Arrhenius parameters curvature

Usually the Arrhenius plot of In k vs. IIT is linear, or at any rate there is usually no sound basis for coneluding that it is not linear. This behavior is consistent with the conclusion that the activation parameters are constants, independent of temperature, over the experimental temperature range. For some reactions, however, definite curvature is detectable in Arrhenius plots. There seem to be three possible reasons for this curvature. 

A more interesting possibility, one that has attracted much attention, is that the activation parameters may be temperature dependent. In Chapter 5 we saw that theoiy predicts that the preexponential factor contains the quantity T", where n = 5 according to collision theory, and n = 1 according to the transition state theory. In view of the uncertainty associated with estimation of the preexponential factor, it is not possible to distinguish between these theories on the basis of the observed temperature dependence, yet we have the possibility of a source of curvature. Nevertheless, the exponential term in the Arrhenius equation dominates the temperature behavior. From Eq. (6-4), we may examine this in terms either of or A//. By analogy with equilibrium thermodynamics, we write... 

Equation (13) cannot be employed to measure A C directly. Even when this parameter has a substantial value, the resulting curvature in the Arrhenius plot, In A against IJT, is difficult to recognise and can certainly... 

Of Interest Is the absolute-rate-constant study carried out by Zellner and Stelnert (163) for the reaction of OH radicals with CH4 over the temperature range 298 to 892°K, where, as may be expected, a priori, from either collision or transition state theory, the Arrhenius plot shows strong curvature. Such effects must be expected to be a general occurrence, and this study points out (a) the need for accurate data over a wide temperature range, and (b) the need for caution In the extrapolation of Arrhenius expressions (which should be viewed as empirical experimental two-parameter fits) beyond the temperature ranges experimentally covered. 

Figure 1 is quite simple but, to our knowledge, no determination of the individual values of the activation parameters for the k(, and kd processes have previously been available. One of the primary purposes of the present work is to discuss an analysis that yields such values. These activation parameters, in turn, are used to illustrate the curvatures that exist in Eyring or Arrhenius treatments of the temperature dependences of the observed rate constants for free radical recombination, trapping and formation by thermolysis of a covalent precursor in solution. 

The curvatures of the Arrhenius plots of y in Fig. 7 are governed mainly by the behavior of a, the dispersion parameter of trap depths. In this model the disc-like molecules of cis-butene-2 produce a [Pg.265]

In alloys presenting an order-disorder transition (case of CuZn see Kuper et at., 1956), the Arrhenius plot of nD versus 1/r shows a change of slope at the transition and a curvature in the LRO state. This has been tentatively explained by (i) an increase of the vacancy migration energy with the LRO parameter S (Girifalco, 1964), (ii) a decrease of the correlation factor in the LRO state (Bakker, 1984), or (iii) different competing mechanisms (Stolwijk et al., 1980). 

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