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 (As 2Se 3). Dissociation enthalpies of AsSe 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 2Se 3, cr) = −(86.1±4.1) kJ·mol −1 and Δ f H m o(As 2Se 3, vit) = −(58.1±4.2) kJ·mol −1. The enthalpy of the transition from the vitreous to the crystalline forms of As 2Se 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 2Se 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 4Se 3(g) and As 4Se 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 2Se 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.