Reaction Smith-Topley

The Smith—Topley (S—T) effect is the characteristic variation of isothermal dehydration rate (da /df)D with prevailing water vapour pressure (PHzo) shown in Fig. 10. (da/df)D first decreases with increasing PH2oi later rises to a maximum value and thereafter diminishes towards the zero rate of water loss that is achieved at the equilibrium dissociation pressure. For many hydrates, the reduction in (da/df)D from that characteristic of reaction in a good vacuum to that at PHzo 0.1 Torr is large (X 0.1) and the subsequent maximum may be more or less sharp. Since the reaction rate is, in general, represented by... 

At least three major interrelated aspects of kinetic behaviom are important in investigating dehydration reactions, (i) Kinetic equations based upon nucleation and growth models have been developed and found application to a wide range of reactants [31], (ii) Theoretical explanations of the magnitudes of calculated Arrhenius parameters have been proposed, (iii) The influence of water vapour pressure on reaction rates has been investigated in detail (the Smith-Topley effect). Topics (ii) and (iii) are expanded below,... 

The reaction of this solid [48] was the first [133] example of Smith-Topley behaviour recognized and studies of this rate process have continued. Flanagan and Kim [133] showed that irradiation decreased the induction period to dehydration and the rate of water evolution rapidly reached a maximum value which was maintained between 0 < nr < 0.4. Water evolution was more rapid than that found for unirradiated salt and the value of , was decreased. Irradiation damage to the crystal promoted nucleation and there was rapid initial establishment of a constant area of reaction interface (the contracting volume equation approximates to zero-order kinetics at low values of nr). There was also evidence [134] that preirradation aided recrystallization during vacuum dehydration. 

A gravimetric study of the thermal dehydration of Y(HC00)j.2H20 was undertaken [149] in various water vapour pressures between 5 x 10" to 8 Torr (387 to 407 K). Following an initial short acceleratory process, the reaction predominantly fitted the contracting volume equation. The reaction rate increased with pQi20), reached a maximum value and thereafter decreased to a constant value. This is a pattern of behaviour similar to the Smith-Topley effect. The results are explained on the basis of the crystallinity of the dehydrated residual product. 

More quantitative measurements of the systematic variations of dehydration rates with/7(H20), referred to as Smith-Topley behaviour, could lead to support for one or more of the several theoretical explanations that have been proposed [2,21,49,54,63] based on recrystallization of sohd product, local self-cooling and/or diffusion (effects expected to occur in all dehydration reactions) and adsorption of the volatile product. Dehydrations may also involve the intervention of a zeolitic residue and/or an amorphous phase, the formation and reciystallization of one or more lower hydrates as intermediates, and diffusive esc e of water through various channels of the barrier layer of product may be slow. 

Prodan et al. [75] studied the low pressure (lO" Torr), low temperature (fi om 273 to 373 K) dehydration of Na5P30,Q.6H20 in the form of fine crystals. Reaction occurred in two stages (with = 56 and 84 kJ mol ) both of which were diffiision controlled. The activation energy increased with extent of reaction. The rate of reaction of this salt was enhanced [76] by water vapour, attributed to its ability to reorganize the diffusion layer. This effect (Smith-Topley behaviour) has been noted in many dehydration reactions (Chapter 7). 

The dehydration that precedes the decomposition of manganese(II) oxalate exhibits the Smith-Topley effect. The residual product of decomposition, MnO, is readily oxidized and the temperature of reaction is decreased in the presence of oxygen. The decomposition of anhydrous manganese(II) oxalate in vacuum [63] can be represented as ... 

Smith-Topley s effect on the influence of water vapor in dehydration reactions... 

In the case of the dehydration reactions, a particular form was announced by Smith-Topley for the influence of the water vapor on the dehydration of manganese oxalate dihydrate [TOP 35] and was found thereafter in other cases by various authors. 

Figure 13.9. Smith-Topley s effect in the case of only one dehydration reaction...

Using the fact that the critical size of the nucleus varies with the pressure and temperature [GRU 74], Bouineau [BOU 98] shows, via a relation such as [13.10], that the equilibrium constant of nucleation of the reaction is modified, and consequently, as in the case of the growth, it is necessary to change curves giving y according to P (or 7). Thus, Smith-Topley s effect would be found on the curves of nucleation for the same fundamental reason (modification of the equilibrium constant) as that on the curves of reactivity. 

Introduction Hydrates of metal salts undoubtedly played a leading part in the history of the kinetics of solid-state reactions. Particularly noteworthy are the first observations of the dehydration of crystalline hydrates, made by Faraday after scratching the crystal surface (Chapter 1), and the anomalous acceleration of the dehydrations of some crystalline hydrates in the presence of water vapour, discovered by Topley and Smith (Chapter 7). 

For thermal decompositions, a decreasing influence of pressure on rate is quickly highlighted. It is rightly ascribed to what one could call an influence of the thermodynamic type the rate decreases with the pressure of produced gas, until canceling itself for the equilibrium pressure. More singular influences were detected, for example, by Smith and Topley, who showed extremums on the curve giving speed versus water pressure for certain dehydrations (see section 13.4.4). Certain influences, of catalytic type, are also ascribed with water vapor even if the gas is not involved in the reaction as a main component. 

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