Reactions between solids

Traditionally one categorizes matter by phases such as gases, liquids and solids. Chemistry is usually concerned with matter m the gas and liquid phases, whereas physics is concerned with the solid phase. However, this distinction is not well defined often chemists are concerned with the solid state and reactions between solid-state phases, and physicists often study atoms and molecular systems in the gas phase. The tenn condensed phases usually encompasses both the liquid state and the solid state, but not the gas state. In this section, the emphasis will be placed on the solid state with a brief discussion of liquids. 

Hydrogen for weather balloons is often supplied by the reaction between solid calcium hydride, CaH2, and water to form solid calcium hydroxide, Ca(OH)2, and hydrogen gas, H ... 

The properties of barrier layers, oxides in particular, and the kinetic characteristics of diffusion-controlled reactions have been extensively investigated, notably in the field of metal oxidation [31,38]. The concepts developed in these studies are undoubtedly capable of modification and application to kinetic studies of reactions between solids where the rate is determined by reactant diffusion across a barrier layer. 

Infrared, X-ray diffraction, DTA, TG, electrical conductivity and solubility measurements have been used to investigate the mechanisms of tribochemical reactions between solids [385,386],... 

Reactions between solids. Parameters requiring consideration in the description of any reaction mixture and in the measurement of interaction kinetics... 

As with the decompositions of single solids, rate data for reactions between solids may be tested for obedience to the predictions of appropriate kinetic expressions. From the identification of a satisfactory representation for the reaction, the rate-limiting step or process may be identified and this observation usually contributes to the formulation of a reaction mechanism. It was pointed out in Sect. 1, however, that the number of parameters which must be measured to define completely all contributory reactions rises with the number of participating phases. The difficulties of kinetic analyses are thereby also markedly increased and the factors which have to be considered in the interpretation of rate data include the following. 

The kinetic principles operating during the initiation and advance of interface-controlled reactions are identical with the behaviour discussed for the decomposition of a single solid (Chaps. 3 and 4). The condition that overall rate control is determined by an interface process is that a chemical step within this zone is slow compared with the rate of arrival of the second reactant. This condition is not usually satisfied during reaction between solids where the product is formed at the contact of a barrier layer with a reactant. Particular systems that satisfy the specialized requirements can, however, be envisaged for example, rate processes in which all products are volatilized or a solid additive catalyzes the decomposition of a solid yielding no solid residue. Even here, however, the kinetic characteristics are likely to be influenced by changing effectiveness of contact as reaction proceeds, or the deactivation of the catalyst surface. 

Two product barrier layers are formed and the continuation of reaction requires that A is transported across CB and C across AD, assuming that the (usually smaller) cations are the mobile species. The interface reactions involved and the mechanisms of ion migration are similar to those already described for other systems. (It is also possible that solid solutions will be formed.) As Welch [111] has pointed out, reaction between solids, however complex they may be, can (usually) be resolved into a series of interactions between two phases. In complicated processes an increased number of phases, interfaces, and migrant entities must be characterized and this requires an appropriate increase in the number of variables measured, with all the attendant difficulties and limitations. However, the careful selection of components of the reactant mixture (e.g. the use of a common ion) or the imaginative design of reactant disposition can sometimes result in a significant simplification of the problems of interpretation, as is seen in some of the examples cited below. 

For the reaction between solid sodium and water, the complete, balanced chemical equation is therefore... 

Consider the surface of a solid. In the interior, we see a certain s mmetry which depends upon the structure of the solid. As we approach the surface from the interior, the symmetry begins to change. At the very surface, the surface atoms see only half the symmetry that the interior atoms do (and half of the bonding as weU). Reactions between solids take place at the surface. Thus, the surface of a solid represents a defect in itself since it is not like the interior of the solid. 

Knochel et al. (1977a) have studied the effect of the structure of the ligand on its ability to catalyse the reaction between solid metal acetates and benzyl chloride dissolved in acetonitrile. Approximate half-lives for the reactions are given in Table 29. For crown ethers, the reactivity sequence decreases in the... 

