The initial solid is a single reactant

We study the reaction in which the initial solid A is a single reactant, but there exists inevitably an external medirrm that either constitutes an inert gas phase in a polymorphic transformation or that contains, at least, gas products of decorrqrositioa 

Most phenomena occur at the internal interface (A/B) however, in the case of decomposition, it is necessary to evacuate gas through layer B, which involves necessarily a diffusion zone (see Chapter 13). If there is gaseous emission, the process of adsorption-desorption is regarded as always present whereas in a gas-solid contact, it is necessary to consider the desorption of produced gas, located either on the surface of A if the gas diffuses through pores of B or on the stuface of B if the gas is produced at this interface after diffusion of point defects in B. 

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To define the zones, we distinguish two categories of transfonnations according to whether the initial solid A is a single reactant or whether it reacts with another phase. 

In what follows we will stiek to the first ease of inward development with internal interface step as rate determining (the laws that correspond to the other cases were not established). Indeed, the laws that correspond to the isotropic growth are primarily known for reactions in which the initial solid A is the single reactant, that is, for the thermal decompositions and the polymorphic transformations. It is known that in these cases, the essence of chemistry proceeds at the internal interface, and as there is no other reactant, the outward development is not conceivable. In the other cases, if the initial solid is not a single reactant, the experiment shows that we have, in the large majority of the cases, either a one-process model with instantaneous nucleation or a two-process model with surface nucleation and anisotropic radial growth developed earlier. 

If only a single gas takes part in the reaction, the mass can be connected to the fractional extent related to this gas and its derivative to the rate relative to this gas. Indeed, one can measure only the amount of gas that is fixed or that is released. If the algebraic stoichiometric number related to gas in reaction [l.R.l] is indicated by (positive for a product, negative for a reactant), it can be easily shown that if is the molar mass of gas, mass of the initial solid A, m and the masses at time... 

The retarding influence of the product barrier in many solid—solid interactions is a rate-controlling factor that is not usually apparent in the decompositions of single solids. However, even where diffusion control operates, this is often in addition to, and in conjunction with, geometric factors (i.e. changes in reaction interfacial area with a) and kinetic equations based on contributions from both sources are discussed in Chap. 3, Sect. 3.3. As in the decompositions of single solids, reaction rate coefficients (and the shapes of a—time curves) for solid + solid reactions are sensitive to sizes, shapes and, here, also on the relative dispositions of the components of the reactant mixture. Inevitably as the number of different crystalline components present initially is increased, the number of variables requiring specification to define the reactant completely rises the parameters concerned are mentioned in Table 17. 

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. 

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