Reaction kinetics at linearly increased temperature. I. Comparative characterization of reactions in solution and solid phase in a unique “mechanistic diagram”

Abstract

A complex reaction can be characterized kinetically in a special “Mechanistic Diagram”. From a time/rate plot recorded at constant heating, the shape index S and the reaction type index M may be calculated. The latter represents a sort of standardized, reciprocal half-width since it has to be referred to the activation parameters of a reference reaction. In practice, it is useful to take those apparent activation parameters which result from a first-order (or second-order) evaluation of either the initial or the total part of the signal. In a plane S versus M, all basic sorts of reaction may be recognized since they yield points (first- or second-order elementary processes), lines (e.g. order kinetics. 0< n < ∞) or regions. Whereas the regions of homogeneous mechanisms involve at least one of the two elementary points and are often extended, i.e. less specific, the usual heterogeneous models give sharper, isolated regions, often near to the n-order line. The diagnostic potential of this diagram is strongly increased if parametric curves due to experimental series, based on systematic changes of conditions (initial concentration of reactant, heating rate, pressure etc.) are considered. Constancy of S and M then will signal a temporary dominance of a part of the prevailing reaction mechanism.