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Bonding
Inter-molecular
(are forces between a
group of molecules, holding
one molecule to the next)
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What are Inter-molecular
forces?
 forces
between two molecules, holding
one molecule to the next molecule
 Intermolecular forces are much weaker
than covalent bonds or ionic bonds
 The strength of the intermolecular forces
determines the physical properties of the
substance; especially melting/boiling
points
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Strength Inter vs. Intra
 Example:
 To
separate one molecule of HCl from the
next molecule of HCl requires 10 Joules of
energy (between),
 BUT to separate the H from the Cl requires
300 Joules of energy (within)
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Three types of Inter bonding:
1.
2.
3.
London-Dispersion Forces (1)
Dipole-Dipole (10)
H-Bonding (100)
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1. London-Dispersion

London-dispersion forces
occur between all molecules

Very weak attraction;
generated by random shift in
a molecule’s electron cloud

This shift pushes & pulls on
neighbouring molecules,
causing a brief attraction

Bigger molecules have more
London-dispersion forces
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Methane CH4
Paraffin C22H46
Bigger molecules
have bigger eclouds surrounding
them. Therefore
more chances for
e- shift, causing
London-Dispersion.
Octane C8H18
The bigger the
molecule, the more
London forces it is
capable of.
Therefore higher MP
and BP
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2. Dipole-Dipole

Polar molecules attract one another when the partial
positive charge on one nears the partial negative
charge on the other

The polar molecules must be in close proximity for the
attraction to be significant

The more polar the molecule, the more Dipole-dipole
forces a substance will undergo
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CH3CN, acetonitrile
C3H8, propane
BP = 82 oC
BP = -42 oC
Uses: solvent for the
manufacture
of pharmaceuticals an
d photographic film
Uses: fuel
The more polar the
molecule, the more
dipole-dipoles will hold the
molecule together.
10
CH3Cl, chloromethane
BP = -24 oC
Uses: Refrigerant
CH3Br, bromomethane
BP = 3 oC
Uses: Pesticide
The more polar the molecule, the more dipole-dipoles will hold
the molecule together. BUT we can only compare molecules of
similar mass. Br is much bigger than Cl, that has more influence
on BP.
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3. Hydrogen Bonding

Only molecules containing at H-F, H-O or H-N bond will
undergo hydrogen bonding

A bond between hydrogen and an electronegative
atom such as F, O or N is very polar; therefore H-Bonds
are very strong

They are very important in the organization of biological
molecules, especially in influencing the structure of
proteins and DNA

This is an important bond responsible for the unique
structure and properties of water
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Water
Hydrogen Sulfide
BP = 100 oC
BP = -60 oC