Thermal stability and thermodynamics of manganese(II) chloride monohydrate

Three tensimetric methods were used to measure the equilibrium water vapour pressure for the reaction MnCl2 · H2O(s) = MnCl2(s) + H2O(g) over the temperature range (353.25 to 500.3) K. Two additional effects were considered, viz.   possibility of a hydrate super disperse phase formation and probability of manganese(II) chloride hydrolysis with water vapour. A linear equation was derived for the temperature dependence of the water vapour pressure: ln(p/p∘)=14.981-7517·(K/T)ln(p/p∘)=14.981-7517·(K/T). The standard enthalpy and entropy of this process were calculated at the mean temperature 425 K: ΔrH∘ (425) = (62,500 ± 200) J, ΔrS∘ (425) = (124.56 ± 0.47) J · K−1. At the standard temperature these quantities were calculated using the approximate constant value of ΔrCP∘=(-8.3±2)J·K-1·mol-1: ΔrH∘ (298) = (63,550 ± 500) J, ΔrS (298) = (127.5 ± 1.2) J · K−1. Standard thermodynamic characteristics of manganese(II) chloride monohydrate were obtained: ΔfHm∘(298)MnCl2·H2O(s)=(-786.6±0.9)kJ·mol-1; Sm∘(298)MnCl2·H2O(s)=(179.5±1.6)J·K-1·mol-1.