A flow-mixing calorimeter has been used to measure the excess molar enthalpy H E m of gaseous (ammonia + water) at the mole fraction y = 0.5, at standard atmospheric pressure, and over the temperature range 383.15 K to 493.15 K. The excess molar enthalpy is negative, indicating that the strength of the ammonia-water hydrogen bond is greater than that of the water-water hydrogen bond. The measurements were analysed first in terms of a purely thermodynamic model which assumes that the second virial cross coefficient B 12 and the related isothermal Joule-Thomson coefficient φ 12 are adequately described by an empirical equation of square well form. Information about the strength of the ammonia-water hydrogen bond was obtained by analysing the measurements on the basis of a quasi-chemical association model. It was shown that hydrogen bonding in ammonia is so slight that to use an association model for the second virial coefficient B 11 would be inappropriate. The second virial coefficient B 22 of water was written B 22 = B ns 22 -R •T • K 22 . The non-specific interaction B ns 22 between water molecules was calculated from the Stockmayer potential with parameters appropriate to a water-nonpolar fluid interaction, and the specific (hydrogen bonding) forces were described by the association model in terms of an equilibrium constant K 22 (298.15 K) = 0.36 MPa -1 and an enthalpy of formation of H 22 = -(16.2 ± 2) kJ • mol -1 for the water-water hydrogen bond. The second virial cross coefficient was written B 12 = B ns 12 -(R • T • K 12 )/2, and from the temperature dependence of ln K 12 the enthalpy of formation of the ammonia-water hydrogen bond was found to be H 12 = -(19.7 ± 3) kJ • mol -1 , and K 12 (298.15 K) = 0.827 MPa -1 . Values of B 12 derived from the measurements are given by the equation:
• mol -1 ) = 38 -43059 • (K/T ) -1.993 • exp{1900 • (K/T )}.