1. Unit 2.3 - Intermolecular forces
Intermolecular forces
• Inter molecular forces are the forces acting
between molecules
• They are much weaker than the bonds within
molecules
• They do, however, significantly affect the physical
properties of substances (melting point etc…)
• We will be looking at three different types of
intermolecular force
1. Dipole- dipole interactions
• Polar molecules like _____ have a permanent
dipole
• In other words: they have ____________ end and
a ______________ end
• Opposite charges ______________ so polar
molecules are drawn together by so-called
permanent dipole- permanent dipole
interactions
• These forces are about 100 times weaker than
covalent bonds
• They do, however, have a big affect on physical properties:
Species Boiling point / °C
Hydrogen chloride (HCl) -85 Dipole-dipole forces
Polar Hydrogen sulphide (H2S) -61 mean that polar
Fluorine (F2) -188 molecules have
Non-Polar Argon (Ar) -186 relatively ________
melting and boiling
temperatures
Use the space below to sketch a quick graph of the data in the above table:
2. London forces
• Permanent dipole-permanent dipole forces only exist
between __________ molecules
• There are, however, forces between non- polar
molecules
• We know that these forces exist because we can liquefy
Fritz London enjoying a cocktail

2. non-polar substances like nitrogen
• These forces are known as London forces
• They are relatively weak
Instantaneous dipole Induced dipole London Forces
• The electron density • The electric field of the • There is attraction between the
in an atom can instantaneous dipole instantaneous and induced
fluctuate effects the electron dipoles
• This can lead to density in nearby atoms • The attraction is weak and
charge separation • This causes induced fluctuates , it is sometimes zero
• This happens very dipoles
briefly and fluctuates
Strength of London forces
Noble gas Boiling temp /
London forces are K • Bigger atoms hold onto their outer
the only Helium 4
Neon 27 electrons _________ strongly
_____________ Argon 87 • Induced dipoles occur more easily in
forces that act Krypton 121
Xenon 166 __________ atoms or molecules
between noble gas Radon 211 • The ___________ the atom or molecule
(group ___) atoms
the stronger the London forces
3. Hydrogen bonds
The third and final type of intermolecular force we will look at is hydrogen bonding, which is present
in many substances including water :

3. A water molecule Hydrogen bonding in water
• Water molecules have a ___________ • The hydrogens have a partially
shape _______________ charge
• Oxygen is __________ electronegative than • The negatively charged lone-pair electrons
hydrogen so it pulls electrons away from of _______________ are attracted to the
it hydrogens
• Oxygen has _____ lone pairs of electrons, • This attraction is a hydrogen bond
which are not involved in covalent bonding
Hydrogen bonding doesn’t just happen in water!
Hydrogen bonding significantly affects physical properties, we will cover this shortly, but first we will
look at what types of compound hydrogen bonding happens in:
Molecules that
Conditions needed for H can form H
bonding… bonds…
1.
2.
3. Relative
strength of
intermolecular
forces:
H bonds
Dipole-dipole
London forces
Trends in physical properties
• The physical properties of a substance ( _____________ and
______________ temperature) are decided by the strength of the
________________ forces present in the substance
• Strong intermolecular forces mean…
• Weak intermolecular forces mean…

4. Melting and boiling temperatures of alkanes
• The table above clearly shows that as
Meltin Boilin
Molecular g g State at
Formula Point Point 25oC
Name (oC) (oC)
methane CH4 -183 -164 gas
ethane C2H6 -183 -89
propane C3H8 -190 -42
butane C4H10 -138 -0.5
pentane C5H12 -130 36
hexane C6H14 -95 69
heptane C7H16 -91 98
octane C8H18 -57 125
nonane C9H20 -51 151 liquid
decane C10H22 -30 174
undecane C11H24 -25 196
dodecane C12H26 -10 216 the length of the hydrocarbon chain
eicosane C20H42 37 343
increases, melting and boiling
triacontane C30H62 66 450 solid
temperatures __________________
• This is because the intermolecular forces are getting ______________
• The intermolecular forces between alkanes are _______________ forces
Sketch an alkane melting/boiling temp
graph:
The longer the chain, the _____________
the ______________ forces
Effect of branching
Displayed formula
Boiling temperature / °C 36 28 10
• Boiling temperature of alkanes _______________ with branching
• They become __________ volatile

