Apparent negative activation energy

Experimentally, the reaction is found to be third order with reaction law of d / df=kii5pS[0] [02]. From the reaction law, it seems that the reaction is an elementary reaction. However, itwas found (Bamford and Tipper, 1972, p. 169) that kii5 = T exp(-0.187 + 1000/T) for 293 Arrhenian behavior of reactions. A reaction mechanism that involves chain reactions that can explain the apparent negative activation energy is as follows. Suppose the above reaction is accomplished by the following two elementary reactions ... 

The plot for 19e shows precisely this behaviour, exhibiting an apparent negative activation energy at temperatures above ca 13 °C and distinct curvature over the -20 to 13 °C temperature range as the rate of product formation (k ) approaches and then surpasses that of reversion of the complex to reactants (k-c). The turnover point where kn = k-c occurs at a temperature of ca — 6°C. Since at this point roh = l/2fcc, it is possible to calculate a value of kc for the reaction of this silene with MeOH at this particular temperature. The value obtained, kc 5 x 109 M 1 s 1, is roughly a factor of three... 

The reactions of oxygenated VOCs with the OH radical proceed mainly by H-atom abstraction from C-H and, to a much lesser extent, from O-H bonds, or by addition to the carbon atoms of any C=C bonds [19,22]. Many of these reactions exhibit apparent negative activation energies, especially at temperatures below room temperature, i.e. the rate constants are observed to decrease with increasing temperature... 

An alternative detailed interpretation is that of Lin and Bauer [439], who investigated the reaction between CO and N2O in a single pulse shock tube at temperatures between 1320 and 2280 K. At the lower end of the temperature range the direct bimolecular reaction between CO and N2O was important, but above 1600 K the dominant reaction path was the dissociation of N2O followed by reactions of O atoms. In the analysis of their results, Lin and Bauer used the rate coefficient from Olschewski et al. [324] for the primary N2O dissociation step, and they obtained an apparent negative activation energy of — 23.4 kcal. mole" for reaction (Ivii)... 

At first sight, one may think that the reaction proceeds via a simultaneous collision of three molecules (2NO + O2), but this occurs very infrequently and cannot explain the observed rate. Furthermore, the rate decreases with temperature, which contradicts the law of Arrhenius. If we use the experimental data given in Figure 6.4.21a, and assume that the rate is inversely proportional to the reaction time, we get an apparent negative activation energy of —20 kj mol by the slope = [EA,apparent/(-RT)] of the plot ofln(rNo) versus 1/T... 

An interesting example of the effect of on a detachment reaction is found in a study of the reaction of electrons with p-benzoquinone (Holroyd, 1982). This reaction is fast and has a positive activation energy in n-pentane but an apparent negative activation energy in tetramethylsilane and neopentane (Fig. 3). The results are attributed to an equilibrium... 

These reactions were measured mass spectrometrically in a discharge flow stemat pressures of 470Nm, ind they thus appear to be true bimoleculcir processes. Both reactions have apparent negative activation energies (-5 kJ/mole for F + HCN and -2 kJ/mole for F + (CN)2>. The reaction of F with HCN is followed by... 

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