Arrhenius activation energies, negative

As for other organics in the atmosphere, the OH radical is a major oxidant for alkenes. Table 6.8 gives the rate constants for some OH-alkene reactions as well as their temperature dependence in Arrhenius form. Several points are noteworthy (1) the reactions are very fast, approaching 10-l() cm3 molecule-1 s-1 for the larger alkenes (2) the rate constants have a pressure dependence (3) the apparent Arrhenius activation energies are negative. ... 

All of the silenes discussed above and in Section III.B.3.a appear to react with MeOH by the stepwise mechanism shown in Scheme 4, as judged by the fact that negative Arrhenius activation energies have been observed in every example whose temperature dependence has been studied44. Those derivatives for which MeOH is on the order of 109 M s-1 or higher exhibit linear dependences of fcdecay on MeOH concentration (i.e. kn Js> fey [MeOH]) over the range of alcohol concentrations for which data are obtainable... 

There was not a clear explanation to the close to zero and negative Arrhenius activation energies, experimentally observed for formaldehyde and acetaldehyde, respectively [95-97]. 

Rate equations state rates of formation (if positive) or consumption (if negative) of species in terms of moles per unit volume and unit time as functions of the local and momentary concentrations of the participants. For gas-phase reactions, partial pressures may be substituted for molar concentrations. Where necessary, rate coefficients are identified by double indices, the first member for the reactant, the second for the product (co-reactants and co-products are disregarded). The temperature dependence of rate coefficients is characterized by Arrhenius activation energies. 

A characteristic feature of the ethylene hydrogenation on nickel is that its rate has a maximum at about 60°C., above which the Arrhenius activation energy is negative (9). Zur Strassen and Schwab (12) ascribed this to a desorption of ethylene from the surface of the nickel catalyst, while Twigg and Rideal attributed it to a desorption of hydrogen held over gaps in the layer of adsorbed ethylene molecules. Both these interpretations of the reversal in the sign of the temperature coefficient are based on the assumption that the rate is controlled by the desorption of the dominant adsorbed molecule. 

The negative Arrhenius activation energy observed for the reaction of OH radicals with CH3CHO (197) Is noteworthy, further discussion being given below In Section G-2-a-l. 

Figure 5.8 demonstrates the temperature dependence of exact and JSA rate constants for the D+H2(t = l,j) reaction. The temperature range is 200 - 1000/if, plotted in inverse Kelvin. The exact rate constants were obtained from Eq. (5.4) and the cross sections in Fig. 5.7, and the JSA rate constants were obtained from Eqs. (5.41)-(5.43) and the J = 0 reaction probabilities in Fig. 5.4. Very good agreement is obtained for all j values. In all cases, the JSA predicts the correct Arrhenius activation energy (i.e. negative of the slope of log k(T) vs. 1/ksT) and is qualitatively correct in predicting the Arrhenius prefactor (i.e. y-intercept). The overall agreement is truly excellent for = 1 and 2. However, the JSA systematically underestimates the rate constant by roughly 40% for j = 0 and 3.

Reaction rates almost always increase with temperature the rare ones that do not have a negative activation energy will be dealt with later. The expression of the temperature dependence is always given for the rate constant, rather than the rate. For now, only elementary reactions will be considered, with composite reactions and other more complicated situations deferred to Section 7.5. Two forms are commonly used to express the rate constant as a function of temperature. The first is the familiar Arrhenius equation,... 

Three possibilities were considered to account for the curved Arrhenius plots and unusual KIEs (a) the 1,2-H shift might feature a variational transition state due to the low activation energy (4.9 kcal/mol60) and quite negative activation entropy (b) MeCCl could react by two or more competing pathways, each with a different activation energy (e.g., 1,2-H shift and azine formation by reaction with the diazirine precursor) (c) QMT could occur.60 The first possibility was discounted because calculations by Storer and Houk indicated that the 1,2-H shift was adequately described by conventional transition state theory.63 Option (b) was excluded because the Arrhenius curvature persisted after correction of the 1,2-H shift rate constants for the formation of minor side products (azine).60... 

Experiments were also performed at various temperatures in the presence of DPPH. Although the data fitted the conventional Arrhenius relationship (Eq. 5.39), it gave an activation energy which was negative i. e. inverse Arrhenius behaviour. 

Mozurkewich M, and Benson, S. W., Negative activation energies and curved Arrhenius plots. 1. Theory of reactions over potential wells, J. Phys. Chem. 88, 6429 (1984). 

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