Activation energies determination

Reactions catalyzed by hydrogen ion or hydroxide ion, when studied at controlled pH, are often described by pseudo-first-order rate constants that include the catalyst concentration or activity. Activation energies determined from Arrhenius plots using the pseudo-first-order rate constants may include contributions other than the activation energy intrinsic to the reaction of interest. This problem was analyzed for a special case by Higuchi et al. the following treatment is drawn from a more general analysis. ... 

Relative reactivity wiU vary with the temperature chosen for comparison unless the temperature coefficients are identical. For example, the rate ratio of ethoxy-dechlorination of 4-chloro- vs. 2-chloro-pyridine is 2.9 at the experimental temperature (120°) but is 40 at the reference temperature (20°) used for comparing the calculated values. The ratio of the rate of reaction of 2-chloro-pyridine with ethoxide ion to that of its reaction with 2-chloronitro-benzene is 35 at 90° and 90 at 20°. The activation energy determines the temperature coefficient which is the slope of the line relating the reaction rate and teniperature. Comparisons of reactivity will of course vary with temperature if the activation energies are different and the lines are not parallel. The increase in the reaction rate with temperature will be greater the higher the activation energy. 

Activation energy (Section 5.9) The difference in energy between ground state and transition state In a reaction. The amount of activation energy determines the rate at which the reaction proceeds. Most organic reactions have activation energies of 40-100 kj/mol. 

Not only were the reaction rates for bromination by bromine and by hypobromous acid very similar, but the corresponding activation energies (determined over a 20 °C range) were between 11.8 and 12.6 (for Br2) and 12.5 and 12.7 (for HOBr). Thus all this kinetic data is consistent with the rapid formation of an intermediate which is identical for both brominating reagents, and from which the slow loss of a proton subsequently occurs. 

Minimize the effects of transport phenomena If we are interested in the intrinsic kinetic performance of the catalyst it is important to eliminate transport limitations, as these will lead to erroneous data. We will discuss later in this chapter how diffusion limitations in the pores of the catalyst influence the overall activation energy. Determining the turnover frequency for different gas flow velocities and several catalyst particle sizes is a way to establish whether transport limitations are present. A good starting point for testing catalysts is therefore ... 

Rate constants governing re-orientation of the glucose transporter, and their activation energies, determined from steady-state and pre-steady-state measurements... 

The activation energies determined for the conversion of 2-, 3- and 4-NAP to 2-, 3- and 4-aminoacetophenone (AAP) are reported in Table 8.2, as is the activation energy for the formation of 1-indoline. As might have been expected the activation energies for the aminoacetophenone isomers are indistinguishable. However, the activation energy for 1-indoline is significantly different. 

Fixing the location of the counterion midway between two identical electrophores has been achieved in the radical anion of the dibenzo[ 18] crown-6 derivative [46] (Mazur et al, 1980). When its radical anion exists as the ion pair with Na + or K+, the intramolecular electron transfer becomes detectable on the esr timescale. The activation energy determined for the electron transfer (1.4 kcal mol-1) clearly demonstrates that in this case a significant contribution to the activation barrier from ion pairing can be negated. 

The effect of the addition of water and molecular solvents such as propylene carbonate, N-methylformamide, and 1-methylimidazole on the conductivity of [C4Cilm][Br] and [C2Cilm][BF4] was measured at 298 K [211]. The mixture of the IL and the molecular solvent or water showed a maximum on the conductivity/mole fraction IL curves. The maximum for nonaqueous solvents was at the level of approximately 18-30 mScm at low mole fraction of the IL and the maximum for water was at level approximately 92-98 mScm [211]. The conductivity of a mixture of these two ILs depends monotonically on the composition. The temperature dependence of the conductivity obeys the Arrhenius law. Activation energies, determined from the Arrhenius plot, are usually in the range of 10-40 kj mol / The mixtures of two ILs or of an IL with molecular solvents may find practical applications in electrochemical capacitors [212]. 

In Fig. 2.7, the TSDC observed in As Sei- for As content 8-15 at.% was shown. A peak at 210K dominates in the depolarization curve. In addition, a peak at 244 K and a shoulder at 197 K are discemable. Release of carriers from shallow states and subsequent trapping on relatively deeper ones transform the TSDC into curves 3 and 4 (Fig. 2.7). Activation energy determined for peaks with 210K and 244K is iit2 = 0.35 eV and Fts = 0.45 eV, respectively. These states are energetically distributed A t2 = 0.05 eV and AFto = 0.1 eV, respectively. 

Fig. 7. Arrhenius plot of the rate constant for hydrogenolysis on nickel/MgAl2O4 and silver/nickel/ MgAl2O4. The rate constant (L) for ethane hydrogenolysis is one order of magnitude lower on the silver/ nickel catalyst than on the nickel catalyst, and the activation energies (determined by the slopes of the Arrhenius plots) are similar for the two.

The presence of a photoconductivity peak at 610 nm at the threshold of the absorption spectrum (curve 4) is a common phenomenon in inorganic semiconductors and is explained by competition between surface and volume recombination processes of the charge carriers. The optical activation energy determined from the spectral photoconductivity threshold is equal to 1.82 + 0.02 eV. The thresholds of the photoelectromotive force and the absorption spectra are likewise in agreement with this value. It is remarkable that the same value has been found for the activation energy of the dark conductivity in this polymer... 

The calculations of Klein et al. predicted a larger activation barrier for the diffusion of m-xylene than for diffusion of o- and p-xylene. Thus, m-xylene may be expected to diffuse more slowly than o- or p-xylene, which is inconsistent with the diffusion coefficients and activation energies determined experimentally (24,100). Of course, one of the main factors that precludes a truly meaningful comparison is that the calculations simulate the... 

The copolymerization of epoxides with cyclic anhydrides is a thermally activated reaction. Table 6 gives a survey of the thermodynamic parameters. The activation energies determined by different authors are in good agreement and vary between 52.8 and 64.9 kJ/mol, depending on the monomer used. Exceptions are only the... 

Boos and Flauschildt90) obtained for the model copolymerization of phenylglycidyl ether with hexahydrophthalic anhydride activation energies of 96 kJ/mol up to 75% conversion and 27 kJ/mol for higher conversions. Frequency factors are also very different (log A = 13.7 and 5.5, respectively). The frequency factors as well as the temperature coefficients of the solution viscosities depended on the initiator concentration. The activation energy determined by the same authors 90) for the curing of epoxy resins at conversions lower than 75% was 86.4 kJ/mol and the frequency factor log A = 11.8 whereas at higher conversions these values were not obtained. 

In conclusion, on the basis of the two activation-energy determinations, the rate-limiting step of terminal hydroformylations is not the dissociation of the trisphosphine complex, but rather a subsequent association of a bisphosphine derivative with the 1-olefin reactant. 

The value of activation energy determines the magnitude of the rate constant. It is approximately related to the enthalpy of formation of the activation complex, an intermediate in any chemical transformation. Thus, reaction between A and B, forming C can be formally written as progressing through the activated complex [ABC]. ... 

The activation energy and entropy of the /i transition in MT are listed in Table 6. The activation energy, a/i> is very similar to the activation energies determined in xly -y polymers for the /i transition. In contrast, the activation entropy is quite low, indicating that the ft transition of MT is less cooperative than the fJ> transition of xTy 11 polymers. 

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