The heat of mixing and second virial cross coefficient of water + oxygen and water + nitrogen

Abstract

A vapour phase flow mixing calorimeter has been used to measure the heat of mixing H m E of (0.5H 2O + 0.5O 2)(g) and (0.5H 2O + 0.5N 2)(g) at the pressure p = 0.101325 MPa and at temperatures from (373.15 to 493.15) K. The H m E measurements were analysed using an association model for water in which the non-specific part of the interaction energy was calculated from the Stockmayer potential with the parameters ε/ k = 233 K, σ = 0.312 nm, and t ∗ = 1.278. The specific part of the energy was calculated from an equilibrium constant for the association of two water molecules K 298 K = 0.36 MPa −1 together with an enthalpy of association Δ H = −16.85 kJ · mol −1. The second virial coefficients of O 2 and N 2 were calculated from the Lennard-Jones potential. Cross coefficients B 12 were calculated from a combining rule containing an adjustable parameter ξ which is known to be satisfactory for mixtures of (water + hydrogen, or nitrogen, or argon, or methane). For all these mixtures the combining rule gives ξ = 0.99, and this value gives a good fit to the new H m E measurements on (water + nitrogen). For (water + oxygen) it was necessary to reduce ξ from 0.99 to 0.83 to fit the measurements. The values of B 12 for both mixtures are close to those obtained from measurements of the solubility of water in compressed oxygen or nitrogen, and confirm that B 12 for water–oxygen is less negative than that for water–nitrogen. This is in accord with recent theoretical calculations of the potential surfaces for water–oxygen and water–nitrogen.