Thermochemistry of inorganic sulfur compounds VIII. Na 2SO 3 and K 2SO 3: standard molar enthalpies of solution and formation at 298.15 K, high-temperature enthalpy increments and heat capacities, and temperatures and enthalpies of transition. A new value for the standard molar enthalpy of formation of SO 32−(aq)

Abstract

Solution calorimetry has been used to determine the enthalpies of reaction with I 3 −(aq) of thoroughly characterized specimens of Na 2SO 3 and K 2SO 3, from which the standard molar enthalpies of formation Δ f H m o of the solid sulfites at 298.15 K have been derived: −(1096.78 ± 0.81) kJ · mol −1 for Na 2SO 3 and −(1123.37 ± 0.96) kJ · mol −1 for K 2SO 3. The standard molar enthalpies of solution in water at 298.15 K of Na 2SO 3 and K 2SO 3 have been determined to be −(13.24 ± 0.09) kJ · mol −1 and −(10.06 ± 0.21) kJ · mol −1, respectively, from which Δ f H m o(SO 3 2−, aq, m o = 1 mol · kg −1, 298.15 K) = −(629.3 ± 0.6) kJ · mol −1, has been deduced. High-temperature enthalpy increments relative to 298.15 K, heat capacities, and the enthalpies of transition have been measured by drop calorimetry. For Na 2SO 3, T trs of the α-to-β transition is (646 ± 2) K and Δ trs H m o = (0.67 ± 0.50) kJ · mol −1; for K 2SO 3. T trs = (696 ± 2) K and Δ trs H m o = (3.15 ± 0.50) kJ · mol −1. Thermodynamic properties are tabulated for M 2SO 3(α, β) and Δ r G m o values are given for the Foerster-Kubel reaction and for reduction of M 2SO 4 to M 2SO 3 by C(graphite) and CO(g).