Reactions in the solid state - metals

Exothermic Decompositions These decompositions are nearly always irreversible. Sohds with such behavior include oxygen-containing salts and such nitrogen compounds as azides and metal styphnates. When several gaseous products are formed, reversal would require an unlikely complex of reactions. Commercial interest in such materials is more in their storage properties than as a source of desirable products, although ammonium nitrate is an important explosive. A few typical exampes will be cited to indicate the ranges of reaction conditions. They are taken from the review by Brown et al. ( Reactions in the Solid State, in Bamford and Tipper, Comprehensive Chemical Kinetics, vol. 22, Elsevier, 1980). 

The metal solutions are sprayed into cold liquids for rapid freezing, after which the droplets are freeze dried and decomposed to metal oxides. Due to the homogeneous distribution of the components, the reactions in the solid state occur at lower temperatures compared with conventionally produced powders. 

In 1937, dost presented in his book on diffusion and chemical reactions in solids [W. lost (1937)] the first overview and quantitative discussion of solid state reaction kinetics based on the Frenkel-Wagner-Sehottky point defect thermodynamics and linear transport theory. Although metallic systems were included in the discussion, the main body of this monograph was concerned with ionic crystals. There was good reason for this preferential elaboration on kinetic concepts with ionic crystals. Firstly, one can exert, forces on the structure elements of ionic crystals by the application of an electrical field. Secondly, a current of 1 mA over a duration of 1 s (= 1 mC, easy to measure, at that time) corresponds to only 1(K8 moles of transported matter in the form of ions. Seen in retrospect, it is amazing how fast the understanding of diffusion and of chemical reactions in the solid state took place after the fundamental and appropriate concepts were established at about 1930, especially in metallurgy, ceramics, and related areas. 

OUR knowledge of reactions in the solid state has been derived mainly from the systematic study of the tarnish reactions and their inverse, dissociation processes. This is due largely to the fact that the product or reactant ionic compounds have been studied by physical methods the nature of the defect, the electronic and ionic processes and the structures are known with some degree of certainty. Moreover, tarnishing reactions are ideal for experimental study, particularly where a coherent film of the product (oxide, halide, etc.) is formed on the metal. 

Oxidative coupling reactions of phenols are usually performed by treatment of phenols in solution with more than an equimolar amount of metal salts such as FeCF. However, the coupling reaction of some phenols with FeCF was demonstrated to proceed much faster and more efficiently in the solid state than in solution and the reaction in the solid state is accelerated by irradiation with ultrasound. For example, the irradiation with ultrasound of a mixture of finely powdered p-hydroquinone and [Fe(DMF)3Cl2][FeCl4] (2 equiv.) in the solid state (50 °C, 1 h) provided 695 in 64% yield (Scheme 137). 

The experimental data of aqueous leaching under constant pH were fitted with Avrami-Erofeev rate law (IS) for reactions in the solid state. The reactions of metal dissoluticm, desorption and transport were combined into one reaction, uhich is expressed as... 

It is with these ideas in mind that we describe here supramolecular approaches to control [2+2] photodimerization reactions in the solid state based on the use of templates. We first describe the use of small organic molecules that act as templates and then move to metal-organic complexes. We end with a treatment of other cycloadditions (e.g., [4+4] cycloaddition, Diels-Alder) that may also be controlled using the template approach. 

The reactivity of the transition metals towards other elements varies widely. In theory, the tendency to form other compounds both in the solid state (for example reactions to form cations) should diminish along the series in practice, resistance to reaction with oxygen (due to formation of a surface layer of oxide) causes chromium (for example) to behave abnormally hence regularities in reactivity are not easily observed. It is now appropriate to consider the individual transition metals. 

These materials have been prepared by polymerisation of p-halothiophenoxide metal compounds both in the solid state and in solution. They have also been prepared by condensation of p-dichlorobenzene with elemental sulphur in the presence of sodium carbonate while the commercial polymers are said to be produced by the reaction of p-dichlorobenzene with sodium sulphide in a polar solvent. 

Crystal structure, crystal defects and chemical reactions. Most chemical reactions of interest to materials scientists involve at least one reactant in the solid state examples inelude surfaee oxidation, internal oxidation, the photographie process, electrochemieal reaetions in the solid state. All of these are critieally dependent on crystal defects, point defects in particular, and the thermodynamics of these point defeets, especially in ionic compounds, are far more complex than they are in single-component metals. I have spaee only for a superficial overview. 

If the major constituents of a solid alloy in contact with a liquid alloy are highly soluble in the latter without formation of compounds, progressive attack by solution is to be expected. If, on the other hand, a stable inter-metallic compound is formed, having a melting point above the temperature of reaction, a layer of this compound will form at the interface and reduce the rate of attack to a level controlled by diffusion processes in the solid state. By far the most serious attack, however, occurs in the presence of stresses, since in this case the liquid alloy, or a product of its reaction with the solid alloy, may penetrate along the grain boundaries, with resultant embrittlement and serious loss of strength. 

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