5. • This is because...
Alkanes vs alcohols
Propane Propanol • Alcohols have much ___________ boiling
Boils at -42 °C Boils at 97 °C temperatures than alkanes because they
contain oxygen and can form
_____________ bonds, whereas alkanes
only have weaker ____________ forces
Some research suggests that geckos’ feet
have complex surface structures that allow
them to use London forces to stick to
smooth surfaces and even walk on ceilings!
Boiling temperatures of the hydrogen halides
We saw earlier that hydrogen bonds can form between molecules that contain _____________
bonded to a more __________________ atom, which draws electron density away from hydrogen
leaving it with a partial ______________ charge that attracts a lone pair of electrons from another
molecule:
Hydrogen bonding in water H bonding in hydrogen H bonding in ammonia
fluoride (HF) (NH 3 )
The hydrogen bonding in hydrogen fluoride has a big effect on its physical properties :
Halide HF HCl HBr HI
Boiling temperature / 19.5 -85.1 -66.4 -34.4
°C
Sketch a graph of the above table…
Electronegativities
Fluorine 4.0
Chlorine 3.0
Bromine 2.8
Iodine 2.5

6. • Fluorine is _________ electronegative than the other halogens
• Because of its high electronegativity fluorine can strongly draw ____________ density away from
hydrogen making it partially _______________
• This means that ______________ bonds can form between HF molecules, which makes the
boiling temperature of hydrogen fluoride unexpectedly _____________
Solubility
Intermolecular forces can also exist between different types of molecule, therefore they have big part
in the dissolving of solids and the mixing of liquids…
Key words
Solubility - The mass of a solute that will dissolve in 100 g of a solvent at a particular temperature
Non-aqueous solvent - A solvent that is not water e.g. hexane or paraffin
Miscible - Liquids that mix completely are miscible
Immiscible - Liquids that do not mix at all and form separate layers are immiscible
Dissolving ionic solids in water
• Ionic solids like _____ and _____ can be soluble in water
• Ionic solids are made up of positive and negative ______, which are held together by strong
_________________ forces of attraction
• The energy required to break down an ionic lattice is known as the _____________ energy
• When ionic solids dissolve the energy to break the lattice up comes from new forces of
attraction between the ions and water molecules:
Hydration - Energy is released
Attractive forces between water Water surrounds Na+ ions Water surrounds Cl- ions

7. and ions break down the lattice
• The process of water molecules surrounding ions is called hydration
• The energy released when this happens is the hydration enthalpy
• If hydration enthalpy is greater than lattice energy a substance will
dissolve in water, if it is not then it will not:
Substance NaCl LiF
Lattice energy / kJmol -1 -770 -1031
Hydration enthalpy / +770 +1025
kJmol -1
Soluble/insoluble in water Soluble Insoluble
Solubility of alcohols in water
• Alcohols and carboxylic acids are soluble in water
• This is because they have polar OH groups, which
can form _____________ bonds with water
molecules
• They become less soluble as chain length
increases
Why are non- polar substances
insoluble in water?
• Oil and water do not mix, they are
___________________
• Two separate layers form with the Weak
_________ on top forces
between oil
• The same happens with other non- molecules
polar organic liquids like
____________ or paraffin Strong
forces
• This is because the ____________ between
water
bonds between water molecules are so molecules
much stronger than the ____________
forces between oil molecules
• The oil molecules cannot disrupt the
structure of the water, they cannot squeeze
between water molecules so the liquids are
__________________
Mixing two organic liquids
• Simple organic liquids like hexane or octane
are ____ - ____________

8. • The only forces between molecules are the weak _____________ forces
• When two non-polar organic liquids are mixed these forces extend throughout the mixture
• Non-polar liquids like hexane and octane are ________________