Types of Solid State Reactions

We can distinguish seven (7) types of phase transformations in any given solid. They include the following  

These seven transformations are familiar to most. We can classify most solid state reactions in terms of the CHANGES given in 3.1.1. This is shown in the following table, given on the next page. 

L G refer to solid, liquid and gas, respectively. Note that just one composition can experience any or all of these phase changes, i.e.- a homogeneous solid state reaction. You will note that only decomposition reactions can be classified as homogeneous. All of the others are heterogeneous in nature. 

Now consider phase transformations between two solids, i.e.- a heterogeneous reaction, and the types of reaction mechanisms which may 

In the case where two particles are involved, those that are not in close proximity will not react. But, those that adjoin each other will undergo solid state reaction with ease. This is due to the fact that cations and/or anions from one structure must be transported, or interchemge by some mechanism, to the other structure in order to form a completely new compound. Thus, the degree of dispersion and mixing of one reacting solid with another is important to the overall mechanism of solid state reaction. This cannot be overemphasized since lack of dispersion, i.e.-incomplete mixing of two solids, is the primary cause for production of 

---

Classify the types of solid state reactions you used to form the compounds in the above problem. 

In general, we can classify solid state reactions as being either homogeneous or heterogeneous. The former involves reactions by a single compound whereas the latter involves reactions between two different compounds. There are at least four (4) types of solid state reactions (as we have already presented in a prior chapter) ... 

Heterogeneous solid state reactions occur when two phases, A and B, contact and react to form a different product phase C. A and B may be either chemical elements or compounds. We have already introduced this type of solid state reaction in Section 1.3.4. The rate law is parabolic if the reacting system is in local equilibrium and the growth geometry is linear. The characteristic feature of this type of reaction is the fact that the product C separates the reactants A and B and that growth of the product proceeds by transport of A and/or B through the product layer. 

The common feature of the internal reactions discussed so far is the participation of electronic defects. In other words, we have been dealing with either oxidation or reduction. We now show that reactions of the type A+B = AB can take place in a solvent crystal matrix as, for example, the formation of double oxides (CaO +Ti02 = CaTi03) in which atomic (ionic) but no electronic point defects are involved. Although many different solvent crystal matrices can be thought of (e.g., metals, semiconductors, glasses, and even viscous melts and surfaces), we will deal here mainly with ionic crystal matrices in order to illustrate the basic features of this type of solid state reaction. 

This type of solid state reaction can be understood in light of the reaction between SiC>2 and CaO (produced by the thermal decomposition of CaC03 into CaO and CO2). See Section 2.2.3. 

Crystalline pyr-FeFs , a metastable cubic form of FeFs with pyrochlore-related structure, was obtained in a topotactical oxidation reaction of NH4Fe2F6 with Br2 in acetonitrile. At this example, the designation "chimie douce (soft chemistry) for this type of solid-state reaction under low-temperature conditions was propagated in the 1980s. ... 

Photocyclodimerization of crystalline olefinic compounds (giving cyclobutane derivatives) thus requires that the C=C bonds be aligned parallel (or antiparallel) to each other and about 4 A apart. It also requires that the formation of new bonds should cause a minimal movement of atoms. This type of solid-state reaction is called a topochemical reaction. It is essentially diffusionless since all that is required is a minor reorganization of atoms rather than a diffusion so that two reactant molecules can approach each other in a suitable manner for reaction. For example, cfs-4a,5,8,8a-dimethyltetrahydronaphthoquinone, forms dimers stereospecifically in the crystalline state upon irradiation (Figure 18.2). By contrast, intramolecular cyclization occurs in solution. 

As we have mentioned earHer, the activation energy is obtained from the temperature dependence of the rate constant. For soHd state reactions, there may not be a transition state that is populated according to the Boltzmarm distribution law. As we consider a few types of solid state reactions, we will see that there is no simple interpretation of k possible in some instances. 

There are three types of solid-state reaction (i) chemical decomposition, (ii) chemical reaction between solids, and (iii) chemical reduction. The first two reactions will be introduced, while the third one is commonly used for nonoxides... 

This chapter will first describe the different types of solid state reactions (the sintering reactions in ceramics processing, e.g., are solid state reactions in materials technology ) and the typical reaction rates that are observed. Then the mechanisms for these reactions or, more precisely, the diffusion mechanisms and how they are studied will be discussed. 

Abstract There are two types of solid-state reactions keeping the single crystal form single crystal-to-single crystal (SCSC) transformations and crystaUine-state reactions. In the former reactions, the crystal structures before and after the reaction are very similar to each other, but the crystallinity is not kept during the reaction. In the latter reactions, the crystallinity is kept in a whole process of the reaction. The reaction cavity was defined to estimate the void space around the reactive group. For the crystalline-state reaction, it was easy to understand the way how the void space is effectively utilized in the process of the reaction, comparing the void space before and after the reaction. 

---