Standard molar enthalpies of formation and transition at the temperature 298.15 K and other thermodynamic properties of the crystalline and vitreous forms of arsenic sesquiselenide (As2Se3). Dissociation enthalpies of AsSe bonds

Abstract Fluorine-combustion calorimetry was used to determine the standard molar enthalpies of formation (at the temperature 298.15 K and with standard pressure p o = 101.325 kPa) of the crystalline and vitreous forms of arsenic sesquiselenide. The following results were obtained: Δ f H m o (As 2 Se 3 , cr) = −(86.1±4.1) kJ·mol −1 and Δ f H m o (As 2 Se 3 , vit) = −(58.1±4.2) kJ·mol −1 . The enthalpy of the transition from the vitreous to the crystalline forms of As 2 Se 3 , −(28.0±3.9) kJ·mol −1 at 298.15 K, is consistent with two of several published values for the enthalpy of fusion of the crystalline sesquiselenide. The present results have been combined with enthalpy increments and the standard entropy has been recalculated from the literature to give, for As 2 Se 3 (cr) only, Δ f H m o and the standard molar Gibbs energy of formation Δ f G m o as functions of temperature. Mean bond enthalpies have been deduced for As 4 Se 3 (g) and As 4 Se 4 (g) on the basis of the new Δ f H m o values, and the thermodynamic results are shown to be consistent with a linear structure for As 2 Se 2 (g) with a central AsAs bond. Bond dissociation enthalpies D m o (As-X) are given, where X = O, S, Se, and Te.