Intermolecular Forces
How are molecules held together?

There are two types of attraction in molecules:
◦ Intramolecular forces
◦ Intermolecular forces

Intermolecular Forces – the forces that
bond molecules to each other

Intramolecular Forces – the forces that
bond the atoms to each other within a
molecule

Until now, we have been discussing
intramolecular forces (ionic, polar, non-polar
bonds)
How are molecules held together?

Intermolecular forces are weak relative to
covalent bonds

Intramolecular forces are strong covalent
bonds

We can conclude that it does not take very
much energy to break the bonds that hold
molecules to each other (intermolecular
forces)

These forces are also referred to as van
der Waals forces after the physicist who
studied them

We will be discussing 4 types of
intermolecular forces
◦
◦
◦
◦
Dipole-Dipole Forces
Ion-Dipole Forces
Dispersion (London) Forces
Hydrogen Bonding
Dipole-Dipole Forces
Dipole is a polar
molecule
 These molecules will
orient themselves so that
oppositely charged ends
of molecules are near to
one another
 The electrostatic
attractions between
these ends are called
dipole-dipole forces

+
+
–
H
Cl
–
+
–
Lets see it!
Result of Dipole-Dipole Forces

Polar molecules will tend to attract one
another more than similarly sized non-polar
molecules

Therefore the energy required to separate
polar molecules from one another is
greater than that needed to separate nonpolar molecules
Ion-Dipole Forces

The force of attraction between an ion and
a polar molecule (a dipole)

Example: NaCl breaks up because the ion
dipole with water is stronger than the
attraction of Na+ to Cl-
Let’s See it!
Dispersion (London) Forces

Weakest intermolecular force that act
between non-polar molecules

Shared pairs of e- can temporarily occupy
positions that create momentary, uneven
distributions of charge

This causes non-polar molecules to
become slightly polar for just an instant

At this instant it is
capable of inducing a
dipole in a nearby
molecule resulting in
an intermolecular
force of attraction

This is called a
dispersion force
Hydrogen Bonding

An attraction between the hydrogen atom,
bonded to a highly electronegative atom,
and the negative end of a dipole nearby
Like Dissolves Like

Ionic solutes dissolve in polar solvents
◦ (ex: NaCl and H2O)

Non polar solutes dissolve in non polar
solvents
◦ (ex: solid I2 and liquid Br2)
Practice

Sheet on polar molecules is for homework!

On this sheet please add this question:
Based on what we know about
intermolecular forces predict the
solubility of this molecule in a) water
and b) acetone