Ammonia–carbon dioxide association. Second virial cross coefficients for (ammonia+carbon dioxide) derived from gas phase excess enthalpy measurements

Abstract A flow mixing calorimeter has been used to measure the excess molar enthalpy H E m of gaseous (ammonia + carbon dioxide) at the mole fraction y =0.5, at standard atmospheric pressure, and over the temperature range 343.15 to 443.15 K. Second virial coefficients B and isothermal Joule-Thomson coefficients φ for ammonia were fitted by the Stockmayer potential, and similar properties for carbon dioxide were fitted by the Kihara potential. Cross terms B 12 and φ 12 were calculated using the arithmetic mean rule for the collision diameter and the rule e 12 =(1− k 12 )( e 11 · e 22 ) 1/2 for the depth of the potential well. Values of H E m were predicted using the combining rule (1− k 12 )=2( σ 11 3 σ 22 3 ) 1/2 ( σ 12 3 )(I 1 I 2 ) 1/2 ( I 1 + I 2 ) −1 which gave (1− k 12 )=0.94. Correction for dipole-induced dipole effects increased this value by 5 per cent to 0.98. At T =353.15 K the calculated value of H E m was 14 J · mol −1 but the experimental value was found to be only 1.5 J · mol −1 . This, and the other experimental values of H E m , could all be fitted by (1− k 12 )=1.47. The difference between the calculated and experimental values of H E m was analysed in terms of a quasi-chemical association model in which the second virial cross coefficient B 12 was written B 12 = B 12 ns −( RTK 12 )/2. The non-specific term B 12 ns was calculated using (1− k 12 )=0.98, 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 Δ H 12 of the ammonia–carbon dioxide complex and this was found to be Δ H 12 =−(8.6±2) kJ · mol −1 . The sum of the specific and non specific contributions is −(11.5±3) kJ · mol −1 .