April 20, 2010 Class

Intermolecular Bonds
-Bonds between molecules
-3 types



1. London Dispersion Force (L.D.F)

• Results from temporary electron dipoles (2 poles)
• Weakest intermolecular force
• Increases as # e- incrase
• occurs in any compound that has e- (ie: everything)
  

2. Dipole-Dipole Force (D.D.F)

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

F-O -> lower boiling point because less electronegativity, strength of 0.5, weaker bonds
F-N -> higher boiling point because more elecronegativity, strength 1.0, stronger bonds


3. Hydrogen Bonds (H-Bonds)

• This is a special type of dipole-dipole bond between H and O, F, or N
• Any molecule that has stronger bond with larger electronegativity difference

H-F
H-O-
HN=

Identify the substances with H-Bonds:
1. CH4 -No
2. CH30H -Yes
3. H2S -No
4. CH3-NH2 -Yes (2)
5. HCl -No
6. Yes (3)

Example: Compare the boiling Points of:
4.Ethanol(C2H5OH), 26 e- (strongest) Actual = 78
2. Ethane (C2H6), 18 e- Actual = -89
3. Methanol (CH3OH), 18 e- Actual= 65
1. Methane (CH4), 10 e- (weakest) Actual = -161