Assigning Contributions of Dispersion Forces, Dipolar Forces, and Hydrogen Bonding to the Enthalpy of Vaporization of N -Pentane, Diethyl ether, 2-Butanone, and 2-Butanol

In this general chemistry laboratory, students determine the standard enthalpies of vaporization for n-pentane, diethyl ether, 2-butanone, and 2-butanol based on measurements of vapor pressure as a function of temperature and the Clausius–Clapeyron equation. Because these molecules have nearly the same molecular weight (72 ± 2 amu), the contribution of dispersion forces to the total intermolecular association energy is approximately the same. The enthalpy of vaporization can be written as sum of dispersion and polar components: ΔH0vap = ΔH0vap(dispersion) + ΔH0vap(polar) = ΔH0vap(pentane) + ΔH0vap(polar). This equation facilitates the determination of ΔH0vap (polar). Based on our student’s results (1999–2003), ΔH0vap (pentane) is 23.8 kJ mol–1 and ΔH0vap (polar) are 3.3 kJ mol–1, 11.5 kJ mol–1 and 22.0 kJ mol–1 for the ether, ketone, and alcohol functional groups, respectively. Except for 2-butanol, the dominant intermolecular forces in n-pentane, diethyl ether, and 2-butanone are dispersion forces. In the case of 2-butanol, dispersion forces and hydrogen bonding contribute about equally. Students also determine the standard entropies of vaporization from which they gain an understanding of Trouton’s rule.