Kinetics of High‐Temperature Hydrolysis of Magnesium Fluoride: I, Evaluation of Reaction Mechanism

Abstract

The rates of hydrolysis of polycrystalline magnesium fluoride slabs were investigated by a gravimetric method as functions of temperature from 745° to 835°C and water‐vapor pressure from 1 to 20 mm Hg. A linear rate law was found, and it was concluded that the rate‐determining process was chemical reaction at the MgF 2 ‐MgO interface. A three‐step reaction mechanism is proposed: (1) the chemisorption of water vapor. image (2) formation of a hydroxyfluoride complex. image and (3) decomposition of the hydroxyfluoride complex to form products. image Assuming that the chemisorption step was in equilibrium and that the HF pressure was negligible, an expression was developed showing that the rate was proportional to the concentration of chemisorbed water vapor. The rate dependence on water‐vapor pressure was expressed in terms of the Langmuir adsorption isotherm equation. Values of –16.9 kcal/mole and–4.2 eu were obtained for the enthalpy and entropy of chemisorption. The activation enthalpy and frequency factor for the decomposition of the chemisorbed intermediate, MgF 2 .H 2 O(chem), were 46.0 ± 0.5 kcal/mole and 1.75 × 1O 10 sec −1 .