Thermodynamic properties of aqueous non-electrolyte mixtures II. Vapour pressures and excess Gibbs energies for water + ethanol at 303.15 to 363.15 K determined by an accurate static method

Abstract

A new apparatus is described in which the vapour pressure of a liquid mixture is determined accurately by a static method. The apparatus has been designed to work in the temperature range 298.15 to 423.15 K for pressures up to 1.5 MPa. Temperatures are measured to within ±1 mK and pressures to ±0.02 per cent or ±2.6 Pa whichever is the greater, over the range 2.7 to 270 kPa. Vapour pressures of “;(1−x) H 2O +x C 2H 5OH ’; have been measured by the static method at 303.15, 323.15, 343.15, and 363.15 K for 24 mixtures with x in the range 0.0044 to 0.9815. Values of the molar excess Gibbs energy G E and the partial molar Gibbs energies G 1 E have been computed by Barker's method. Positive deviations from ideality were found at all four temperatures with maxima in G E of 770, 837, 889, and 925 J mol −1 at 303.15, 323.15, 343.15 and 363.15 K respectively; these maxima occur at values of x about 0.440, 0.428, 0.418, and 0.406 respectively. The excess Gibbs energies have been combined with experimental excess enthalpies H E to give excess entropies S E. The curves of TS E against x all have the form of smooth skewed parabolas with large negative minima. The minimum values of TS E are −1289, −1076, −858, and −648 J mol −1 at 303.15, 323.15, 343.15, and 363.15 K. These minima all occur at a value of x about 0.28: partial molar excess entropies are presented. Isobaric liquid-vapour equilibrium data for “;(1−x) H 2O +x C 2H 5OH ’; at a pressure of 101.325 kPa have been calculated from the isothermal data. The results are compared with data from the literature. The composition of the normal-boiling positive azeotrope, x = (0.8933±0.0005) at T = (351.320±0.002) K, was determined from an analysis of the experimental vapour pressures. The minimum temperature for azeotrope formation was (305.7±0.5) K. Azeotrope compositions at 323.15, 343.15, and 363.15 K were x = 0.9324, 0.9002, and (0.8845±0.0005) respectively.