Intermolecular forces
“attraction between one molecule and
a neighbouring molecule”
Intermolecular forces

Van Der Waal’s
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London forces
Dipole-dipole
Dipole- induced polarity
Hydrogen bonding
London forces

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Between non-polar molecules.
Instantaneous polarity caused by the random.
movement of electrons within a non-polar molecule
(=temporary dipole).
Instantaneous polarity induces polarity in
neighbouring molecules.
London forces is the attraction as a result of the
polarity in neighbouring molecules.
Weakest of the intermolecular forces and only acts
over a short distance.
London forces
http://www.chemguide.co.uk/atoms/bonding/vdw.html
.
Strength depends on
Number of electrons/molecular size
Inert gas
Boiling
point (C)
Halogen
Boiling
point (C)
He
-269
F
-188
Ne
-246
Cl
-34.4
Ar
-186
Br
58.8
Kr
-152
I
184
Xe
-108
At
337
Rn
-62
Also depends on
http://www.chemguide.co.uk/atoms/bonding/vdw.html
Molecular shape
Dipole-dipole
http://www.teachmetuition.co.uk/Chemistry/Intermolecular/dipole.htm
.
Intermolecular forces between
polar molecules
Hydrogen bonding: evidence
http://www.chemguide.co.uk/atoms/bonding/hbond.html#top
Boiling points of group 4 hydrides
Hydrogen bonding: evidence
http://www.chemguide.co.uk/atoms/bonding/hbond.html#top
Boiling points of group 5, 6 and 7 hydrides
Hydrogen bonding: evidence
http://www.chemguide.co.uk/atoms/bonding/hbond.html#top
Boiling points of compounds with similar mass:
ethanol, C2H5OH, (78 °C), methoxymethane,
CH3OCH3 (-24.5 °C), propane, C3H8 (-44.5
°C), ethanal, CH3CHO (20.8 °C),
Hydrogen bonding: evidence



High solubility of ammonia in water (89.9
g/100 mol of water at 0°C) as hydrogen
bonding occurs between ammonia and
water molecules
Strong forces between polymer molecules
Ethanoic acid forms dimers in non-polar
solvents so it has a high melting and
boiling point
Hydrogen bonding: water!!
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high surface tension
high m.p./b.p. (water is a
liquid over a wide range of
temperatures)
greater specific heat capacity than almost
any other liquid
high heat of vaporization
structure of ice (lower density than water)
Ice
http://www.elmhurst.edu/~chm/vchembook/122Adensityice.html
Ice
http://chemistry.umeche.maine.edu/CHY132/Ice2.html
Hydrogen bonding

Strong attraction between
 highly positive hydrogen atom (part of a large
dipole or polar bond i.e. bonded with N,O or
F)
 the lone pair of a highly electronegative atom
(usually N, O or F) of a neighbouring
molecule (=intermolecular hydrogen bonds)
or the same molecule (=intramolecular
hydrogen bonds as in proteins).

Strongest intermolecular force
Hydrogen bonding
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/H/HydrogenBonds.html
Strength depends on:

Polarity of H atom which depends on difference
in electronegativity between H and N or O or F

Number of hydrogen bonds
most important factor

e.g. water 2 per molecule
Intermolecular forces


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Weakest: London forces (typically
around 3 kJ/mol)
Strongest: hydrogen bonding
(typically around 30 kJ/mol)
Molecules which can make hydrogen
bonds will also make London and
dipole-dipole; dipoles will also have
London.