TOPIC: Intermolecular Forces
Part 2: Dipole- Dipole and Hydrogen Bonding
Do Now: List the 4 categories of Nonpolar Molecules –
all of these have DISPERSION FORCES

Noble Gas –group 18:


7 diatomic elements


H2, N2, O2, Cl2, F2, I2, Br2
Pure Hydrocarbons – molecules with only C and H


He, Ne, Ar, Kr, Xe, Rn
General formula CxHy : examples = CH4, C2H6, C3H8
these 3 small symmetrical molecule

CO2, CF4, CCl4
All molecules have Dispersion
forces (the regents calls
these Van der Waals)
2 other types of forces (IMF):
1. Dipole-Dipole forces
2. Hydrogen bonds
-if one of these are present,
they are more important.
2.
•
•
•
Dipole-dipole forces:
Stronger then dispersion forces
occur between polar (asymmetrical)
molecules (they have a partial charge at
each pole – one is typically much larger
than the other)
Click here for animation (slide 3 of 13)
Dipole-dipole Forces & Polar Molecules
Polar Molecule shows
permanent separation
of charge; has poles:
one end partially (-) &
one end partially (+);
Asymmetrical
3.
•
•
Hydrogen bonds:
strongest IMF
occur between molecules that have an :
H-F H-O or H-N bonds ONLY
Strongest
Intermolecular Force
Hydrogen Bonding
Dipole-Dipole
Dispersion
Hydrogen Bonding
H-O
N-H
Occurs between molecules with H-F, H-O, or H-N
bonds
Hydrogen Bonding
 Hydrogen
bonding is extreme case of
dipole-dipole bonding
 F, O, and N are all small and electronegative
 strong
electrons attraction
 H has only 1 electron, so if being pulled away H
proton is almost “naked”
H
end is always positive &
F, O, or N end is always negative
Strength of Hydrogen Bonding
 Fluorine
most electronegative element, so
 H-F
bonds are most polar and exhibit
strongest hydrogen bonding, so strongest IMF

H-F is stronger than H-O which is stronger than H-N
(H-bonding…sound like FON to me!!!)
H
H
O H
O H
H-Bonding = strongest IMF
much harder to “pull” molecules apart
H
H
C
H
H
H
H
C
H
H
Dispersion Forces= weakest IMF
much easier to “pull” molecules apart
Hydrogen bonding:
• strongest IMF
• influences physical props a great deal
H-F > H-O > H-N
IMF vs Physical Properties
 If
IMF  then:
point 
 Melting point 
 Heat of Fusion  Change from solid to liquid w/o changing temp
 Heat of Vaporization  Change from liquid to gas w/o changing temp
 Boiling
while:
 Evaporation
Rate 
Rate at which conc. will go from liquid to gas
Why do some substances exist
as gases, some as liquids, and
some as solids at room temp?
#1 reason = IMF
 If
IMF are strong,
substance will be solid
or liquid at room temp
Particles want to
clump together
 If
IMF are weak,
substance will be gas at
room temp
Particles free to
spread apart
Why do some substances exist
as gases, some as liquids, and
some as solids at room temp?
#1 reason = IMF
#2 reason =
temperature (avg. KE)
 Temp
= average KE
 If we change T we change KE
 Increase KE will help “pull”
molecules apart (overcome IMF)
Indicate type of IMF for each molecule:
 NH3
•
 Ar
•
 N2
•
 HCl
•
 HF
•
 Ne
•
 O2
•
 HBr
•
 CH3NH2
•
Hydrogen bonding
Dispersion forces
Dispersion forces
Dipole-dipole forces
Hydrogen bonding
Dispersion
Dispersion
Dipole-dipole
Hydrogen bonding