Crystolysis reaction

References to the formulation of reaction mechani sms throughout this chapter have emphasized the possibility that the transition state theory of reaction kinetics may not be appUcable to chemical changes proceeding in the solid state and crystolysis reactions in particular. For many of the rate processes of interest, little information is available concerning interface structures at the molecular scale. The reaction... 

The identification of similarities and of differences of behaviour of reactant hydrates may provide insights into not only the mechanisms of dehydration, but also a wider range of crystolysis reactions. Inconsistencies of behaviour may be recognized and directions for future research identified. The progress achieved towards a general classification of solid state decompositions is discussed in Chapter 18,... 

Major bond redistribution steps during decompositions of solids (crystolysis reactions) These may be rate controlling and may also be reversible... 

The steps which may contribute to a crystolysis reaction are summarized in Table... 

Infrared and other spectral measurements have been widely used [25] in experimental studies of crystolysis reactions, usually as an analytical tool for the identifications of reactants, products and, sometimes, intermediates [28]. There are difficulties in associating spectrd features with interface species and structures because these may be present at very low concentrations, or their absorptions may obscured by other stronger absorptions, or by the opacity of a sohd product (perhaps a metal or carbon). Nevertheless, the approach offers the possibility, in favourable systems, of identifying or confirming steps controlling, or participating in, interface reactions. 

Reports of compensation behaviour appear to have been mainly empirical findings which have not contributed much to the theory of solid state reactions. Identification of the effect as a direct consequence of isokinetic behaviour [57,58] does, however, offer a theoretical foimdation for interpretation of the observed correlated changes in apparent Arrhenius parameters. The fit of Arrhenius parameters for crystolysis reactions to the expression ... 

Thermochimica Acta was identified in a previous literature review [25] as the international journal containing the largest number of articles concerned with crystolysis reactions. Seventy-one relevant entries were found in an extended search that located altogether 368 contributions to the topic dated 1981. A more recent search of this same journal for the interval, January 1994 to December 1995, Volumes 232-270, resulted in the identification of 262 articles on subjects falling within the scope of the present book. These research reports were concerned with decompositions of diverse compounds stated, or presumed, to proceed in the solid state, together with aspects of the relevant instrumentation and theory, particularly theoretical aspects of reaction kinetics. This selected set of relevant articles is sufficiently large to highlight trends and topics of particular interest and importance. In the analysis that follows, bracketed numbers indicate the number of articles principally concerned with each topic discussed for the (262 entries) that constitute the source set. 

This book, together with much of the literatme devoted to crystolysis reactions, predominantly seeks correlations within the confines of the topic. (The Arrhenius equation, and attendant theory, is an obvious exception). Cross-fertilization of ideas witii neighbouring fields may be hampered by the jargon existing within each subject area. Reviews addressed to wider readerships, cross-disciplinary conferences and plenary lecturers capable of communication beyond the narrow confines of their speciality are required. 

Of all crystolysis reactions, understanding of dehydrations appears to be most advanced and it remains to be seen whether the mecharustic patterns of behaviour recently proposed (Chapter 7) are of wider applicability. Systematic comparisons... 

The formulation of mechanisms of individual crystolysis reactions is more difficult than might be expected. Investigations invaiiably require detailed interpretations of the many observations obtained from complementary experimental techniques. Little is known, at present, about the controls and chemistry of processes at interfaces. The subject lacks theoretical principles and models capable of accounting systematically for the diversity of behaviour of the many thermal decompositions described in the literature. In the absence of such models, classification of behaviour has not made the necessary progress. 

The recognition of behaviour patterns, both similarities and differences, amongst crystolysis reactions is necessary in any attempt to identify the factors that control reactivity. Establishment of such patterns requires detailed reviews of the vast literature (of which a small but representative part has been surveyed in Chapters 7 to 17). As an agenda for the future, some approaches which might provide a basis for a system of classification are discussed below. 

The character and role of reaction interfaces. This approach was introduced in Chapter 6 and developed for dehydrations in Chapter 7. Such a classification scheme could be extended to a wider range of crystolysis reactions. In interpreting interfacial phenomena, cooperative interactions between species within the reaction... 

Kinetic parameters. The hterature contains numerous reports of the rate equations identified for particular crystolysis reactions, together with the calculated Arrhenius parameters. However, reproducible values of (Section 4.1.) have been reported by independent researchers for relatively few solid state decompositions. Reversible reactions often yield Arrhenius parameters that are sensitive to reaction conditions and can show compensation effects (Section 4.9.4.). Often the influences of procedural variables have not been carefully identified. Thus, before the magnitudes of apparent activation energies can be compared, attempts have to be made to relate these values to particular reaction steps. 

These examples demonstrate conclusively that the availability of volatile product in the immediate vicinity of the site of a reversible dissociation can markedly influence the apparent kinetic characteristics of crystolysis reactions. Some systems are highly sensitive to such effects but others, such as chrome alum dehydration, are markedly less affected. The kinetics of CaC03 dissociation vary considerably with reaction conditions (83), and here the pattern of reaction rates is also influenced by heat flow during the endothermic, reversible reaction. It follows that it cannot be assumed, without examination of the influence of the procedural variables, that measured kinetic parameters are determined by a slow rate-limiting step. 

Note The term crystolysis reaction was proposed for use in describing the thermal decomposition of a solid reactant (31,72). This specific label is of value as a keyword and index entry, while also confirming that the reaction of interest proceeded in the crystalline state. This important aspect of behavior is not always explicitly stated in reports of many thermal investigations, where the phase in which the change occurs is not positively identified. 

Vyazovkin, S. (2003). Reply to "What is meant by the term variable activation energy when ap>phed in the kinetics analyses of sohd state decompositions (crystolysis reactions) ", Themochimica Acta, Vol. 397, pp. 269-271 ISSN 0040-6031 Vyazovkin, S. (2010). Thermal Analysis, Analytical Chemistry, Vol. 82, pp. 4936-4949 ISSN 0003-2700... 

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