Benzene–chloroform association Excess molar enthalpy of (cyclohexane+chloroform)(g) and (benzene+chloroform)(g) at temperatures from 353.2 to 423.2 K

A flow-mixing calorimeter has been used to measure the excess molar enthalpy HmE of (cyclohexane+chloroform)(g) and (benzene+chloroform)(g) at standard atmospheric pressure over the temperature range 353.2–423.2 K. The non-ideality of the cyclohexane and benzene was fitted using the Kihara potential, and that of the chloroform using the Stockmayer potential. Cross-terms were calculated using the equation e12=(1-k12)(e11e22)1/2 and to fit the measurements on (cyclohexane+chloroform)(g) the value (1-k12)=0.965 was needed. This value was used to calculate HmE for (benzene+chloroform)(g), and the calculated values were found to be positive and to be similar to those for (cyclohexane+chloroform)(g). However, the experimental values are negative, and about 35 J mol-1 below the values for (cyclohexane+chloroform)(g). The difference between the calculated and experimental values was described in terms of a quasi-chemical model that, for the benzene–chloroform interaction, yielded a value of the equilibrium constant K12(298.15 K)=0.373 MPa-1 and an enthalpy of association ΔH12=-(16.1±2) kJ mol-1. This value of ΔH12 is attributed to a charge transfer between the benzene and the chloroform that is not present in the cyclohexane–chloroform interaction.