Arrhenius plots slightly curved

Two sets of experimental data are available, one set measured by NMR spectroscopy in the range 200-300 K [8, 9] and another set measured by optical spectroscopy in the range 95-130 K [10]. Arrhenius plots of the data shown in Fig. 29.11 are slightly curved at low temperature the slope approaches a constant value close to the calculated cis-trans energy difference of 8.3 kcal moh. The observation that double proton transfer remains thermally activated by roughly this amount down to low temperatures, immediately suggests that the process proceeds stepwise. 

In the prediction of degradation rate according to the Arrhenius equation for degradations exhibiting slightly curved Arrhenius plots, nonisothemal prediction may provide an... 

Figure 45. Slightly curved Arrhenius plots used for Monte Carlo simulation studies. (1,2) Concave plots (3) linear plots (4, 5) convex plots. (Reproduced from Ref. 348 with permission.)...

Table6. Estimatedf90 Values from Slightly Curved Arrhenius Plots 6...

In the foregoing discussion, it has been supposed that the height of the potential barrier will be the same at aU temperatures. This is probably not so. As the temperature increases the lattice will expand and, in general, E would be expected to decrease. Moreover, some of the other constant terms in the preceding equations may vary with temperature. For example, an expansion of the crystal is fikely to lead to an increase in the vibration frequency, v. The Arrhenius plots reveal this by being slightly curved. 

Arrhenius equation. If the reaction was zero order, the slope of the temperature-time plot would continue to increase until the reaction was complete, as shown by the dashed line in Figure 1.7. However, for orders of V2, 1, or 2, the curves are quite similar in shape, and very accurate data would be needed to distinguish between different orders. Furthermore, many reactions do not follow the Arrhenius equation exactly, and slight differences in activation energy should not be used to decide between possible orders or mechanisms. The correlation of adiabatic kinetic data is discussed by Rodriguez [5]. 

In Fig. 3 are shown the results obtained with a 1% alkali chitin solution. The cloud point can be determined as the temperature at which a very slight increment in the absorbance arises. From optical measurements it could be appreciated that the cloud point is centred at 30 C (Fig. 3a). The appearance of a slight opalescence in the solution at this temperature is in very close correspondence with the onset of changes in q equilibrium values. The latter are manifested by a well defined break-point in the Arrhenius-type plots (Fig. 3c). The dependence of the break-point values of each ti (T ) plot with frequency, CD, is also evident. The behaviour of the system was also monitored by the ratio of fluorescence intensities of pyrene at 384 and 372 nm, when excited at 343 nm. From the curve depicted in Fig. 3b, it can be appreciated a decrease in the fluorescence ratio when the system is heated with an inflexion point at 21.5°C (as determined by the first derivative method), signalling the onset of self-aggregation of the polymer. 

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