio of ln(H/T) and ln(D/T) (the exponent of equation 4) at 25°C has been included. The different GO glycoforms are denoted by their molecular weight.

D/T KIE s on the Arrhenius Pre-Exponential Factors and Enthalpies of Activation for ll- Hl-2-deoxvglucose with Glucose Oxidase Glycoforms ... 

Rates of bimolecnlar collisions are calculated using the kinetic theory of gases to arrive at the Arrhenius pre-exponential factor and the activation energy is measured experimentally. 

Note An equivalent SI unit for the liter is dm. Rate constants and Arrhenius pre-exponential factors incorporating volume units are sometimes reported using dm or cm instead of liters. Those incorporating amount units are sometimes reported in units of molecules instead of moles. For example, a second-order rate constant of 1.2 x lO" L-moP -s" is equivalent to 1.2 X lO" dm -moP -s" or 2.0 x 10" ° cm molecule" -s". ... 

The total surface area of a decomposing solid must be distinguished from the effective area of the active reactant/product interface which, in most cases, is an internal structure. This active area is not usually accessible to measurement, but if it could be determined, this would enable the Arrhenius pre-exponential factor to be expressed in dimensions of area, as in catalytic studies [49]. 

Table 2.1 Ranges of Arrhenius pre-exponential factors for different reaction types...

Arrhenius pre-exponential factor (units of the corresponding rate constant) reactor length (m)... 

Reaction (28.3) was found to have a surprisingly low Arrhenius pre-exponential factor (log (Ah/s 1) = 5.3). l Br was synthesized in which the three tert-butyl groups had been essentially fully deuterated ( H content > 99%) [4]. Under similar conditions in the EPR an even more persistent IJ, radical was obtained. This also decayed with first-order kinetics and yielded kf/kf 50 at -30 °C. It was thought probable that reaction (28.3) would provide one of the first clear and unequivocal examples of QMT in an H-atom transfer. This reaction and related reactions were therefore examined in considerable detail [4, 5]. 

To overcome this problem, in the new work the (CF3)2NO radical itself was employed [27]. Rate constants for H-atom abstraction from 11 new substrates yielded Arrhenius pre-exponential factors ranging from a low of lO - s i for diethyl ether (temperature range 297-178 K) to a high of lO - s i for 1,4-cyclo-hexadiene (296- 192 K). In addition, the rate constants were measured for a reaction in which QMT could not be involved. This was the addition of (CF3)2NO to CH2=CCl2. 

Stern, M. J., Weston, R. E. J. (1974) Phenomenological manifestations of quantum-mechanical tunneling. 11. Effect on Arrhenius pre-exponential factors for primary hydrogen kinetic isotope effects, J. Chem. Phys. 60, 2808-2814. 

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