Formation activation energies

Fig. 13. Arrhenius plots of the kinetics of H atom recombination on a Ni77Cu23 alloy film catalyst. Above room temperature—active NiCu film with low activation energy. Below room temperature—film deactivated owing to a 0-hydride phase formation activation energy markedly increased. After Karpinski el al. (65).

Catalyst % Dispersion of Ir CO conversion Activation energy (kJ/mol) for COz formation Activation energy (kJ/mol) for H2 formation... 

The Arrhenius diagrams gi for the rate ccxistants pertaining to Uk reacticxi of (V,f) with NQ the values reported in Table 23. The comparison of our kinetic data with those available for the cyclopentadiene-NQ adduct shows that (V,f)-NQ adduct is less stable by 9.1 Kcal/mole, but its formation activation energy is lower by 5,7 Kcal/mole. Also the frequency factor is lower in the case of (VJF)-NQ formation, and aU the data suggest that the steric hindrance existing on tl cyclopentadienyl ring of (V/) exerts its influence on the di odation reaction rather than on the adduct formation. 

Methane. The methane evolution profiles for all five shale samples are surprisingly similar, but occur at significantly higher temperatures than has been observed (2) for the Green River shale. Although some methane evolution accompanies the oil formation, the major part is formed in the secondary pyrolysis region. At least three major processes with maxima in the vicinity of 500, 580 and 700°C appear to contribute to the total methane formation. Activation energies for these processes were determined for Condor carbonaceous shale and are summarised in Table 6. 

Following a similar approach to that of pyrene exclmer formation, activation energies for pyrene- 4. exciplex formation can be obtained from expression of Eqn. 9 in an Arrhenius form and differentiation by 1/T. ki+k2 are obtained from data taken in eyelohexane (32), and A3 and E3 from the lifetime taken at f CA1 - 0. E3 can also be obtained from the slope of the phase dependent dynamic Stern-Volmer plots. As seen in Table 1 the data from each method are in good agreement. The small differences in activation parameters measured in the cholesteric and isotropic phases probably reflect changes in viscosity that accompany phase transitions. 

The stability of polyrotaxanes was accounted for by the macrocycle tendency to assume such a conformation which would assure the maximum dipole-dipole interaction between oxyethylene linls of the chain and ring. This vms confirmed by kinetic studies which demonstrated the polyrotaxane dissodation activaticm energy to amount to 15.9 kcal/mole and the ring-on-chain threading i.e.pofyrotaxane formation, activation, energy to 3.4 kcal/mole. 

Here, r is positive and there is thus an increased vapor pressure. In the case of water, P/ is about 1.001 if r is 10" cm, 1.011 if r is 10" cm, and 1.114 if r is 10 cm or 100 A. The effect has been verified experimentally for several liquids [20], down to radii of the order of 0.1 m, and indirect measurements have verified the Kelvin equation for R values down to about 30 A [19]. The phenomenon provides a ready explanation for the ability of vapors to supersaturate. The formation of a new liquid phase begins with small clusters that may grow or aggregate into droplets. In the absence of dust or other foreign surfaces, there will be an activation energy for the formation of these small clusters corresponding to the increased free energy due to the curvature of the surface (see Section IX-2). 

Colgan E G 1995 Activation energy for Pt2Si and PtSi formation measured over a wide range of ramp rates J. Mater. Res. 10 1953... 

Like tert butyloxonium ion tert butyl cation is an intermediate along the reaction pathway It is however a relatively unstable species and its formation by dissociation of the alkyloxonium ion is endothermic Step 2 is the slowest step m the mechanism and has the highest activation energy Figure 4 8 shows a potential energy diagram for this step... 

The activation energy for this step is small and bond formation between a posi tive ion and a negative ion occurs rapidly... 

