Kinetic behaviour

In his valence bond theory (VB), L. Pauling extended the idea of electron-pair donation by considering the orbitals of the metal which would be needed to accommodate them, and the stereochemical consequences of their hybridization (1931-3). He was thereby able to account for much that was known in the 1930s about the stereochemistry and kinetic behaviour of complexes, and demonstrated the diagnostic value of measuring their magnetic properties. Unfortunately the theory offers no satisfactory explanation of spectroscopic properties and so was... 

In this case study, the kinetic behaviour of the immobilised system was analysed while the following parameters were taken into account ... 

Tenet (iv). The influence of a barrier layer in opposition to the progress of reaction may be expected to rise as the quantity of product, and therefore the thickness of the interposed layer, is increased [35,37,38]. Thus, the characteristic kinetic behaviour of the overall process may be expected to include contributions from both geometric factors and the barrier effect, though in specific instances one or other of these may be dominant. 

Mass loss determinations refer to the total change resulting from reactant decomposition and usually include contributions from a mixture of product compounds, some of which would normally be condensed under conditions used for accumulatory pressure measurements. Such information concerned with the overall process is, however, often usefully supplemented by evolved gas analyses (EGA) using appropriate analytical methods. Sestak [130] has made a detailed investigation of the effects of size and shape of reactant container on decomposition kinetics and has recommended that the sample be spread as a thin layer on the surfaces of a multiple plate holder. The catalytic activity of platinum as a reactant support may modify [131] the apparent kinetic behaviour. 

Doping of solid reactant involves the introduction of a controlled amount of an impurity into solid solution in the host lattice. Such impurities can be selected to cause the generation or destruction of those electronic or structural defects which participate in the rate process of interest. Thus, the influence of the additive on kinetic behaviour can provide evidence concerning the mechanism of reaction [46,47]. Even if the... 

Fig. 1. Generalized a—time plot summarizing characteristic kinetic behaviour observed for isothermal decompositions of solids. There are wide variations in the relative significance of the various stages (distinguished by letter in the diagram). Some stages may be negligible or absent, many reactions of solids are deceleratory throughout. A, initial reaction (often deceleratory) B, induction period C, acceleratory period D, point of inflection at maximum rate (in some reactions there is an appreciable period of constant rate) E, deceleratory (or decay) period and F, completion of reaction.

Zero-order kinetic behaviour, in an unusual dehydration reaction [62], has been shown to be due to the constant area of reaction interface and this interface has been identified as original surfaces of the reactant crystallites which do not advance. Water molecules are mobile within the... 

This is known as the Prout—Tompkins equation and has found application to many systems, in addition to the thermal decomposition of potassium permanganate [465] with which it is often associated. The kinetic behaviour of silver permanganate was somewhat different and in a variation of... 

De Tar and Day [498] considered problems which arise in applying the first-order expression [eqn. (15)] to kinetic data of limited accuracy. If a is known to a high degree of accuracy (e.g. 0.1%), then small components (5—10%) of higher or lower order behaviour are detectable. When the accuracy of a is reduced to 1%, a second-order component of 25% could escape detection, and perhaps an even larger contribution might be missed within a limited a interval. Using exact values of a from known kinetic behaviour, the apparent rate coefficient, ft, was found to depend on both the proportion of the non-first-order component and on the extent of reaction considered. 

A constant rate (zero-order kinetic behaviour) maintained during all, or the greater part of the process may be accounted for [487] by the following reaction models, illustrated in Fig. 5. These alternatives may be distinguished by microscopic observations. 

Hydrated metal sulphates have long been used to study water removal processes, and characteristic kinetic behaviour is conveniently illustrated by reference to these substances. Frost and co-workers [602,603] have investigated the structures, stabilities and adsorption properties of various intermediate amorphous phases, the immediate reaction products which can later undergo reorganization to yield crystalline phase. 

Boddington and Iqbal [727] have interpreted kinetic data for the slow thermal and photochemical decompositions of Hg, Ag, Na and T1 fulminates with due regard for the physical data available. The reactions are complex some rate studies were complicated by self-heating and the kinetic behaviour of the Na and T1 salts is not described in detail. It was concluded that electron transfer was involved in the decomposition of the ionic solids (i.e. Na+ and Tl+ salts), whereas the rate-controlling process during breakdown of the more covalent compounds (Hg and Ag salts) was probably bond rupture. 

Diverse and sometimes contrasting types of kinetic behaviour have been described for the decompositions of salts in this class, which includes the metal carbonates, sulphates, nitrates and nitrites, phosphates, oxyhalides, permanganates and chromates (Sects. 3.1—3.7, respectively). It is con-... 

The kinetic behaviour of metal salts of oxyacids may be influenced by water of crystallization. Where complete-dehydration precedes decomposition, the anhydrous material is the product of a previous rate process and may have undergone recrystallization. If water is not effectively removed, there may at higher temperature be the transient formation of a melt prior to decomposition. The usual problems attend the identification of partial or transient liquefaction of the reactant in the mechanistic interpretation of kinetic data. 

Metal carbonate decompositions proceed to completion in one or more stages which are generally both endothermic and reversible. Kinetic behaviour is sensitive to the pressure and composition of the prevailing atmosphere and, in particular, to the availability and ease of removal of C02. The structure and porosity of the solid product and its relationship with the reactant phase controls the rate of escape of volatile product by inter-and/or intragranular diffusion, so that rapid and effectively complete withdrawal of C02 from the interface may be difficult to achieve experimentally. Similar features have been described for the removal of water from crystalline hydrates and attention has been drawn to comparable aspects of reactions of both types in Garner s review [ 64 ]. 

Isothermal a—time curves for the decomposition of U02(CH3C02)2 in air (513—573 K) [1018] showed two approximately linear regions, 0.0 < a < 0.2 and 0.2 < a < 0.9, for which the values of E were 107 and 165 kJ mole-1, respectively. In nitrogen, the earlier portion of the curve was not linear and E = 151 kJ mole-1 for the later interval. The zero-order kinetic behaviour was explained by growth of nuclei in thin, plate-like crystals, which were shown by microscopic and surface area measurements to fragment when a > 0.85. The proposed initial step in the decomposition was fission of bonds between the U02+ and the (OCO CH3) species [1018]. 

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