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

✍ Scribed by C.J. Wormald; B. Wurzberger

• Publisher: Elsevier Science
• Year: 2002
• Tongue: English
• Weight: 117 KB
• Volume: 34
• Category: Article
• ISSN: 0021-9614
• DOI: 10.1006/jcht.2002.0980

✦ Synopsis

A flow mixing calorimeter has been used to measure the excess molar enthalpy H E m 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 B 12 and φ 12 were calculated using the arithmetic mean rule for collision diameters and the rule

for the depth of the potential well. The H E m measurements on (ammonia + cyclohexane) were fitted to within experimental error by the choice (1 -k 12 ) = 0.92. At the temperatures 363.15 K and 493.15 K the values of H E m for (ammonia + benzene) were found to be 9 J • mol -1 and 5 J • mol -1 less positive than the values calculated using (1 -k 12 ) = 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 B 12 was written B 12 = B ns 12 -(RT K 12 )/2. The non-specific term B ns 12 was calculated using (1 -k 12 ) = 0.92, and values of the equilibrium constant K 12 were determined from the difference between the calculated and experimental excess enthalpies. A plot of ln K 12 against reciprocal temperature yielded the enthalpy of formation H 12 of the ammonia-benzene complex and this was found to be H 12 = -(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.