Equimolar decomposition mode

Another important conclusion is that it is possible to use the relationship Tin/E = 3.62 0.38K mol kJ as an approximate estimate of E values from measured values of (with a mean relative error of about 10%), and as a validation criterion in the analysis of experimental results. Clearly, this relationship is valid for the equimolar decomposition mode only, and is subject to the requirement that the initial decomposition temperature must correspond to the equilibrium pressure of the gaseous products within the limits of 10 - 10- bar. 

The ratio of the E parameters or molar enthalpies for the isobaric and equimolar decomposition modes satisfies the condition ... 

Different Decomposition Modes As follows from the above kinetic equations (Sect. 3.6), the presence of a gaseous product in the reactor should cause variations of the A and E parameters in full conformity with Eq. 12.1. This can be shown using the example of the decomposition of a binary substance into two products in the equimolar and isobaric modes, when one of the products (A) is a low-volatility one. 

These achievements can he considered as a reliable confirmation of the CDV mechanism underlying the studies described. New trends have been revealed in the course of these studies, such as deceleration of the decomposition upon melting of the reactant, increase in the enthalpy of the reaction with temperature (for decomposition reactions yielding a solid product), correlation of the decomposition temperature with the molar enthalpy and existence of two basic decomposition modes (equimolar and isobaric). 

The method of absolute rate of decomposition has appeared [51[. Substantiation of the equimolar and isobaric modes of evaporation [52]. 

The next step in the calculation of absolute decomposition rates with Eqs. 3.14 and 3.16 consists in determining the equilibrium pressures of the products, Pa and Pb, through the reaction equilibrium constant and the corresponding ther-mod3mamic functions (the entropy and the enthalpy). The mode, equimolar or isobaric, in which the reactant is decomposing, should also be taken into account. 

Equimolar and Isobaric Modes These key concepts were introduced into the kinetics of decomposition reactions more than 20 years ago [24]. Equimolar is the mode in which the actual pressure of the primary gaseous product in the reactor jg lo gj- than its equilibrium value (Peqp), i.e., P < Peqp. 

Similar discontinuities in Arrhenius plots are observed in thermal analysis (TA) as well, in particular, in the dehydration of crystalline hydrates performed in humid air. For illustration. Fig. 3.2 reproduces an Arrhenius plot for the dehydration of calcium oxalate monohydrate in an air flow, carried out under non-isothermal conditions by Dollimore et al. [28]. The equilibrium pressure of water vapour Pgqp measured at temperatures of up to 400 K and comparatively moderate decomposition rates turns out to be lower than its partial pressure in air which implies that the decomposition occurs in the isobaric mode. Above 400 K, the equilibrium pressure of H2O becomes higher than p with the process becoming equimolar. The slope of the plot decreases to one half of its former value in full agreement with theory (see Sect. 3.7). 

The equations presented in Tables 3.1 and 3.2 permit the estimation of both the absolute values of parameters A and their ratio for the isobaric and equimolar modes of decomposition. This ratio has the same form irrespective of the actual decomposition conditions (vacuum or foreign gas environment) ... 

The final values of the molar enthalpy (parameter E) for decomposition of CaCOs, SrCOs, and BaCOs in the equimolar and isobaric modes are summarized in Table 5.8. The mean value of the ratio is 1.98 0.03, which... 

Table 5.8 Experimental values of the E parameter for the decomposition of carbonates in the isobaric [9] and equimolar [17, 23] modes...

The T-S effect becomes apparent under extreme conditions of such kind. The temperature for experiments on the T-S effect is chosen to be much higher than temperatures typical for usual experiments on the decomposition kinetics in the absence of an excess of gaseous product. For crystalline hydrates this excess of temperature may reach 30-50 K, and for calcium carbonate, 100-150 K. This is because the decomposition in the isobaric mode is slower than in the equimolar mode. However, for initial points of the T-S curve corresponding to the absence of gaseous product or to a very low pressure of this gaseous product, this temperature is obviously much higher than the optimal value. That is why self-cooling appears to be well above the common value. 

Limitations Despite apparent advantages (simpler evacuation and weaker self-cooling), a low vacuum (about 10 bar) cannot be used in the equimolar mode of decomposition if the equivalent pressure of the gaseous product is... 

Experimental The kinetics and mechanism of the dehydration of kaolinite, muscovite, and talc was studied by L vov and Ugolkov within the framework of the CDV mechanism [70]. The main goal of that study was determination of the enthalpy of decomposition by the third-law method in a high vacuum (equimolar mode) and in the presence of water vapour (isobaric mode). In the isobaric mode, the measurements were performed in air, to eliminate or minimize the effect of self-cooling on the results (see Sect. 15.4). 

Table 16.32 Average values of the reaction enthalpy and the E parameter for the equimolar and isobaric modes of clay decompositions Reactant Primary Products of Decomposition...

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