The SnI mechanism is generally accepted to be correct for the reaction of tertiary and secondary alcohols with hydrogen halides It is almost certainly not correct for methyl alcohol and primary alcohols because methyl and primary carbocations are believed to be much too unstable and the activation energies for their formation much too high for them to be reasonably involved The next section describes how methyl and primary alcohols are converted to their corresponding halides by a mechanism related to but different from S l... 

Step 4 of the thermal treatment process (see Fig. 2) involves desorption, pyrolysis, and char formation. Much Hterature exists on the pyrolysis of coal (qv) and on different pyrolysis models for coal. These models are useful starting points for describing pyrolysis in kilns. For example, the devolatilization of coal is frequently modeled as competing chemical reactions (24). Another approach for modeling devolatilization uses a set of independent, first-order parallel reactions represented by a Gaussian distribution of activation energies (25). 

The overall requirement is 1.0—2.0 s for low energy waste compared to typical design standards of 2.0 s for RCRA ha2ardous waste units. The most important, ie, rate limiting steps are droplet evaporation and chemical reaction. The calculated time requirements for these steps are only approximations and subject to error. For example, formation of a skin on the evaporating droplet may inhibit evaporation compared to the theory, whereas secondary atomization may accelerate it. Errors in estimates of the activation energy can significantly alter the chemical reaction rate constant, and the pre-exponential factor from equation 36 is only approximate. Also, interactions with free-radical species may accelerate the rate of chemical reaction over that estimated solely as a result of thermal excitation therefore, measurements of the time requirements are desirable. 

This reaction is catalyzed by iron, and extensive research, including surface science experiments, has led to an understanding of many of the details (72). The adsorption of H2 on iron is fast, and the adsorption of N2 is slow and characterized by a substantial activation energy. N2 and H2 are both dis so datively adsorbed. Adsorption of N2 leads to reconstmction of the iron surface and formation of stmctures called iron nitrides that have depths of several atomic layers with compositions of approximately Fe N. There is a bulk compound Fe N, but it is thermodynamically unstable when the surface stmcture is stable. Adsorbed species such as the intermediates NH and NH2 have been identified spectroscopically. 

When [NO] temperature dependence, an activation energy of 316 kj/mol (75.5 kcal/mol). Unfortunately, the rate becomes appreciable just in the range of typical hydrocarbon—ak flame conditions. If it is also assumed that [O] = [O], the observed rate in most lean-flame products in which N2 is roughly 75 mol % of the gas can be approximated by... 

An overview of some basic mathematical techniques for data correlation is to be found herein together with background on several types of physical property correlating techniques and a road map for the use of selected methods. Methods are presented for the correlation of observed experimental data to physical properties such as critical properties, normal boiling point, molar volume, vapor pressure, heats of vaporization and fusion, heat capacity, surface tension, viscosity, thermal conductivity, acentric factor, flammability limits, enthalpy of formation, Gibbs energy, entropy, activity coefficients, Henry s constant, octanol—water partition coefficients, diffusion coefficients, virial coefficients, chemical reactivity, and toxicological parameters. 

In theory two carbanions, (189) and (190), can be formed by deprotonation of 3,5-dimethylisoxazole with a strong base. On the basis of MINDO/2 calculations for these two carbanions, the heat of formation of (189) is calculated to be about 33 kJ moF smaller than that of (190), and the carbanion (189) is thermodynamically more stable than the carbanion (190). The calculation is supported by the deuterium exchange reaction of 3,5-dimethylisoxazole with sodium methoxide in deuterated methanol. The rate of deuterium exchange of the 5-methyl protons is about 280 times faster than that of the 3-methyl protons (AAF = 13.0 kJ moF at room temperature) and its activation energy is about 121 kJ moF These results indicate that the methyl groups of 3,5-dimethylisoxazole are much less reactive than the methyl group of 2-methylpyridine and 2-methylquinoline, whose activation energies under the same reaction conditions were reported to be 105 and 88 kJ moF respectively (79H(12)1343). 

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