Thermochemistry of inorganic sulfur compounds V. Peroxymonosulfate revisited: standard molar enthalpies of formation of KHSO 5·H 2O(cr), KHSO 5(cr), and HSO 5−(aq)

Abstract

The enthalpy of reaction of pure KHSO 5·H 2O with HI(aq) has been determined calorimetrically. This result has been combined with auxiliary enthalpy-of-solution measurements to yield the standard molar enthalpy of formation: Δ f H m o(KHSO 5·H 2O, cr, 298.15 K) = −(1359.88±0.57) kJ·mol −1. The standard molar enthalpies of solution in water of KHSO 5·H 2O and KHSO 5 have also been measured: Δ sln H m o(KHSO 5·H 2O, cr, 298.15 K) = (46.24±0.34) kJ·mol −1, Δ sln H m o(KHSO 5, cr, 298.15 K) = (33.68±0.24) kJ·mol −1. From these results, Δ f H m o(HSO 5 −, aq) with standard molality m o = 1 mol·kg −1, has been calculated to be −(775.64±0.68) kJ·mol −1. The standard potential for the aqueous half-reaction: HSO 5 − + 2H + + 2e − = HSO 4 − + H 2O, is estimated to be (1.85±0.03) V. Also reported are Δ f H m o(KHSO 5, cr, 298.15 K) = −(1061.49±0.71) kJ·mol −1; the molar enthalpy of dehydration at 298.15 K for the reaction: KHSO 5·H 2O(cr) = KHSO 5(cr) + H 2O(l), Δ f H m o = (12.56±0.42) kJ·mol −1; and the equilibrium H 2O(g) vapor pressure at 298.15 K for the dehydration reaction: KHSO 5·H 2O(cr) = KHSO 5(cr) + H 2O(g), p p o = (7.1±1.0) × 10 −3 . Taking p o = 101325 Pa, the vapor pressure is (720±100) Pa.