Intermolecular Forces
Chapter 4.7
Intramolecular vs. Intermolecular
• An intramolecular bond is the chemical bond within a
molecule
• An intermolecular force is a force that occurs between
molecules
States of Matter
3 Types of Intermolecular Forces
1. London Dispersion Force
2. Dipole-Dipole Force
3. Hydrogen Bonds
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
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
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
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
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
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)
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
Intermolecular Forces and
Physical Properties
Intermolecular Forces and
Physical Properties
explain
the trend
explain
the trend
Intermolecular Forces and
Physical Properties
explain the
trend
explain the
trend
Intermolecular Forces and
Physical Properties
explain the
trend
H3C N CH3
CH3
trimethylamine, b.p. 3.5°C
explain the
trend
CH3CH2
N CH3
CH3CH2CH2
N H
H
ethylmethylamine, b.p. 37°C propylamine, b.p. 49°C
H
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
OBSERVATION
EXPLANATION
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)
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
HOMEWORK
Required Reading:
p. 239-247
(remember to supplement your notes!)
Questions:
p. 244 #1-2
p. 247 #1-8