TOPIC: Intermolecular Forces
Part 1: Dispersion Forces
Do Now: How do particle
diagrams of liquids & solids
compare to those of gases?
SOLID
LIQUID
GAS
Why do some substances exist
as gases, some as liquids, and
some as solids at room temp?
 Part
of answer has to do with
forces between separate
molecules (called intermolecular
forces)
Intermolecular forces between molecules.
They are weaker. Intramolecular forces are
between individual atoms (we will learn this later)
Intermolecular
forces
Intramolecular
forces
Intermolecular Forces=IMF
means “between” or “among”
 Intermolecular forces = forces
between neighbouring compounds
 Inter
***Separation of charge is
responsible for the forces between
the Molecules***
…
Most atoms don’t have a charge, unless they are
ions, so we often refer to them as having partial
charges and write them like this
1. Dispersion Forces (van der waals):
●
weakest IMF
●
occur between nonpolar (symmetrical) molecules
•
•
•
Nonpolar = no poles (no + or -)
Can’t tell one end of molecule
from other end
electrons are evenly distributed
Click here for animation (slide 4 of 13)
●
•instantaneous and momentary
•fluctuate
•results from motion of electrons
if charge cloud not symmetrical will induce asymmetry in neighbor’s
charge cloud!
4 categories of Nonpolar Molecules
- all these have DISPERSION FORCES
(you need to memorize)
 Noble
 He,
7
Ne, Ar, Kr, Xe, Rn
diatomic elements
 H2,

Gas –group 18:
N2, O2, Cl2, F2, I2, Br2
Pure Hydrocarbons – molecules with only C and H
 General

formula CxHy : examples = CH4, C2H6, C3H8
these 3 small symmetrical molecule
 CO2,
CF4, CCl4
Dispersion Forces and Size
 The
larger the molecule, the great
the Dispersion forces = stronger
IMF
B/c, the larger the electron cloud, the
greater the fluctuations in charge
 Noble Gases: Rn has greater
dispersion forces = strongest IMF
 Diatomic Elements: I2 is larger then F2,
so I2 is larger (way more electrons) so
greater dispersion forces, I2 is a solid at
room temp. F2 is much smaller (less
electrons) weaker dispersion forces, F2
is a gas at room temp.
You try…
 Which
has the greatest dispersion forces
between it’s molecules?
C3H8
C8H18
CH4
C5H12
 Which
is most likely a liquid/solid (not a gas) at
room temp?
C3H8
 Which
C3H8
C8H18
CH4
C5H12
is most likely a gas at room temp?
C8H18
CH4
C5H12
The weaker the IMF, the lower the boiling
point (BP)
Br2 = boils at 58.8°C, 137.8°F
Compared to
Water = boils at 100°C, 212°F
So water must have stronger IMF
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