Kinetics of dehydration of single crystals of copper formate tetrahydrate

Abstract

Reproducible kinetic data for the dehydration of individual single crystals of copper formate tetrahydrate have been obtained from –45° to +25°C in vacuo. Although structural studies suggest the presence of two differently coordinated types of water molecule, all of the water molecules behave identically with respect to their kinetics of dehydration. The reactant/product interface penetrates into the crystal at a constant rate in only the two dimensions parallel to the planes (001) which contain the copper and formate ions. A two-dimensional contracting envelope equation describes the α(fraction dehydration) against time curves. The energy of activation, 11.2 ± 0.2 kcal/mol H₂O, for penetration of the interface is less than the overall heat of dissociation, 12.5 kcal/mol H₂O. The kinetics of dehydration are unaffected by an antiferroelectric transition at –37.7°C. The inhibition of the dehydration process by water vapour has been examined and discussed quantitatively.