- The boiling point of a liquid is the temperature at which the vapor pressure of the
liquid equals the pressure of the atmosphere above it.
- Boiling points of liquids are pressure dependent.
- The normal bp given for a liquid is its bp at 1 atm (760 torr).
- The intermolecular van der Waals attractions increase as the size of the molecule
increases because the surface areas of heavier molecules are usually much
greater. - For example: the bp of methane (–162 °C), ethane (–88.2 °C), and decane (174
°C) becomes higher as the molecules grows larger.
Table 2.5 Attractive Energies in Simple Covalent Compounds
Attractive energies
(kJ mol–^1 )
Molecule moment (D) Dipole Dipole-Dipole van der Waals Melting point (°C) Boiling point (°C)
H 2 O 1.85 36 a 88 0 100
NH 3 1.47 14 a 15 –78 –33
HCl 1.08 3 a 17 –115 –85
HBr 0.80 0.8 22 –88 –67
HI 0.42 0.03 28 –51 –35
a These dipole-dipole attractions are called hydrogen bonds.
- Fluorocarbons have extraordinarily low boiling points when compared to
hydrocarbons of the same molecular weight.
- 1,1,1,2,2,3,3,4,4,5,5,5-Dodecafluoropentane (C 5 F 12 , m.w. 288.03, bp 28.85 °C)
has a slightly lower bp than pentane (C 5 H 12 , m.w. 72.15, bp 36.07 °C). - Fluorine atom has very low polarizability resulting in very small van der Waals
forces. - Teflon has self-lubricating properties which are utilized in making “nonstick”
frying pans and lightweight bearings.