Ammonia–benzene association. Second virial cross coefficients for ammonia–benzene and ammonia–cyclohexane derived from gas phase excess enthalpy measurements

Abstract

A flow mixing calorimeter has been used to measure the excess molar enthalpyHmE of gaseous (ammonia + benzene) and (ammonia + cyclohexane) at the mole fraction y= 0.5, at standard atmospheric pressure, and over the temperature range 363.15 K to 493.15 K. Second virial coefficients B and isothermal Joule–Thomson coefficients φ for benzene and cyclohexane were fitted by the Kihara potential, and similar properties for ammonia were fitted by the Stockmayer potential. Cross terms B12 andφ12 were calculated using the arithmetic mean rule for collision diameters and the ruleϵ12= (1 −k12 )(ϵ11·ϵ22 )1/2 for the depth of the potential well. TheHmE measurements on (ammonia + cyclohexane) were fitted to within experimental error by the choice (1 −k12) = 0.92. At the temperatures 363.15 K and 493.15 K the values ofHmE for (ammonia + benzene) were found to be 9J · mol−1 and 5 J·mol−1 less positive than the values calculated using (1 −k12 ) = 0.92. This difference was attributed to a specific interaction between ammonia and benzene. The difference was analysed in terms of a quasi-chemical association model in which the second virial cross coefficient B12 was written B12=B12ns− (RTK12)/2. The non-specific term B12ns was calculated using (1 −k12 ) = 0.92, and values of the equilibrium constant K12 were determined from the difference between the calculated and experimental excess enthalpies. A plot oflnK12 against reciprocal temperature yielded the enthalpy of formationΔH12 of the ammonia–benzene complex and this was found to beΔH12=−(4.6 ± 2)kJ· mol−1. The sum of the specific and non specific contributions is −(8.5 ± 3)kJ·mol−1 , and this is in reasonable agreement with values of the binding energy of the ammonia–benzene van der Waals complex computed by ab initio methods.