The thermodynamic properties of elementary phosphorus The heat capacities of two crystalline modifications of red phosphorus, of α and β white phosphorus, and of black phosphorus from 15 to 300 K

Abstract

A study of time and temperature conditions for the preparation of several crystalline modifications of red phosphorus has been made. The heat capacities of two crystalline modifications of red phosphorus and the heat capacity of black phosphorus have been measured from 15 to 300 K. The heat capacity of white phosphorus has been measured from 15 to 320 K. White phosphorus has a first-order transition at 195.4 K. The enthalpy of transition, (124.6±0.2) cal mol −1, enthalpy of fusion, (157.4±0.5) cal mol −1, and the melting point, (317.3±0.1) K of white phosphorus were measured. The barrier to rotation of P 4 molecules in the α-phase of white phosphorus is low enough to allow the contribution to the heat capacity associated with hindered rotation to approach the classical value for free rotation at the melting point. The sublimation pressure of red phosphorus (form IV) was determined as (1.22±0.05) atm at 444.6 °C. The entropies of the two red forms, of black, and of white phosphorus were calculated from the heat capacities. Standard Gibbs energy and enthalpy changes for white and red (IV) phosphorus have been calculated from the entropies and other data concerning phosphorus.