Kinetics of nucleation and growth during dehydrations

The dehydration reactions of Cu(HC02)2 4 H20 [213] and of Mn(CH02)2 2 H20 [91,212] are characterized by the rapid initial production of a complete and coherent reactant—product interface at 

U02(N03)2 6 H20 showed unusual behaviour [62] in that there was no induction period to dehydration, the generation of specialized nuclei was apparently unnecessary since water evolution occurred by desorption at existing crystal surfaces and no migratory interface was developed. 

The initiation of dehydration at the first-formed nuclei does not necessarily preclude the continued production of further nuclei elsewhere on unreacted surfaces. During dehydration of CuS04 5 H20, the number of nuclei was shown [426] to increase linearly with time, whereas during water removal from NiS04 7 H20 [50] the number of nuclei increased with the square of time, Nt = kN(t — t0)2. (The latter behaviour contrasts with the instantaneous nucleation of NiS04 6 H20 mentioned above.) 

Gamer and Jennings [431] studied nucleation during the dehydration of potassium and ammonium chromium alums. Detailed kinetic measurements were made for the relatively enhanced rate of nucleation which followed admission of water vapour to the solid after a period of vacuum nucleation. This catalytic effect of water vapour is ascribed to its participation in the reorganization of the lattice which had collapsed during previous treatment in vacuum. 

It is usually assumed in the derivation of isothermal rate equations based on geometric reaction models, that interface advance proceeds at constant rate (Chap. 3 Sects. 2 and 3). Much of the early experimental support for this important and widely accepted premise derives from measurements for dehydration reactions in which easily recognizable, large and well-defined nuclei permitted accurate measurement. This simple representation of constant rate of interface advance is, however, not universally applicable and may require modifications for use in the formulation of rate equations for quantitative kinetic analyses. Such modifications include due allowance for the following factors, (i) The rate of initial growth of small nuclei is often less than that ultimately achieved, (ii) Rates of interface advance may vary with crystallographic direction and reactant surface, (iii) The impedance to water vapour escape offered by 

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