- BP is directly proportional to increasing MW-dispersion force (van der Waals force).
- Greater MW results in greater dispersion forces.
- Strength of dispersion force depends on how readily electrons can be polarized.
- Large molecules are easier to polarize than small, compact molecules. Hence, for
comparable MW, compact molecules have lower BP. - Polar compounds have slightly higher BP than nonpolar compounds of
comparable MW. - Hydrogen bonds are very strong intermolecular forces, causing very high BP.
Lowest BP: F 2
•F 2 is nonpolar; the only intermolecular attraction present is due to dispersion forces.
•F 2 has a MW of 38 g/mole.
•F 2 is covalently bonded.Intermediate BP: PH 3
•PH 3 is polar; geometry is trigonal pyramidal; presence of lone pair of electrons.
•PH 3 is primarily covalently bonded; two nonmetals.- There are dipole forces present between PH3 molecules because PH3 is polar.
•PH 3 has a MW of 34 g/mole (even though PH3 has a lower MW than F2 and
might be expected to have a lower BP, the effect of the polarity outweighs any
effect of MW).
Highest BP: H 2 O
•H 2 O is covalently bonded.
•H 2 O is a bent molecule; hence, it is polar.
•H 2 O has a MW of 18 g/mole.- Between H 2 O molecules there exist hydrogen bonds.
- Even though H 2 O has the lowest MW of all three compounds, the hydrogen bonds
outweigh any effects of MW or polarity.
Part IV: AP Chemistry Practice Test