Thermodynamic properties of the n-hexane and benzene system at elevated temperatures

The total vapour pressure and the composition of the liquid of the n-hexane and benzene mixtures have been measured under equilibrium conditions at 90, 110, 130, 150, 170 and 190°C. The corresponding vapour phase compositions were estimated using an expression of the liquid phase activity coefficient as a function of composition of the liquid phase. Several expressions were compared and the three-constant Redlich and Kister equation was found to give the best fit to the experimental vapour pressure data. The virial equation of state as well as a modified Redlich and Kwong equation were used to express the deviation of the vapour phase from the ideal. Results were compared and the discrepancy was found to be within the experimental tolerance. The low values of the liquid phase activity coefficients indicated that the behaviour of this particular system was not far from ideal. Also, it was found that the degree of deviation from the ideal decreased with the increase in temperature. The excess Gibbs free energy of mixing was calculated and it was found to be non-symmetrical with liquid composition. On increasing the temperature of the system, the liquid composition corresponding to the maximum GE tended to shift to a lower n-hexane concentration, while at fixed liquid composition, GE initially decreased and then remained stationary or increased again. This was thought to be due to differences in the molar volumes of the two components.