Gas Imperfections Determined from the Heat of Vaporization and Vapor Pressure

The imperfections of many gases may be obtained by combining available heats of vaporization and vapor pressure data with the Clausius-Clapeyron equation. The accuracy of this method in some cases is comparable to that of good direct P.V.T. measurements and therefore may be useful in preliminary investigations. If P is the vapor pressure, T is the absolute temperature, ΔH is the heat of vaporization, V and Vl are the molal volumes of the vapor and liquid phases: PVRT=−ΔHR(1−Vl/V)[dlnP/d(1/T)]=1+βV.This equation determines the gas imperfection β. The value of β calculated in this manner is compared with experimental second virial coefficients for H2O, NH3, H2, O2, A, CH4, propane, n-butane, n-heptane, ethene, propene, CH3Cl, CCl3F, CH3OH, C6H6, and Hg. Values of β are determined for HI, HBr, HCl, CH3NH2, (CH3)2NH, HCN, isobutane, tetramethylmethane, 2,2,4, trimethylpentane, C2N2, COS, PH3, H2S, CH3Br, and C2H5OH. Evidence of hydrogen bond formation is shown for CH3OH, C2H5OH, CH3NH2, and (CH3)2NH. It is shown that HI and HCN contain a considerable fraction of instantaneously formed double molecules. The gas imperfection of benzene is perfectly normal. The number of instantaneously formed double molecules is estimated for all of the above molecular species.