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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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  • July 2, 2019
  • 6
  • 2017/2018
  • Summary
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INTERMOLECULAR FORCES

Intermolecular = 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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