IMFA’s
intermolecular forces of attraction
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**London Dispersion Forces
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Also know as Van der
What affects the strength of a dispersion force?
Waals forces
Molecules must be very close together for
A momentary non
these attractive forces to occur.
Polarizability is the ease with which an
symmetrical electron
electron cloud can be deformed.
distribution that can
The larger the molecule (the greater the
develop giving a
number of electrons) the more
temporary dipole
polarizable it is.
arrangement of
charge – called an
LD forces increase as molecular weight
INSTANTANEOUS
increases.
DIPOLE
LD forces depend on the shape of the
Weak and short lived
molecule.
Noble gases freezing
The greater the surface area available
point goes up as you
for contact, the greater the dispersion
increase atomic
forces.
number because of
London dispersion forces between
spherical molecules are smaller than
the size of the atoms
between more cylindrically shaped
and the number of
molecules.
electrons.
Strength of LD forces
What affects the strength of a dispersion force?
Molecules must be very close together for these attractive forces
to occur.
Polarizability is the ease with which an electron cloud can be
deformed.
The larger the molecule (the greater the number of electrons)
the more polarizable it is.
LD forces increase as molecular weight increases because there
are more electrons.
LD forces depend on the shape of the molecule.
The greater the surface area available for contact, the
greater the dispersion forces.
Hydrogen bonding
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A special type of bonding
between hydrogen and the
NOFs or FONs – nitrogen,
oxygen and fluorine
A special type of dipole
dipole
Occurs with atoms of high
electro negativity and small
size
Cause of the spaces in
crystal lattice of water so
that water as a solid
EXPANDS – most solids do
not.
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Dipole-dipole forces exist
between neutral polar molecules.
Polar molecules attract one
another.
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The partially positive end of one
molecule attracts the partially
negative end of another.
Polar molecules need to be close
together to form strong dipoledipole interactions.
Dipole-dipole forces are weaker
than ion-dipole forces.
If two molecules have about the
same mass and size, then dipoledipole forces increase with
increasing polarity.
Crystals
Crystal Lattice Energy:
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Solids have a crystal lattice
Smallest repeating unit is a
UNIT CELL
Determined by X-ray diffraction
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Coulombs law
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Attractive and repulsive forces
lattice energy INCREASES
as the charges on the ions
increase and as their radii
decrease.
Types of Solids
Type of
Solid
Melting
Point
Conductivity
Physical Unit at
Properties Lattice
Sites
Force
Eg.
between
Units
Ionic
Very high
Electrical and
thermal conductors
Hard,
malleable,
ductile
+ and ions
Coulombs,
electrostatic
forces
salts
Very low
insulators
soft
Non polar
molecules
London
dispersion
forces (LD)
CO2, HCs
moderate
insulators
intermediate
Polar
molecules
LD, Hbonds,
dipole
Water,
sugar
Network
covalent
Extremely high
Insulators or
semiconductors
Extremely
hard
atoms
Covalent
bonds
Diamond,
quartz,
sand
Metallic
variable
conductors
variable
Positive
ions
Sea of
electrons
Cu, brass,
alloys
Atomic
Extremely low
insulators
atoms
LD
Ne, He
Molecular
Nonpolar
Polar
Polar Liquids
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Surface tension – the
resistance of a liquid to
an increase in surface
area – sphere has least
amount of surface area
Capillary action Spontaneous rising of
liquid in a narrow tube
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Viscosity – the measure of a
liquid’s resistance to flow
Cohesive forces are intermolecular forces that bind molecules to one
another.
Adhesive forces bind molecules to a surface
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Illustrate this by looking at the meniscus in a tube
filled with liquid.
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The meniscus is the shape of the liquid surface.
If adhesive forces are greater than cohesive forces, the liquid
surface is attracted to its container more than to the bulk
molecules. Therefore, the meniscus is U-shaped (e.g., water in
a glass).
If cohesive forces are greater than adhesive forces, the
meniscus is curved downward (e.g., Hg(l) in glass)
Volatile Liquids
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Weak imfa’s
Low boiling point
Evaporates readily
High vapor pressure
Liquid A is volatile
n  normal boiling point is at 68 oC
n Liquid C is not volatile
n normal boiling point is at 100 oC
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Strength of imfa’s
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Rated from strongest to weakest:
Network covalent
n  Ionic
n  Metallic - large variety
n  Molecular n 
hydrogen bonding (a special type of dipole force)
n  Dipole
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LD * can be stronger than dipole forces
Molecular non polar
n  Atomic
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