Kinetics and mechanism of the isothermal dehydration of zinc acetate dihydrate

Article abstract of DOI:10.1016/S0040-6031(97)00264-5

The overall kinetics of the thermal dehydration of zinc acetate dihydrate was investigated by means of isothermal mass-change measurements, complemented by microscopic observations of the reaction geometry and morphological change during the reaction. Under isothermal conditions, the compound loses its water of crystallization in a well-defined single step; Zn(CH₃COO)₂ · 2H₂O → Zn(CH₃COO)₂ + 2H₂O. The microscopic observations for the single crystals confirm that the reaction initiates at the edge surfaces of the hexagonal thin plate by nucleation and growth processes, consequently forming the reaction interface which advances inward, toward the center of the hexagon. The kinetic results obtained from the thermoanalytical measurements indicated agreement to the first-order law, in spite of the two-dimensional shrinkage of the reaction interface. This discrepancy is discussed in connection with the interactions of the elementary nucleation and growth processes at the reaction interfaces with the self-generated water vapor. The overall kinetic behavior of the crushed crystals of different particle-size fractions, under various atmospheric conditions, was investigated. The apparent kinetic results varied systematically with the sample and atmospheric conditions, accompanied by changes of the roles of surface reaction, diffusion of evolved water vapor from the reaction interface and gross diffusion of water vapor through the assemblage of sample particles.