Solvents can increase reaction rates by dispersing reactant molecules and increasing the collision frequency (Figure 1.7a). In solution, all of the solutes are potential reactants. Reactions between solids, however, tend to be much slower than reactions in liquids as there is only a small amount of contact between the solid reactants. Even fine powders will have a relatively small surface area-to-mass ratio, so the bulk majority of the reactant is not in the right place to react (Figure 1.7b). 

Figure 1.7 (a) In the hquid phase, molecules of both compounds are dispersed and all molecules are potentially reactive (b) when two sohds react, reaction can only occur at the point where the two crystallites are in direct contact (c) reaction between solids may form a thin liquid layer which increases the rate of reaction... 

Heating, often up to high or very high temperatures, is a common procedure needed for melting or speeding up a reaction between solid or liquid components. 

Some examples of reactions between solid-state components and of their different uses and applications are reported in a few paragraphs in the following. 

Write balanced half-reactions from the net ionic equation for the reaction between solid aluminum and aqueous iron(in) sulfate. The sulfate ions are spectator ions, and are not included. 

Chloro-, Bromo-, and lodo-complexes. The effect of pressure on the kinetics of the reaction between solid NaCl and ZrC or HfCl4 vapours has been investigated. The i.r. and Raman spectra of solid ZrCl4,2NOCl and HfCl4,2NOCl have shown that they consist of NO and octahedral hexachlorometallate ions. °° Normal-co-ordinate analyses have been reported for the [ZrX ] " and [HfXg] ions (X = Cl or Br). ° ... 

This chapter will deal with both types of processes, namely reactions between solids and reactions between solids and gases. In the first section we will discuss examples of gas-solid reactions between molecular solids (operatively taken as crystals formed by neutral molecules or molecular ions) and vapours of small molecules, while the second section will deal with solid-solid reactions between molecular crystals to yield co-crystals. Even though examples will come mainly from our own work with organometallic molecules, coverage of the work of many scientists in the field will be attempted. The reader should be warned, however, that this chapter has no review-type ambitions. 

Component exchange between phases is controlled by mass transfer. Between solid phases, mass transfer is through diffusion where the exchange of components may be used as a geospeedometer (Lasaga, 1983). Convection rather than diffusion may play a dominant role if fluid phases are involved. In reactions between solid and fluid phases, diffusion in the solid phase is usually the slowest step. However, dissolution and reprecipitation may occur and may accomplish the exchange more rapidly than diffusion through the solid phase. 

Now according to the rule of Dulong and Petit the specific heat of a compound is usually nearly equal to that of the sum of its components. The change in entropy AS, in a reaction between solids only, is usually, therefore, very small, and even if the heat effect is relatively low, the temperature will not cause the compound that is stable at low temperatures to decompose at higher, the... 

The amazing evolution of solid state physics and chemistry over the last 30 years induced an intensive study of various solid state processes, particularly in the context of materials science. Materials have always been an important feature of civilization and are the basis of our modern technical society. Their preparation is often due solely to reactions between solids. Solid state reactions are also often responsible for the materials adaptation to a specific technical purpose, or for the degradation of a material. 

In the past, most solids were prepared on a large scale by standard ceramic techniques, in which accurate control of the composition, as well as uniform homogeneity of the product, were not readily achieved. Unfortunately, this has sometimes led to uncertainty in the interpretation of the physical measurements. In recent years more novel methods have been developed to facilitate the reaction between solids. This is particularly true for the preparation of polycrystalline samples, on which the most measurements have been made. It is of utmost importance to prepare pure single-phase compounds, and this may be very difficult to attain. Even for a well-established reaction, careful control of the exact conditions is essential to ensure reproducible results. For any particular experiment, it is essential to devise a set of analytical criteria to which each specimen must be subjected. It will be seen from the solid-state syntheses included in this volume that one or more of the following common tests of purity are used to characterize a product. 

In recent years, more novel methods have been developed by chemists in order to hasten the reaction between solids or to grow single crystals of new and exotic solids. Among those included in the following syntheses are electrolysis of fused salts, chemical transport, and hydrothermal crystal growth. 

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