Vaporization of FeI2(s) and the thermochemistry of FeI2(g), (FeI2)2(g), (FeI2)3(g), and FeI3(g)

Abstract The evaporation of FeI2(s) was investigated by high-temperature mass spectrometry with a Knudsen cell in the temperature range from 605 to 725 K. The mass spectrum of the vapour over FeI2(s) consists of (with appearance potentials): Fe+, (15.2±0.3) V; I+, (10.5±0.3) V; FeI+, (11.5±0.3) V; I2+, (9.4±0.3) V; FeI2+, (9.9±0.3) V; Fe2I2+, (12.2±0.3) V; FeI3+, (9.7±0.3) V; Fe2I3+, (10.2±0.3) V; Fe2I4+, (9.1±0.3) V; Fe3I4+, (10.2±0.3) V; Fe2I5+, (7.7±0.3) V; Fe3I5+, (9.4±0.3) V; Fe3I6+, (8.3±0.3) V; Fe2I+, Fe2+, Fe2I6+, and Fe4I6+. The species FeI2(g), (FeI2)2(g), (FeI2)3(g), FeI3(g), I(g), I2(g), as well as Fe2I5(g) and/or (FeI3)2(g) were identified in the equilibrium vapour over FeI2(s). Equations for the partial pressures of these species are given, with the exception of Fe2I5(g) and (FeI3)2(g). Enthalpy changes were determined for: The values are: ΔrHmo(298.15 K) = (193.2±2.5) kJ·mol−1, equation (1), i = 1; ΔrHmo(298.15 K) = (229.3±4.0) kJ·mol−1, equation (1), i = 2; ΔrHmo(298.15 K) = (283.1±7.0) kJ·mol−1, equation (1), i = 3; ΔrHmo(298.15 K) = −(157.1±6.4) kJ·mol−1, equation (2), n = 2; ΔrHmo(298.15 K) = −(297.5±10.3) kJ·mol−1, equation (2), n = 3; ΔrHmo(298.15 K) = (21.3±2.4) kJ·mol−1, equation (3); and ΔrHmo(298.15 K) = −(63.2±4.1) kJ·mol−1, equation (4). The second-law method, or both the second- and third-law methods, were used for the determination of the aforementioned enthalpy changes. The molar enthalpy of formation of FeI2(s) and the dissociation enthalpy referred to the reaction: were computed as ΔfHmo(298.15 K) = −(109.5±3.5) kJ·mol−1 and ΔrHmo(298.15 K) = (395.0±2.5) kJ·mol−1. The partial pressures and reaction enthalpies obtained are discussed in the context of literature values.