between
 Intra-
› strong forces that hold the atoms in a
molecule together
 e.g. – it takes 464 kJ/mol to break the H-O
bonds within a water molecule
› responsible for chemical properties
 burning, toxic, reactivity…
 Inter-
› weak forces that holds molecules to one
another
 e.g. – it takes only 19 kJ/mol to break the
bonds between water molecules
› the strength of the intermolecular forces
determines the physical properties of the
substance
 melting, boiling, solubility, conductivity,
volatility
3
main types of intermolecular
forces
1. London forces (also called
dispersion forces, instantaneous
induced dipole forces, or Van
strength
der Waals forces)
increases
2. dipole-dipole forces (polar
molecules)
3. a stronger type of dipole-dipole
bonding called hydrogen
bonding
London Forces YouTube (:20)
 https://www.youtube.com/watch?v=S8QsL
UO_tgQ
 occurs in all molecules, even diatomic
molecules (HOFBrINCl)
 the distribution of electrons around an
individual atom, at a given instant in time,
may not be perfectly symmetrical

› this can produce temporary/instantaneous
dipole (polar molecule)
› this can then induce a nearby molecule to
be polar and therefore a very weak
attraction between the two molecules and
so on…
› magnitude of the force depends on…
1. number of electrons and size of the
electron cloud
 increases with more electrons, valence
electrons are farther away from the
nucleus and can be polarized more easily
2. shape of molecules
 molecules with shapes that have more
contact area have greater forces
between them than those that don’t
these round
shapes do
NOT allow
them to stick
to one
another
this flat
shape allows
it to stick to
one another
better
boiling point increases
attractive forces between the positive end
of one polar molecule and the negative
end of another polar molecule
 the more polar the molecule, the greater
the dipole-dipole force
 stronger than London forces

YouTube Hydrogen Bonding (1:40)
 YouTube Hydrogen Bonding Video (:58)
 a specific type of dipole-dipole type
interactions
 stronger than other dipole-dipole and
London forces
 the hydrogen (H) in a molecule is
intermolecularly bonded to a small,
highly electronegative element (usually
an N, O or F atom) on another molecule

H-NOF

the greater polarity of a molecule, the
higher the boiling point
› In HF, H is 2.1 and F is 4.0, difference of 1.9
› In HCl, H is 2.1 and Cl is 3.0, difference of 0.9
BP is 20°C
BP is -85°C
H-NOF ?
› H2O vs. H2S?
 In H2O, H is 2.1 and O is 3.5, difference of 1.4
 water molecules can hydrogen bond (and
London forces) to each other
 BP is 100°C
 In H2S, H is 2.1 and S is 2.5, only a difference
of 0.4
 H2S can dipole-dipole bond (and London forces)
to each other
H-NOF ?
 BP is -60°C
no
H-NOF ?
yes


CH3OCH3 vs. CH3CH2OH ?
-24°C
dipole – dipole


78°C
has hydrogen bonding
NH3 vs. PH3 ?
-33°C
-87°C
has hydrogen bonding
dipole – dipole
H-NOF ?

CH3CH2 CH3 vs. CH3CHO vs. CH3CH2OH ?
only London forces
(VDW) low BP
VDW & dipole-dipole
medium BP
VDW, D-D, & hydrogen
bonding
highest BP