Intermolecular Forces
Questions
• Why do some solids dissolve in water
but others do not?
• Why are some substances gases at
room temperature, but others are liquid
or solid?
• What gives metals the ability to conduct
electricity, what makes non-metals
brittle?
• The answers have to do with …
Intermolecular forces
Intermolecular Forces(IMFs)
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)
•
Generally, intermolecular forces are much weaker than
intramolecular forces.
Intramolecular force
Intermolecular force
• Ionic
• Ion-dipole
• covalent (network solids)
• dipole - dipole
• metallic
• H-bonding
• London dispersion forces
Properties of Liquids
Surface tension is the amount of energy required to stretch
or increase the surface of a liquid by a unit area.
Strong
intermolecular
forces
High
surface
tension
Capillary Action:
Spontaneous rising of a
liquid in a narrow tube.
Properties of Liquids
Cohesion is the intermolecular attraction between like molecules
Adhesion is an attraction between unlike molecules
Adhesion
attracted to glass
Cohesion
attracted to each other
Properties of Water
Surface Tension is the amount of energy required to
stretch or increase the surface of a liquid by a unit
area.
The stronger the IMFs the molecules have with
each other, the higher is its surface tension.
This is because of the cohesive forces inside the
liquid (Polar-Polar forces) become much greater than
the adhesive forces between the liquid (polar) and the
air (non-polar) molecules outside.
Properties of Liquids
Viscosity is a measure of a fluid’s resistance to flow.
Strong
intermolecular
forces
High
viscosity
Bond Polarity
• Nonpolar Cl2
(Electronegativity
difference is zero)
• Polar HCl
(Electronegativity difference
is not zero)
• Ionic NaCl
Types of Intermolecular Forces
1. Ion-Dipole Forces
Attractive forces between an ion and a polar molecule
Ion-Dipole Interaction
Types of Intermolecular Forces
2. Dipole-Dipole Forces
Attractive forces between polar molecules
Orientation of Polar Molecules in a Solid
Types of Intermolecular Forces
3. Hydrogen Bond (strongest)
The hydrogen bond is a special dipole-dipole interaction
between the hydrogen atom in a polar N-H, O-H, or F-H bond
and an electronegative O, N, or F atom. IT IS NOT A BOND.
A
H…B
or
A
A & B are N, O, or F
H…A
Hydrogen Bond
Why is the hydrogen bond considered a
“special” dipole-dipole interaction?
Decreasing molar mass
Decreasing boiling point
Types of Intermolecular Forces
4. Dispersion Forces – van der Walls forces/London forces
(weakest)
Attractive forces that arise as a result of temporary
dipoles induced in atoms or molecules
Non-polar molecules do not have dipoles
like polar molecules.So London dispersion
force exists in non polar molecule.
ion-induced dipole interaction
dipole-induced dipole interaction
Dispersion forces usually increase with molar
mass.
Intermolecular Forces
4. Dispersion Forces Continued
Polarizability is 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
Dispersion
forces usually
increase with
molar mass.
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.
S
SO2
SO2 is a polar molecule: dipole-dipole forces. There are
also dispersion forces between SO2 molecules.
Types of IMF
• The hydrogen bonds in water explain its relatively high
boiling point, considering that it is a small molecule. The Hbonds hold the water molecules together as a liquid, so you
have to heat it a lot before it will change to a gas. Compare
boiling points of these molecules:
Molecule
IMF (s) present
Molar Mass
(g/mol)
Boiling Point
(oC)
CH4
London Disp.
16.05
- 164
HCl
London Disp.
36.46
- 85
18.02
100
H2O
Dipole-Dipole
London Disp./DipoleDipole/Hydrogen
Bonding
Water, H20
Which is related to…
And Glycerine (Glycerol)
Compare the three structures
Which of these molecules has the
greatest polarity?
Which will exhibit the strongest
intermolecular forces?
Ethanol. Polar or Non-Polar?
Methanol. Polar or Non-Polar?
Methane. Polar or Non-Polar?
Chloro-Methane. Polar or NonPolar?
Sulfur Hexafluoride. Polar or NonPolar?
Nitrogen Trihidride, aka Ammonia.
Polar or Non-Polar?