Chapter 3.5
Pages 54 - 59
Learning outcomes
 State the three types of intermolecular force.
 State what is needed for hydrogen bonding to occur.
 Describe how van der Waals and dipole-dipole
forces arise.
 Describe how van der Waals forces affect
temperature.
 Explain why NH3, H2O and HF have higher boiling
temperatures than might be expected.
Specification ref: 3.1.3
Practical
 Complete the worksheet to investigate the properties of
the various substances
Intermolecular forces
 Molecules and separate atoms are attracted to each
other by weaker intermolecular forces. (inter means
between).
 If intermolecular forces are strong enough, the molecules
can be held closely enough together and can form liquids
(e.g. bromine) or solids (e.g. iodine) at room
temperature
Types of intermolecular
force
 There are three types of intermolecular force:
 van der Waals forces
 Acts between all atoms and molecules
 Dipole – dipole forces
 Acts only between certain types of molecules
 Hydrogen bonding
 Acts only between certain types of molecule
Weakest
Strongest
Dipole – dipole forces
 Polar molecules = permanent dipoles
 δ+ of one molecule attracts the δ– of a different polar
molecule to form a permanent dipole-dipole force.
 For example HCl
Permanent dipole–dipole
forces
If molecules contain bonds with a permanent dipole, the molecules may align so there
is electrostatic attraction between the opposite charges on neighbouring
molecules.
Permanent dipole–
dipole forces (dotted
lines) occur in hydrogen
chloride (HCl) gas.
The permanent dipole–dipole forces are approximately one hundredth the
strength of a covalent bond.
 Complete the starter
activity
Check your answers!
 In molecules with more than one polar bond, the effects
of each bond may cancel out, leaving a molecule that is
non-polar (with no dipole moment).
 E.g. carbon dioxide is a linear molecule and the dipoles
cancel.
 Both are equal and acting in the opposite direction
 Tetrachloromethane is tetrahedral and the dipoles
cancel.
 Dichloromethane, the dipoles do not cancel because of
the shape of the molecule
Identifying polar molecules
van der Waals’ forces
 Or... Induced dipole-dipole interactions...
 Exists between all molecules whether polar or non-polar
 Weak intermolecular attractions between very small,
temporary dipoles in neighbouring molecules
Van der Waals forces
Strength of van der Waals
forces
200
The strength of van
der Waals forces
increases as
molecular size
increases.
This is illustrated
by the boiling
points of group 7
elements.
boiling point (°C)
150
100
50
0
-50
-100
-150
-200
F2
Cl2
Br2
I2
element
Atomic radius increases down the group, so the outer electrons
become further from the nucleus. They are attracted less strongly
by the nucleus and so temporary dipoles are easier to induce.
Strength of van der Waals
forces
The points of contact between molecules also affects the strength of van der Waals
forces.
butane (C4H10)
boiling point = 272 K
2-methylpropane (C4H10)
boiling point = 261 K
Straight chain alkanes can pack closer together than branched alkanes, creating more
points of contact between molecules. This results in stronger van der Waals forces.
Boiling points of alkanes
van der Waals
 van der Waals forces act between all atoms or molecules
at all times
 They are in addition to any other intermolecular forces
 The dipole is caused by the changing position of the
electron cloud, so the more electrons there are, the larger
the instantaneous dipole.
 The size of the van der Waals force increases with the
number of electrons present.
Boiling points
 Can you explain why the boiling points of the noble
gases increases?
Noble gas
Boiling point/˚C
Number of
electrons
He
-269
2
Ne
-246
10
Ar
-186
18
Kr
-153
36
Xe
-108
54
Rn
-62
86
Why does ice form such
ccol shapes?
 https://www.youtube.com/watch?v=UukRgqzk-KE
Hydrogen bonding
 https://www.youtube.com/watch?v=fWaSutDUpl4
What is hydrogen bonding?
When hydrogen bonds to nitrogen, oxygen or fluorine, a larger dipole occurs than
in other polar bonds.
This is because these atoms are highly
electronegative due to their high nuclear charge
and small size. When these atoms bond to hydrogen,
electrons are withdrawn from the H atom, making it
slightly positive.
The H atom is very small so the positive charge is more concentrated, making it easier to
link with other molecules.
Hydrogen bonds are therefore particularly strong examples of permanent dipole–
dipole forces.
Hydrogen bonding
In molecules with OH or
NH groups, a lone pair of
electrons on nitrogen or
oxygen is attracted to the
slight positive charge on
the hydrogen on a
neighbouring molecule.
hydrogen
bond
lone pair
Hydrogen bonding makes the melting and boiling points of
water higher than might be expected. It also means that
alcohols have much higher boiling points than alkanes of a
similar size.
Hydrogen bonding and
boiling points
The boiling point of
hydrogen fluoride is much
higher than that of other
hydrogen halides, due to
fluorine’s high
electronegativity.
boiling point (°C)
Boiling points of the hydrogen
halides
40
20
0
-20
-40
-60
-80 HF
HCl
HBr
HI
-100
HF
HCl
HBr
The means that hydrogen bonding between molecules of hydrogen fluoride is
much stronger than the permanent dipole–dipole forces between molecules of
other hydrogen halides. More energy is therefore required to separate the
molecules of hydrogen fluoride.
HI
Permanent dipole–dipole
forces
Intermolecular Snap
In pairs, take a pack of 20 cards.
Shuffle these and deal out into two piles face down.
Turn over a card each at the same time.
It is snap if consecutive molecules have the same type of
intermolecular attraction. Say which type of intermolecular
attraction is present between each molecule.
When snap is called, take it in turns to explain how it arises
for the actual molecules on each card.
e.g. water and ammonia snap since both form hydrogen
bonds
Water and methane don’t snap because methane cannot
hydrogen bond.
After ten goes reshuffle the cards and repeat.