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  • Author or Editor: B. L’vov x
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Abstract  

Elementary thermochemical calculations show that in all cases of formation of solid product in the process of the congruent dissociative vaporization of reactants, the equilibrium partial pressure of the main product greatly exceeds its saturation vapour pressure, and therefore causes the appearance of vapour oversaturation. The oversaturation is responsible for the formation and growth of nuclei, their shape and position, the transfer of condensation energy to the reactant, the existence of induction and acceleration decomposition periods, the reaction localization, the epitaxial/topotaxy effects and the nanocrystal structure of the solid product. Variations in the energy transfer explain an increase of the molar enthalpy with temperature and the decelerating influence of melting on the rate of decomposition.

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Abstract  

The novel thermochemical and traditional Arrhenius approaches to solid-state decomposition reactions have been evaluated from the standpoints of the number and the validity of the assumptions introduced in the theories, and achievements obtained over the last decade in the frameworks of both approaches. As it follows from the analysis, in both respects, the thermochemical approach is preferable. The so-called ‘controversial’ problem of the use of thermodynamic concepts and thermochemical data for the quantitative evaluation of decomposition kinetics in the thermochemical approach has also been discussed.

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Abstract  

The main purpose of this paper is to prove the applicability of the mechanism of congruent dissociative vaporization (CDV) to the solid-state decomposition kinetics through the comparison of the fundamental theoretical relationship E i/E e=(a+b)/a resulted from this mechanism with experiment. It has been shown that the ratios of E i and E e parameters of the Arrhenius equation measured in the isobaric and equimolar modes (in the presence and absence of H2O vapour) for 22 reactants with the general formula aSalt⋅bH2O or aOxide⋅bH2O are in agreement with the values of (a+b)/a. The relative standard deviation is only 17% and the correlation coefficient is close to 0.99. A probability of accidental correlation for all set of the E parameters taken from the literature is lower than 4⋅10–16 . This strongly supports the validity of the CDV mechanism. The problem of stability of polyatomic molecules of inorganic salts in the gaseous state, which are the primary decomposition products of crystalline hydrates, was also discussed on the basis of recent mass spectroscopy studies. It was concluded that any doubts in the applicability of the CDV mechanism as a general mechanism of solid-state decomposition reactions are unsound.

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Abstract  

The apparent increase of the reaction enthalpy ΔH with temperature due to the self-cooling and condensation effects is responsible for the fundamental restrictions of the second-law and Arrhenius plot methods related to determination of this parameter. Theoretical analysis and a comparison with the experimental data indicate that the systematic underestimation of ΔH magnitudes determined by the second-law method equals 10–25% for the reactants decomposed to gaseous products and 15–50% for reactants decomposed to solids. Therefore, the use of these methods in decomposition kinetics is hardly acceptable. The replacement of the Arrhenius plot and second-law methods to the much more precise and accurate third-law method is desirable or even obligatory.

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Abstract  

This is an overview of the early investigations into the mechanism of solid decompositions since the fundamental studies by Ostwald on catalysis during 1890–1902 and the first experimental study of the autocatalytic decomposition of Ag2O by Lewis in 1905. In order to explain the formation mechanism of the solid product, Volmer suggested in 1929 that the decomposition of Ag2O includes two sequential stages: first, a thermal decomposition of the oxide into gaseous silver atoms and oxygen molecules and second, the condensation of the supersaturated silver vapor. This revolutionary idea was immediately used by Schwab to explain the autocatalytic peculiarity of solid-state decomposition reactions. However, this mechanism did not receive the acceptance of the scientific community. On the contrary, as can be seen from the results presented at the conference “Chemical reactions involving solids” in Bristol in 1938, this model was dismissed as unrealistic and, as a result, was since forgotten. Instead, considerable attention at this conference was devoted to the disorder theory proposed earlier by Wagner and Schottky, and to the mechanism of ion transport in the solid crystals. During the subsequent 70 years, decomposition mechanisms have been interpreted, without visible progress, on the latter basis. The mechanism of congruent dissociative vaporization proposed independently in 1990 turned out to be in complete agreement with the Volmer–Schwab model. It has been treated with the same distrust. These historical events, in the author’s opinion, are responsible for the prolonged stagnation in the development of solid-state decomposition theory.

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Abstract  

The third-law method has been applied to the results of kinetic studies reported in the literature and obtained in this work to determine the E parameters of the Arrhenius equation and investigate the impact of self-cooling on the dehydration kinetics of Li2SO4H2O, CaSO42H2O and CuSO45H2O. The values obtained (104, 98 and 88 kJ mol-1, respectively) are about 20% higher compared to the literature data calculated by the Arrhenius-plots method. This discrepancy is connected with the severe effect of self-cooling, which can reach several ten degrees at maximum temperatures of experiments.

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Summary  

The third-law method has been applied to determine the enthalpies, Δr H T 0, for dehydration reactions of kaolinite, muscovite and talc. The Δr H T 0values measured in the equimolar (in high vacuum) and isobaric (in the presence of water vapour) modes (98015, 371039 and 279334 kJ mol-1, for kaolinite, muscovite and talc, respectively) practically coincide if to take into account the strong self-cooling effect in vacuum. This fact strongly supports the mechanism of dissociative evaporation of these compounds in accordance with the reactions (primary stages): Al2O32SiO22H2O(s)→Al2O3(g)↓+2SiO2(g)↓+2H2O(g); K2O3Al2O36SiO22H2O(s) →K2O(g)↓+3Al2O3(g)↓+6SiO2(g)↓+2H2O(g) and 3MgO4SiO2H2O(s) →3MgO(g)↓+4SiO2(g)↓+H2O(g). The values of the Eparameter deduced from these data for equimolar and isobaric modes of dehydration are as follows: 196 and 327 kJ mol-1for kaolinite, 309 and 371 kJ mol-1for muscovite and 349 and 399 kJ mol-1for talc. These values are in agreement with quite a few early results reported in the literature in 1960s.

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