Thermal stability and thermodynamic characteristics of crystalline hydrates of cobalt(II) fluoride

Abstract

By interaction of freshly-precipitated cobalt(II) hydroxo-carbonate (CoOH)2CO3 with 35% water solution of hydrofluoric acid the cobalt (II) fluoride tetrahydrate CoF2 · 4H2O was synthesized; its composition is confirmed by results of chemical analysis. The enthalpies of (CoOH)2CO3 interactions with hydrofluoric and nitric acids in solutions was determined using the method of dissolution calorimetry. On the basis of these data, as well as on the basis of data for byside processes observed in the systems considered together with available reference data, previously unknown values of enthalpy of formation (ΔH0 298) of CoF2 · 4H2O and (CoOH)2CO3 have been determined, namely −1881 ± 20 and −1428 ± 31 kJ/mol, correspondingly. It has been revealed using the tensimetry method with membrane null manometer that thermal dehydration of CoF2 · 4H2O takes place in the interval of temperatures ΔT = 295 ÷ 345 K and it means the loss of three (from four) moles of crystalline water with the formation of CoF2·H2O. The process of CoF2·H2O dehydration occurs within the temperature interval ΔT = 350 ÷ 380 K. Using the barograph equations the thermodynamic characteristics of dehydration processes have been determined. The thermodynamic characteristics for CoF2 · 4H2O were calculated based on the aforementioned data and reference information for the components of these systems, namely: ΔH0 298 = −1865 ± 18 kJ/mol; S0 298 = 400 ± 21J/(mol·K) were calculated. The same parameters for CoF2 · H2O are the follows: ΔH0 298 = −901±19 kJ/mol; S0 298 = 229±21 J/(mol·K). Refs 21. Tables 6.