2. Intramolecular vs. Intermolecular
• An intramolecular bond is the chemical bond within a
molecule
• An intermolecular force is a force that occurs between
molecules
4. 3 Types of Intermolecular Forces
1. London Dispersion Force
2. Dipole-Dipole Force
3. Hydrogen Bonds
5. London Dispersion Force
Attraction resulting
from a random
momentary non
symmetrical electron
distribution
You can also think of it as the negative
electrons from one molecule being
attracted to the positive nucleus of a
adjacent molecule
6. London Dispersion Force
• All molecules experience London Dispersion
Forces, but they have the greatest impact in
non-polar molecules
• London Dispersion Force is the weakest
intermolecular force
• The strength of the London force increases
with:
– Increasing size
– Increasing surface area of contact or proximity
7. Dipole-Dipole Force
• The dipole-dipole force is the electrostatic attraction
that is caused when the dipoles of polar molecules
position their positive and negative ends near each other
8. Dipole-Dipole Force
• Only polar molecules experience Dipole-Dipole
forces
• Dipole-Dipole forces are stronger than London
Dispersion forces (but are still only 1% as strong
as covalent or ionic bonds)
• The strength of the Dipole-Dipole force increases
with:
– Increasing polarity
– Decreased distance between molecules
9. Hydrogen Bonds
• A hydrogen bond is the strong dipole-dipole
interaction that occurs when a hydrogen atom
bonded to a small highly electronegative atom
(oxygen, nitrogen, or fluorine) is attracted to a
partially negative atom on a nearby molecule
10. Hydrogen Bonds
• Only molecules that have O, N, or F attached
to H experience hydrogen bonding
• Hydrogen bonds are the strongest type of
intermolecular force (but they are still 10-20
times weaker than covalent bonds)
11. Intermolecular Forces and
Physical Properties
• The stronger the intermolecular forces are, the more energy it will
require to pull the molecules apart
• Molecules that have strong intermolecular forces also have high
melting points and boiling points
• Intermolecular forces can also affect solubility
15. Intermolecular Forces and
Physical Properties
trimethylamine, b.p. 3.5°C
N CH3H3C
CH3
propylamine, b.p. 49°C
CH3CH2CH2 N
H
H
ethylmethylamine, b.p. 37°C
N CH3CH3CH2
H
explain the
trend
explain the
trend
16. Physical Properties of Liquids
• Surface Tension
is the resistance
of a liquid to
increase its
surface area
• The stronger the
intermolecular
forces, the
greater the
surface tensoin
18. Physical Properties of Liquids
• Capillary action is the spontaneous
rising of a liquid in a narrow tube
due to adhesive and cohesive forces
Glass (SiO4)
19. Physical Properties of Liquids
• Viscosity is the measure of a liquid’s resistance to flow
• Liquids with strong intermolecular forces tend to be
more viscous
• Ex: Vegetable oil is more viscous than water