Reactions between inorganic solids

Other factors have been identified as rate controlling in other types of solid—solid interaction, and some of these are described in subsequent sections. These include, for example, the decomposition of a solid catalyzed by a (different) solid and rate processes in which one reactant is volatilized, e.g. reaction of carbon (- C02) with a solid oxidizing agent. 

This account of the kinetics of reactions between (inorganic) solids commences with a consideration of the reactant mixture (Sect. 1), since composition, particle sizes, method of mixing and other pretreatments exert important influences on rate characteristics. Some comments on experimental methods are included here. Section 2 is concerned with reaction mechanisms formulated to account for observed behaviour, including references to rate processes which involve diffusion across a barrier layer. This section also includes a consideration of the application of mechanistic criteria to the classification of the kinetic characteristics of solid-solid reactions. Section 3 surveys rate processes identified as the decomposition of a solid catalyzed by a solid. Section 4 reviews other types of solid + solid reactions, which may be conveniently subdivided further into the classes 

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Direct Reaction between the Solid Support and the Metal Complex 9.9.2.2.1 Inorganic supports... 

There are significant differences between the kinetic models for reactions carried out in the solid phase and those taking place in gases or solutions. Therefore, it is appropriate to describe briefly some of the kinetic models that have been found to be particularly applicable to reactions in inorganic solids. 

Mechanochemically activated reactions between molecular solids and inorganic salts to obtain complexation and ion segregation in the solid state. 

Another example of the use of percolation in inorganic chemistry is the SHS reaction as described in Chapter 6. The reaction between two solid powders (such as the reaction between titanium and boron particles) can propagate after local ignition only if the heat of reaction is high enough and loss of heat low enough. 

In addition to chemical reactions, the isokinetic relationship can be applied to various physical processes accompanied by enthalpy change. Correlations of this kind were found between enthalpies and entropies of solution (20, 83-92), vaporization (86, 91), sublimation (93, 94), desorption (95), and diffusion (96, 97) and between the two parameters characterizing the temperature dependence of thermochromic transitions (98). A kind of isokinetic relationship was claimed even for enthalpy and entropy of pure substances when relative values referred to those at 298° K are used (99). Enthalpies and entropies of intermolecular interaction were correlated for solutions, pure liquids, and crystals (6). Quite generally, for any temperature-dependent physical quantity, the activation parameters can be computed in a formal way, and correlations between them have been observed for dielectric absorption (100) and resistance of semiconductors (101-105) or fluidity (40, 106). On the other hand, the isokinetic relationship seems to hold in reactions of widely different kinds, starting from elementary processes in the gas phase (107) and including recombination reactions in the solid phase (108), polymerization reactions (109), and inorganic complex formation (110-112), up to such biochemical reactions as denaturation of proteins (113) and even such biological processes as hemolysis of erythrocytes (114). 

To a stirred solution containing 49 gm (0.11 mole) of lead tetraacetate (supplied as a 90% solution in acetic acid) in 200 ml of methylene chloride is added a solution of 14.8 gm (0.10 mole) of acetone phenylhydrazone in 25 ml of methylene chloride over a 15 min period while maintaining the reaction temperature between 0° and 10°C with an ice bath. After the addition has been completed, the reaction temperature is raised to 20°-25°C and stirring is continued for an additional 15 min. Then to the reaction mixture is added 200 ml of water, the inorganic solids are filtered off, and the methylene chloride layer is separated. This product layer is washed in turn with water and with dilute aqueous sodium bicarbonate until all the acetic acid has been removed. After drying the methylene chloride solution with anhydrous sodium sulfate, the solvent is evaporated off at reduced pressure. The residue is distilled under reduced pressure. The product has b.p. 89°C (1 mm Hg) yield 17.0 gm (83 %). 

Until this point, we have focused on cases in which we could neglect chemical bond formation between the sorbate and materials in the solid phase. However, at least two kinds of surface reactions are known to be important for sorption of some chemicals (referred to as chemisorption). Simply, some organic substances can form covalent bonds with the NOM in a sediment or soil (see Fig. 9.2) other organic sor-bates are able to serve as ligands of metals on the surfaces of inorganic solids (Fig. 11.le). We discuss these processes below. 

The development of solvent-free organic synthetic methods has thus become an important and popular research area. Reports on solvent-free reactions between solids, between gases and solids, between solids and liquid, between liquids, and on solid inorganic supports have become increasingly frequent in recent years. 

Based on some interesting reactions in certain inorganic crystalline compounds, Kohlschutter [9,10] proposed that the nature and properties of the products obtained take place on the surface or within the solid state. Indeed, he coined the term topochemistry for such reactions in the solid state. However, systematic investigations of photoinduced reactions in crystals began from 1964 onward by Schmidt and Cohen [11], Their studies on the 2tt + 2tt photoreaction of cinnamic acid derivatives in the crystalline state and correlation with the molecular organization in these crystals led to what are now known as Topochemical Principles. The most important conclusions reached by them are as follows (1) The necessary conditions for the reactions to take place are that the reactive double bonds are parallel to one another and the center-to-center distance be within 4.1 A (2) there is one-to-one correspondence between the stereochemistry of the photoproduct and the symmetry relationship between the reactants. The centrosymmet-ric relationship (called the a-form) leads to centrosymmetric cyclobutane (anti-HT), whereas the mirror symmetric arrangements (called the (5-form) produce mirror symmetric dimer (yy -HH). 

The best solvent from an ecological point of view is without doubt no solvent. There are many great reactions that can already be carried out in the absence of a solvent, for example numerous industrially important gas-phase reactions and many polymerizations. Diels-Alder and other pericyclic reactions are also often carried out without solvents. Reports on solvent-free reactions have, however, become increasingly frequent and specialized over the past few years. Areas of growth include reactions between solids [5], between gases and solids [6], and on supported inorganic materials [7], which in many cases are accelerated or even made possible through microwave irradiation [8]. 

Refractory dust particles not only serve as condensation nuclei for ices (see above), but are also necessary for energy dissipation during gas phase reaction between energetic atoms and molecules. Furthermore, solid inorganic particles (mostly oxides and sulphides) may play a role of photocatalysts in transformations of organic materials in both space and the primitive soup [5, 31]. 

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