Cliffs AP Chemistry, 3rd Edition

(singke) #1

  • 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.


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