Fluorine bomb calorimetry. Part 18.—Standard enthalpy of formation of phosphorus pen afluoride and enthalpies of transition between various forms of phosphorus. Thermodynamic functions of phosphorus pentafluoride between 0 and 1500°K

Abstract

The energies of combustion in fluorine of α-white phosphorus and three modifications of red phosphorus—amorphous I, crystalline IV and crystalline V—were measured. With the adoption of α-white phosphorus as standard state, the standard enthalpy of formation of PF₅(g) at 25° was calculated to be –380.8±0.3 kcal mole^–1. The enthalpies of transition at 25° for the following processes are reported: P(α-white)→P(amorphous red I), ΔH_t=–3.9±0.4 kcal g-atom^–1; P(α-white)→P(red V), ΔH_t=–3.9±1.5 kcal g-atom^–1; P(red IV)→P(red V), ΔH_t=–1.2±1.0 kcal g-atom^–1. High-temperature thermodynamic functions for PF₅(g) are also reported.