Arrhenius rate constant

The symmetry coefficient = —P d nk/dAE is usually close to j, in agreement with the Marcus formula. Turning to the quantum limit, one observes that the barrier transparency increases with increasing AE as a result of barrier lowering, as well as of a decrease of its width. Therefore, k grows faster than the Arrhenius rate constant. At 7 = 0... 

The constant Ar00 is the pre-exponential constant in the high-pressure Arrhenius rate constant for decomposition of the (stable) C molecule to form products D and E. 

With this demonstration that the kinetics of the phenolysis reaction are second order in nature, an Arrhenius rate constant model... 

Lignin Sulfonate in Phenol (61.5%) using the Arrhenius Rate Constant Model (15)... 

This value of the activation energy, together with the corrected rate constants for each temperature, was employed to evaluate the pre-exponential constant, k0, in the Arrhenius rate constant model. The value thus derived was... 

Arrhenius rate constants show a point of inflection at T = E/2R, Above this temperature the curves are, of course, concave to the T axis. However, for any reaction of possible interest here, this would correspond to temperatures of 10,000°K or more. 

The Arrhenius rate constant varies from 0, when T =... 

AlA pre-exponential factor in equilibrium constant Arrhenius rate constants of forward and back reactions constants in a form of f = [I + 2... 

Comparing Eq. (6) with the usual Arrhenius rate constant gives a linear dependence of the activation energy on potential... 

Here t is time, x the coordinate directed along the catalytic element axis, T, To the temperature of the catalyst and the reaction medium, C, Cq the reactant concentrations at the catalyst surface and in the bulk-flow, m, k the heat capacity per unit volume and thermal conductivity of the catalyst, D the reactant diffusion coefficient for the gaseous phase, k = koexp — EIRT) the Arrhenius rate constant for a first-order reaction, q the thermal effect of the reaction, 3 the characteristic size of the effective film, " a and b numerical coefficients of the order of unity related to the element geometry (for simplicity, let a = f> = 1). 

In our own laboratories, a new technique called very low pressure pyrolysis (VLPP) (60) has permitted us to make direct studies on high temperature rate processes at sufficiently low pressures that secondary processes are negligible. Under such conditions (1200°K, 10 4 torr), we have found that the four-center elimination of HI from isopropyl iodide can be as much as 1/1000 as fast as one would estimate from extrapolating the Arrhenius rate constant from lower temperatures. 

In the above kinetic expression, the Arrhenius rate constant, 1, is modified by the two adsorption equilibrium constants of methanol and water during the steady state kinetic studies. During the transient TPRS experiments, however, only the Arrhenius rate constant, kj, is measured since there is no vapor phase methanol or water to be equilibrated. The similar TPRS results for the oxidation of the surface methoxy intermediate to formaldehyde over the different supported vanadia catalysts reveal that all the catalysts possess the same kj. Consequently, the dramatic differences in the steady state TOFs during methanol oxidation over the different supported vanadia catalysts must be associated with the methanol and water equilibrium adsorption constants. Both methanol and water can be viewed as weak acids that will donate a proton to a basic surface site, but methanol is more strongly adsorbed than water on oxide surfeces, K, > [16]. Furthermore, the methanol oxidation TOFs... 

Another reaction in solution where curved Arrhenius plots have been ascribed to the tunnel effect is the protonation of the anion of 2,4,6-trinitrotoluene by hydrofluoric (25a) and acetic (24) acids. The Arrhenius plots started to deviate from linearity at —20 and — 90°C, respectively. From the slopes of the straight part, values of Ea and hence of the predicted Arrhenius rate constants at low temperatures, were calculated and compared with the experimental results. This is shown in Fig. 8. Data for monochloroacetic acid are included (24) where the effect was very small even at — 114 C. 

K is the reactivity rate constant, D is a constant, and is the critical extent of curing at which the glassy state is attained. Because switching from a reactivity-controlled reaction takes place gradually, the overall rate constant k (x, T) can be expressed using the Rabinovitch model [13] in terms of the Arrhenius rate constant (k) or reactivity rate constant (k ) and diffusion rate constant (k ) as follows [19, 20] ... 

TABLE 5.2 Arrhenius Rate Constants and Reaction Orders for the Pyrolysis of PTFE and the Relevant Gas-Phase Reactions of the Pyrolysis Products... 

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