Intermolecular Forces

1
Intermolecular Forces
2
Intermolecular forces are attractive forces
between molecules.
Intramolecular forces hold atoms together in a
molecule.

• Intermolecular vs Intramolecular
• 41 kJ to vaporize 1 mole of water (inter)
• 930 kJ to break all O-H bonds in 1 mole of water
  (intra)

Measure of intermolecular force boiling
point melting point DHvap DHfus DHsub
11.2
3
Intermolecular Forces

• 1. London Forces (Dispersion Forces)
• Dipole-Dipole Interactions
• 3. Ion-Dipole Interactions (Salt dissolving in
  solution)
• 4. Hydrogen Bonding

4
Dispersion Forces
Occur between every compound and arise from the
net attractive forces amount molecules which is
produced from induced charge imbalances
Figure 10-8 Olmsted Williams
The larger the molecule the greater its
Dispersion Forces are.
Figure 10-9 Olmsted Williams
5
The boiling point of long molecules increase with
the length of the carbon chain.
Olmsted Williams Fig 10-10
Pg 437
6
How molecular shape affects the strength of the
dispersion forces The shapes of the molecules
also matter. Long thin molecules can develop
bigger temporary dipoles due to electron movement
than short fat ones containing the same numbers
of electrons.                                
                                                  
                                                  
                          Butane has a higher
boiling point because the dispersion forces are
greater.
http//www.chemguide.co.uk/atoms/bonding/vdw.html
7
Polarizability
the ease with which the electron distribution in
the atom or molecule can be distorted.

• Polarizability increases with
• greater number of electrons
• more diffuse electron cloud

11.2
8
Is the Molecule Polar?
The more Electronegative atom will pull the
electron density of the bond Closer to itself
giving it a partial negative charge leaving the
other Atom with a partially positive charge. This
is a dipole moment.
9
Molecules with 3 Atoms
Even though the C-O bond is polar, the bonds
cancel each other out because the molecule is
linear the dipole moments are equal and in
opposite directions.Therefore CO2 is non-polar.
CO2
The dipole moment between H-C points in the
direction of C. The dipole moment points between
C-N points in the direction of the N. Therefore
the dipole vectors are additive and HCN is polar
HCN
SO2
SO2 is a polar molecule because the S-O dipole
Moments dont cancel each other out due to the
angle
10
Molecules with 4 Atoms
CCl4 is non-polar
CHCl3 is polar
11
How to Determine if a Molecule Is Polar

• Draw Lewis Structure
• If all of the regions of electron density are
• bound to the same thing (CCl4 CO2 ) than the
• molecule is non-polar
• If the regions of electron density are not bound
  to
• the same thing than the molecule is polar (HCN
  SO2)

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dipole moment polar molecule
dipole moment polar molecule
no dipole moment nonpolar molecule
no dipole moment nonpolar molecule
10.2
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Dipole-Dipole Forces
Attractive forces between polar molecules
11.2
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Dipole Forces occur between molecules containing
a dipole moment. The positive end of the dipole
moment on one mole is attracted to the Negative
end of the dipole moment on a nearby molecule.
2-methyl propane (left) and acetone (right) Both
compounds are about Equal in size and shape
Figure 10-11
Olmsted Williams
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Ion-Dipole Forces
Attractive forces between an ion and a polar
molecule
The larger the charge the stronger the force
11.2
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Fig 10-34
Olmsted Williams
A molecular picture showing the ion-dipole
Interaction that helps a solid ionic crystal
dissolve in water. The arrows indicate
ion-dipole interactions.
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What type(s) of intermolecular forces exist
between each of the following molecules?
HBr
HBr is a polar molecule dipole-dipole forces.
There are also dispersion forces between HBr
molecules.
CH4
CH4 is nonpolar dispersion forces.
SO2
SO2 is a polar molecule dipole-dipole forces.
There are also dispersion forces between SO2
molecules.
11.2
18
The Hydrogen Bond
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The melting point, boiling point, heat of fusion
and heat of vaporization of water are extremely
high and do not fit the trend of properties
relative to molar mass within Group VIA.
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Water exhibits these unusual properties because
of hydrogen bonding between water molecules.
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• A hydrogen bond is an intermolecular bond.

• A hydrogen bond is formed between polar molecules
  that contain hydrogen covalently bonded to a
  small, highly electronegative atom F, O, N.

FH
OH
NH
22

• it will be attracted to another F, O, or N, on
  another molecule.

• A dipole-dipole bond bond will be formed between
  the two molecules which is called a hydrogen bond.

hydrogen bond
covalent bond
covalent bond
23
Water in the liquid and solid states exists as
groups in which the water molecules are linked
together by hydrogen bonds.
13.8