Arrhenius plots double

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. 

Boum describe their use of the line shape method in the temperature range —24° to — 82°C (deuterium decoupling was used so that only the unperturbed single proton resonance was observed) and of a double-resonance method in the temperature range —97° to — 116°C. This involved the observation of recovery of magnetization of one of the two lines in the spectrum after a saturating r.f. field applied to the other line was removed. Consistent rates of inversion were found from both methods as evidenced by linearity of the Arrhenius plot. The results do not agree with the spin-echo results of Allerhand et al In this type of work, while fairly consistent results of rate constants may be obtained, there is dispute as to how the thermodynamic parameters should be derived, even in the relatively simple case of cyclohexane. ... 

C., C2, and particularly C3 are the dominant species in the equilibrium vapor in the temperature range of 2400 - 2700 K as shown in the Arrhenius plot of the partial pressure of these species in Fig. 2.19.f l[ l The contribution of Cg and Ictrger molecules to the vapor pressure is small and generally of no practical import. The general structure of these carbon molecule is believed to consist of double carbon bonds, C=C C=C (the so-called cumulene structure) which have delocalized bondings and an axial symmetry. Larger molecules, i.e., the fuiierenes, are reviewed below. 

Experimental determination of Ep, and ko is carried out based on Eq. (4.3.4) by a plot of ln(k) versus 1/T (Arrhenius plot). As a rule of thumb, a rise in temperature by 10 K doubles k (e.g., for Ep, of lOOkJmol and 140 °C, see Topic 4.3.5 for more details). The common range of a values for catalytic and thermal processes is about 30-200kJmol . Caution should he apphed for < 30 kj mol, indicating that we may have mass transport hmitations. Extrapolation of experimentally determined rates far outside the measured range is also problematic, as other mechanisms may prevail that result in different Ea values. 

In 1966 we noted that a log tJq - plot published by Spencer and Dillon for an ar-PS fraction consisted of two straight lines intersecting near T . Later studies by ourselves using both first derivatives and regression analysis with residuals revealed double or triple Arrhenius plots (depending on the temperature range of the data) for ar-PSs of 1.1, for a broad... 

The kinetics of the reaction of OH with 2,2-dimethyl-1-propanol has only been investigated by Tully (1990) at the time of this writing. The measurements are summarized in table ll-B-23 and shown in figure n-B-18. The reaction has been studied using the laser photolysis-laser induced fiuorescence at 750 Torr of helium pressure over the temperature range 293-764 K. As seen from figure ll-B-18, there is pronounced curvature in the Arrhenius plot. A fit of the double Arrhenius expression to... 

If the Arrhenius function is valid, the plot of In k versus T shows a straight line and the slope is -Ea/9I. When determining the activation energy for an ozone reaction, it is important to keep in mind that by increasing the temperature of the water, the solubility of ozone decreases. The same liquid ozone concentration should be used at the various temperatures, which can be a problem in systems with fast reactions. Simplifying the temperature dependency, one could say that the increase of the temperature by 10 °C will double the reaction rate, the so-called van t Hoff rule (Benefield et al., 1982). 

If one plots log kf against 1 IT for a great many reactions, one obtains a straight line. Since the slope is —El2.303 R, E is readily evaluated and is called the Arrhenius activation energy. R has the value of 1.987 cal/deg when E is in cal/mol. A great many reactions double their rate for a 10°C rise in temperature. If we take only two temperatures, it is convenient to use the expression... 

The value of Gtotalgas was found to be 0.87 [385]. Methane and hydrogen are the main products [386, 387]. A new absorption at 11.25 pm, assigned to vinylidene double bonds, was observed [386]. Total unsaturation was measured by titration with iodine monochloride. It corresponds to the formation of 1.87 double bonds per main-chain scission irrespective of temperature. Main-chain scissions were determined at various temperatures (Table 28). An Arrhenius-type plot of Gscission versus temperature gives an activation energy of 2.5 kcal mole-1 above 0°C. Below 0°C, the temperature coefficient is negligible. 

---