Chem Class - April 20, 2010

Intermolecular Bonds
- bonds between molecules
- 3 types

1) London Dispersion Force (L.D.F)
- Results from temporary electron dipoles
- Weakest intermolecular force
- Increases as the $ e- increases
- Occurs in any compound that has e- (ie: everything)

2) Dipole-Dipole
- Results from a permanent dipole in molecules
Polar molecules experience this force
Polarity depends how much elements want e- (electronegativity)
- Electronegativity increases to the right and up
- The strength of a dipole- dipole  bond depends on the difference in electronegativity between the two atoms
- Only polar molecules experience this

Substance          Boiling Point              # of e-
N2                     -196 degrees C          14
O2                     -183 degrees C          16
NO                   - 152 degrees C          15
ICl                       97 degrees C            70
Br2                     59 degrees C             70

(The more electons, the higher the boiling point. The type of intermolecular bond also plays a role.)

3) Hydrogen Bonnds (H-bonds)
-This is a special type of dipole- dipole bond between H and O, F, or N
- Any molecule that: H-F, H-O or H-N

Identify the substances with H-Bonds:
1) CH4
2) CH3OH
3) H2S
4) CH3-NH2
5) HCl
6) CH2-OH-OH2
       /      /       /
    OH  OH  OH

Answer: Number 2, 4, and 6

Compare the boiling points of:
- Ethanol (C2H5OH)
- Ehtane (C2H6)
- Methanol (CH3OH)
- Methane  (CH4)

The actual boilingpoints: Ehtanol = 78 degrees Celcius, Ethane= -89 degrees Celcius, Methanol = 65 degrees Celcius and Methane = 161 degrees Celcius. Remember London Forces are the weakest intermolecular force and hydrogen bonds are the strongest. Also, the more electrons, the higher the boiling point.