Thermal stability and thermodynamics of manganese(II) chloride monohydrate

Abstract

Three tensimetric methods were used to measure the equilibrium water vapour pressure for the reaction MnCl 2 · H 2O(s) = MnCl 2(s) + H 2O(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 ) . The standard enthalpy and entropy of this process were calculated at the mean temperature 425 K: Δ r H ∘ (425) = (62,500 ± 200) J, Δ r S ∘ (425) = (124.56 ± 0.47) J · K −1. At the standard temperature these quantities were calculated using the approximate constant value of Δ r C P ∘ = ( - 8.3 ± 2 ) J · K - 1 · mol - 1 : Δ r H ∘ (298) = (63,550 ± 500) J, Δ r S (298) = (127.5 ± 1.2) J · K −1. Standard thermodynamic characteristics of manganese(II) chloride monohydrate were obtained: Δ f H m ∘ ( 298 ) MnCl 2 · H 2 O ( s ) = ( - 786.6 ± 0.9 ) kJ · mol - 1 ; S m ∘ ( 298 ) MnCl 2 · H 2 O ( s ) = ( 179.5 ± 1.6 ) J · K - 1 · mol - 1 .