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Summary Inter-molecular Forces
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Covers Inter-molecular forces that will be assessed in your Grade 11 year in the chemistry section of your Physical Science module/syllabus
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INTERMOLECULAR FORCESIntermolecular = between molecules (forces)
Intramolecular = within molecules between atoms (bonds)
Intermolecular force – a weak force of attraction between molecules, ions, or
atoms of noble gases
PHASES OF MATTER
In a solid or a liquid there must be forces holding them together otherwise the
molecules would move apart and it would become a gas
The physical properties of a substance (e.g. melting and boiling points)
are related to the strength of the intermolecular forces
TYPES OF INTERMOLECULAR FORCES
Referred to as Van Der Waals forces
LONDON FORCES
H2 molecules are non-polar (distribution of electron cloud)
When two H2 molecules collide the electron clouds repel each other
creating a distortion and a momentary dipole
This momentary dipole can induce a nearby molecule
The oppositely charged ends attract each other forming weak
intermolecular forces AKA London force
Induced dipoles are continually loosing charge
London fores occur between non-polar molecules (CH 4, CO2)
London forces are very weak forces resulting in very low melting and
boiling points
The greater the mass and the greater the surface area the greater the
strength of the London force
, DIPOLE – DIPOLE FORCES
Forces between polar molecules
Oppositely charged ends of the dipoles attract each other
Dipole-dipole forces are stronger than London forces which results in a
higher melting and boiling point
The strength of the force increases with increasing molar mass, electron
cloud density and surface area
HYDROGEN BOND FORCES
A special case of dipole – dipole force
Two criteria:
Hydrogen bonds covalently to a small atom with a high electronegativity
Polar molecules are formed that have relatively large δ+δ- charges
The only atoms that fit this criteria = N, O, F
The small electron cloud around the δ+ (due to the electronegativity
pulling the electron cloud) end of the molecule allows the H side of the
molecule to get very close to the neighbouring H2O
The hydrogen bond force is much stronger than normal dipole – dipole
forces which results In a higher melting and boiling point
Hydrogen bond forces only occur between HF, H2O, NH3
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