Enthalpy self-cooling influence

MgCOj has the calcite structure [2]. Britton et al. [30] concluded that the decomposition of magnesite (MgCOj) was an inter ce process, initiated at boundary surfaces and thereafter advancing inwards. The value of E found (150 kJ mol between 813 and 873 K) was appreciably greater than the enthalpy of dissociation (101 kJ mol ). They considered the possible influences on the reaction rates of factors such as self-cooling, the recombination process at the reaction interface, the restriction of escape of carbon dioxide, and the rate of the nucleation step. 

Reactant temperature within the reaction zone may be appreciably different from that measured for the controlled furnace reaction vessel because of local self-cooling or self-heating as a consequence of the reaction enthalpy. The significance of self-cooling in dehydrations has been discussed by Bertrand et al. (19). L vov et al. (20) have developed a computer model to represent the effect with reference to the endothermic dehydration of Li2S04 H20. Not all research reports discuss the possible consequences of reaction enthalpy in influencing reactant temperature. 

Unresolved Problems Against the background of above listed achievements, there are still some unanswered questions that appear in the course of the research. These include, in particular, the mechanism of condensation energy transfer from a low-volatility product to a reactant, and the influence of the symmetry of the reactant crystal structure on the composition of the gaseous decomposition products. It would be worthwhile to perform a more thorough analysis of the dependence of the t coefficient and the sizes of the condensate particles on the vapour oversaturation of the low-volatility product, as well as of the relative contributions of the condensation and self-cooling effects to the underestimation of enthalpies determined by the second-law and Arrhenius plot methods. 

In this monograph, the kinetics of carbonate decompositions have been considered in several sections concerning the formation of oversaturated vapour and nucleation (Sect. 2.4), the structure of the solid product (Sect. 2.6), the influence of the reaction mode and stoichiometry on the molar enthalpy (Sect. 5.4), the experimental estimation of the self-cooling (Sect. 6.3), the T-S effect (Sect. 7.3), the variation of the enthalpy of decomposition with temperature (Sect. 8.2), the compensation effect (Chapter 12), and the determination of the absolute rates of decomposition of single crystals and powders in a vacuum and in air (Sects. 15.1 and 15.5). 

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