Solid state in reactions

Copper(II) oxide, CuO. Black solid formed by heating Cu(OH)2, Cu(N03)2, etc. Dissolves in acid to Cu(II) salts, decomposes to CU2O at 800 C. Forms cuprates in solid state reactions. A cuprate(III), KCUO2, is also known. 

In solid state reactions, the rate of nucleation may be given by either of the expressions dN/dt = const, or dN/dt = t° const. For both expressions, the probability (pdf) is proportional to the total volume of the spherical layers at the instant t at the peripheries of nuclei which originated at time r. The radii of the spheres at the inner and outer boundaries of these layers are... 

The slope of the Arrhenius plot has units (temperature) 1 but activation energies are usually expressed as an energy (kJ mol 1), since the measured slope is divided by the gas constant. There is a difficulty, however, in assigning a meaning to the term mole in solid state reactions. In certain reversible reactions, the enthalpy (AH) = E, since E for the reverse reaction is small or approaching zero. Therefore, if an independently measured AH value is available (from DSC or DTA data), and is referred to a mole of reactant, an estimation of the mole of activated complex can be made. 

We have Investigated the structure of solids In the second chapter and the nature of point defects of the solid in the third chapter. We are now ready to describe how solids react. This will Include the mechanisms Involved when solids form by reaction from constituent compounds. We will also describe some methods of measurement and how one determines extent and rate of the soUd state reaction actually taking place. We will also show how the presence and/or formation of point defects affect reactivity In solid state reactions. They do so, but not In the memner that you might suspect. We will also show how solid state reactions progress, particularly those involving silicates where several different phases appear as a function of both time and relative ratios of reacting components. 

THE ROLE OF PHASE BOUNDARIES IN SOLID STATE REACTIONS... 

Note also that we have just introduced the concepts of nuclei and nucleation in our study of solid state reaction processes. Our next step will be to examine some of the mathematics used to define rate processes in solid state reactions. We will not delve into the precise equations here but present them in Appendices at the end of this chapter. But first, we need to examine reaction rate equations as adapted for the solid state. 

Determination of Rate Processes in Solid State Reactions... 

The Role of Phase Boundaries in Solid State Reactions 132... 

Gomes, W. (1961). "Definition of Rate Constant and Activation Energy in Solid State Reactions," Nature (London) 192, 965. An article discussing the difficulties associated with interpreting activation energies for reactions in solids. 

The solid state reactions are extremely complex due to intervention of many physical parameters and it becomes often necessary to make some generalizations in the complex reactions. The rate in solid state reactions cannot be defined in the same way as that for a homogeneous reactions because the concept of concentration in solid state reactions has no significance. The energy of activation in a solid state reaction has also no significance, except in some rare cases. 

When mixtures of substances are investigated, e.g. in solid state reactions, the mixture should be completely homogeneous and of uniform particle size. Smaller particle size, i.e. higher ratio surface area/volume, are important for such types of reaction ... 

The characteristic melting points and IR frequencies of 55 indicate well-defined compounds. The second molecule HX in the salts d, d, e, e is probably firmly included, forming hydrogen bonds to the available oxygen atoms. These salts are easily obtained and should be versatile building blocks in solid-state reactions or reactions in dry aprotic solvents [9]. 

Diffusion is ubiquitous in nature whenever there is heterogeneity, there is diffusion. In liquid and gas, flow or convection is often present, which might be the dominant means of mass transfer. However, inside solid phases (minerals and glass), diffusion is the only way of mass transfer. Diffusion often plays a major role in solid-state reactions, but in the presence of a fluid dissolution and recrystallization may dominate. 

Diacyl peroxides proved to be a fortunate choice, because when other techniques suggested that reaction-induced lattice stress played an important role in solid-state reactions, FTIR spectroscopy of the C02 product provided a method for measuring the stress. In the process of applying this method we... 

Metal carbonates are often used as reagents in solid-state reactions because heating them creates finely divided metal oxides as a result of the release of C02(g) from the crystal lattice. These finely divided particles, which are made freshly in situ, are often more reactive in heterogeneous solid-state reactions than are the metal oxides found on the laboratory shelf because they have a higher surface area that is more reactive because of its fresh preparation. Determine the minimum temperature necessary to achieve spontaneous thermal decomposition of BaC03. 

These differential heating effects may also be used in solid-state reactions involving metal powders that are made into a fluidised bed by a counter stream of gas. The metal particles interact very strongly with the microwaves and rapidly heat, whereas the gases are transparent to microwaves therefore, the reaction is induced by a very selective interaction between the metal particles and microwaves. 

In the solid-state reaction, nucleation and growth have a fundamental role, because, in essence, the solid-state reaction is a phase transformation. In this type of reactions, nucleation and growth follow similar principles as those previously analyzed in Section 3.1 the principal difference being the increased role of diffusion in solid-state reactions [30],... 

See also Retro-Hantzsch Hard acid or base, see HSAB theory Heating curves, in solid-state reactions,... 

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