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 center dot 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: In(p/p degrees) = 14.981 - 7517 - (K/T). The standard enthalpy and entropy of this process were calculated at the mean temperature 425 K: Delta H-r degrees (425) = (62.500 +/- 200) J. Delta S-r degrees (425) = (124.56 +/- O.47)J . K-1. At the standard temperature these quantities were calculated using the approximate constant value of Delta C-r(P)degrees = (-8.3 2) J . K-1 . mol(-1): Delta H-r degrees (298) = (63,550 +/- 500)J. Delta S-r (298) = (127.5 +/- 1.2)J . K-1. Standard thermodynamic characteristics of manganese(H) chloride monohydrate were obtained: Delta H-f(m)degrees (298)MnCl2 center dot H2O(s) = (-786.6 +/- 0.9) kJ.mol(-1); S-m degrees (298)MnCl2 - H2O(s) = (179.5 +/- 1.6) J . K-1 - mol(-1). (c) 2008 Elsevier Ltd. All rights reserved.