Kinetic parameters

Abstract

The wide variations of calculated activation energies for solid decomposition suggests that there is no discrete activated state. Further, the statistical distribution on which the calculations are based is not a realistic concept. The lowest energy possible --and most frequently occurring --is the energy of the bulk crystal. Within the crystalline solid, vibrational interactions transfer energy so rapidly that a substantial difference from the average energy is not achievable within the crystal. The lack of a statistical distribution rules out the use of the Arrhenius equation unless it is independently verified for the particular system. * Du-Pont-ICTA Award address presented at the Fifth International Conference on Thermal Analysis Kyoto, Japan, 1 August 1977. J. Thermal Anal. 13, 1978 GARN:KINETIC PARAMETEKS either process happening so rapidly that it is not a rate limiting step. If the relative probability of forming product remains unchanged, the rate is determined by the fraction of reactant reaching the activated state. In homogeneous systems, the distribution of energy states can be defined with reasonable certainty by statistics, and plotted as in Fig. 2a. As the average energy increases, the fraction having some particular extra energy increases. Under both these influences, the fraction having total energy enough to reach the activated state (E* or TR) increases with temperature. In effect, we are measuring the temperature dependence of this rate.

Kinetic compensation effects observed during oxidation of carbon monoxide on γ‐alumina supported palladium, platinum, and rhodium metal catalysts: Toward a mechanistic explanation

“…Such reactions may be formally regarded as termolecular, or even higher molecularity, processes, which have negligible probabilities in gaseous encounters, but are possible, in principle, on a densely occupied surface. Second, Garn [42,43] has concluded that the energy distribution function for solids is different from that expressed by the Maxwell-Boltzmann equation, so that the Arrhenius equation, and mechanistic conclusions based on this approach, may be inapplicable. The same argument may be applicable to surfaces.…”

Isoparametric kinetic relations for chemical transformations in condensed substances (Analytical survey). II. Reactions involving the participation of solid substances

“…It was noted in [ 15,38] that the Arrhenius law can hardly be used for solid-phase reactions; nevertheless, it is usually satisfied, since the unit chemical changes of which the total process is composed are of an activation nature. Just as in homogeneous processes, it is advisable to take into account the pre-exponent temperature-dependence [82].…”

“…Although interesting approaches to the problem have been found of late [14], on the whole there is no satisfactory theory to explain the activation energy and pre-exponential factor as applied to solid-phase reactions. According to [ 15], the decomposition of solids does not give rise at all to a discrete activated state, and the absence of statistical energy distribution precludes the use of the Arrhenius equation. One may hope that the analysis of isoparametric relations in the kinetics of reactions in solids will promote progress in this field, just as it has in the kinetics of homogeneous reactions.…”

Kinetics of phase transformations in Cu-Al and Cu-Zn-Al alloys

“…It is uncertain which part of the thermal effect is connected with the phase transformation and can be used for kinetics computing, and which part is connected with other phenomena such as heat transition. Thus, DTA is still of questionable validity, because a representative value which would unambiguously define the change in the system is not available from the DTA peak [3]. The following question may be posed: if the kinetic parameters obtained from DTA measurements can be explained analogously as for homogeneous reactions, then do these parameters have any physical sense at all?…”

“…The procedure most often used is to obtain the 9 factor on the basis of the increase in area of the DTA effect. However, the shape of the thermal effect is a function not only of the reaction kinetics, but also of the geometry and thermal diffusivity within the furnace, sample and thermocouple assembly [1][2][3]. It is uncertain which part of the thermal effect is connected with the phase transformation and can be used for kinetics computing, and which part is connected with other phenomena such as heat transition.…”