Activation energy and A factor

Letort determined the initial rate coefficient, ko, in the temperature range 473-577.5 °C and at initial pressures of 50-450 torr, obtaining the expression 

Reinterpreting the results of Hinshelwood and Hutchison (temperature range 430-592 °C and initial pressures of about 400 torr), he derived the following value 

Later Boyer et suggested (on the basis of the data of Hinshelwood et as well as on those of Letort ) the following expression for the temperature range 

Eusuf and Laidler determined a rate coefficient in the temperature range 480-525 °C, which is in reasonable agreement with the one given by Boyer et al., viz. 

From the measurement of the initial rate of formation of methane at 480, 500 and 520 °C, Come et derived the equation 

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In general, intramolecular isomerization in coordinatively unsaturated species would be expected to occur much faster than bimolecular processes. Some isomerizations, like those occurring with W(CO)4CS (47) are anticipated to be very fast, because they are associated with electronic relaxation. Assuming reasonable values for activation energies and A-factors, one predicts that, in solution, many isomerizations will have half-lives at room temperature in the range 10 7 to 10 6 seconds. The principal means of identifying transients in uv-visible flash photolysis is decay kinetics and their variation with reaction conditions. Such identification will be difficult if not impossible with unimolecular isomerization, particularly since uv-visible absorptions are not very sensitive to structural changes (see Section I,B). These restrictions do not apply to time-resolved IR measurements, which should have wide applications in this area. 

Although quantitative measurements of the rate constants for these three types of reactions are not available, they appear to have very high efficiencies, i.e., low-activation energies and A factors near 1010 liter/... 

Although complex reactions can be classified as non-chain and chain, the type of experimental data collected and the manner in which it is analysed is common to both. The ultimate aim is to produce a mechanism, to determine the rate expression and to find the rate constants, activation energies and A-factors for all of the individual steps. 

TABLE 3a Activation Energies and A-Factors for Thermal cis - tram Isomerization... 

Kineticists have been excited by the prospect of obtaining detailed information on the dynamics of chemical reactions at molecular level from molecular beam studies. The experimental difficulties of the technique are formidable and the results have been limited mainly to the reactions of alkali metal atoms. Molecular beam studies of transfer reactions are omitted here, where the emphasis is on bulk kinetics and the measurements of rate coefficients and Arrhenius parameters (activation energies and A-factors). 

If we consider the above isomerization process to have a low activation energy and A factor, while the radical chain process proposed earlier proceeds via a higher activation energy process, with a higher A factor, then the molecular (first order) process should be observed at lower temperatures with a gradual transition being observed to the radical chain process as the temperature is increased, providing the rates for... 

The activation energy and A factor derived from Figure 2 indicate k 109 e 31/-. These values are somewhat higher than those reported in the litera-tiu e for the dimerization of butadiene ( ) ... 

In 2010, the research groups of Lewis and Wasielewski examined the hole migration process in DNA hairpins conjugated with St and stilbenediether (Sd) by means of the laser flash photolysis. The rates were found to be 4.3 X 10 and 1.2 x 10 s for the single-step G-to-G and A-to-A hopping, respectively. Furthermore, from the temperature dependence, the activation energy and A-factor... 

The gas phase decomposition A B -r 2C is conducted in a constant volume reactor. Runs 1 through 5 were conducted at 100°C run 6 was performed at 110°C (Table 3-15). Determine (1) the reaction order and the rate constant, and (2) the activation energy and frequency factor for this reaction. 

Fourth, the model of a rigid cage for a bimolecular reaction in the polymer matrix helps to explain another specific feature. This model explains the simultaneous increase in activation energy and preexponential factor on transferring the reaction from the liquid (Eh At) to solid polymer matrix (Es, As). In the nonpolar liquid phase / obs = E = gas but in the polymer matrix [3,21] it is... 

The role of the isothermal and pseudo-first-order reaction assumptions on the observed value of activation energy was assessed to allow comparison of our data to previous work by modifying Malkin s autocatalytic equation so that the autocatalytic term b is equal to zero. The values of the activation energy and front factor were calculated using short-time, low-conversion data. By making the autocatalytic term equal to zero, the modified Malkin autocatalytic model becomes a first-order rate reaction. Table 1.2 shows that by assuming a... 

The adsorbed hydrogen coverage ( H(a)) is at present unknown. The activation energy and preexponential factor can be taken from the work of Christmann et al. (396a). These are 9 kcal mol-1 and 0.075 s 1, respectively. 

In either case, one has to proceed through a four-centered complex for which there is no appreciable driving force and for which all evidence indicates an order of 20 to 26 kcal. of strain energy. In addition, the A factor for such a reaction, because it involves a tight transition state with loss of internal rotation, is expected to be low by about a factor of 10. The over-all result is that one expects a four-centered transition state, a relatively high activation energy, and a relatively low A factor. 

The accuracy of any of the above-mentioned methods of analytically determining the rate of propagation of a deflagration wave depends finally on the validity of the rate laws used, and on the values of the physical constants of the gases under consideration. In particular, the activation energy, and steric factor for any combustible are very important parameters. Much work is being done on the kinetics of chemical reactions, so that more accurate data on reaction rates will be available. It is hoped that this work will lead to better agreement between theoretical and experimental results. 

One of the important limitations in the use of DSC for the study of expls is that decompn is often accompanied by, or is a consequence of, melting or sublimation. Data analysis of such systems results in kinetic orders which have no significance. The problem was examined by Rogers (Ref 32) who noted that organic expls decomp normally more rapidly in the melt and, therefore, show very high apparent activation energies and preexponential factors, and that, therefore, compds which decomp without autocatalysis decomp in a DSC at a rate which is max when the melt is complete. For this reason Rogers used only the data above the ATmax peak. He performed the decompn iso thermally and ob-... 

It also follows from their arguments that the activation energy and proportionality factor e should be less, the more exothermic the reaction, and that the transition state should be correspondingly closer in structure to the reactants. In our case it is clear that the partial bond to JR in VII will be weaker, and the n bonds between atoms r, s and atom t will be stronger, the more exothermic the formation of III. This will correspond to a larger value of / . Comparison of Eqs. (62) and (86) shows that the proportionality factor e is here given by ... 

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