Congruent dissociative vaporization

Foundation Thermal decomposition of a solid reactant, R, is usually described by two types of reaction by congruent dissociative vaporization of a reactant R into gaseous products A(g) and B(g), i.e., according to the scheme ... 

Table 3.1 Factors of the Arrhenius A parameter for congruent dissociative vaporization in a vacuum... 

Mechanism Kinetics Methodology Congruent dissociative vaporization Langmuir vaporization equations Third-law method R(s/1) A g)i + B(g) feqp = f J) ArHf = - uR nPeqp)... 

The terminology used has undergone the usual changes or refinements with time and experience. The terms mechanism of dissociative vaporization , physical approach , and specific enthalpy , introduced in previous communications by the author, have been replaced in this book by mechanism of congruent dissociative vaporization , thermochemical approach , and molar enthalpy . The notation of some physical quantities has likewise undergone changes. 

Considering that dissociation occurs upon volatilization, the temperatures can be correlated extremely well on a In P vs (1/rd.voi) plot, where P is the total system pressure and T a.voi decomposition temperature, as the case dictates. Such a plot is shown in Fig. 11. Since the Clausius-Clapeyron relation for vapor pressure of pure substances shows an exponential dependence on temperature, TVoi was considered a pseudo-boiling point at the respective system pressure. For a substance that vaporizes congruently to its gaseous state, the slope of lines on a In P vs (l/Tvoi) plot represents the enthalpy of vaporization. Indeed, the enthalpy of vaporization calculated from the slope on a In P vs (l/TVoi) plot for the B-O2 system (360 kJ/mol) agrees exactly with the value calculated by using... 

The second example concerns those situations where the solid phase containing the solvated compound is in equilibrium with both its liquid and its vapor phase. This system would result from the congruent melting of the solid phase, which was in turn accompanied by the simultaneous volatilization of the included solvation molecules. The equilibrium therefore consists of two components (substance and solvent) present in three phases (initial solvate, fused liquid, and solvent vapor). According to the phase rule, this constitutes a univaiiant system, so just as for the system described in Section A, for each temperature there will correspond a definite vapor pressure. This is still a dissociation pressure and will be independent of the relative or absolute amounts of phases present. 

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