Ch 11 Intermolecular Forces
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Transcript Ch 11 Intermolecular Forces
Chapter 11: Intermolecular
Forces, Liquids, and Solids
Tadas Rimkus
Period 2
AP Chemistry
Intermolecular Forces
Intermolecular forces are the forces
that exist between molecules.
They include:
Ion-dipole forces
Dipole-dipole forces
London Dispersion forces
Hydrogen bonding
The last 3 are called van der Waals
forces as well
Intermolecular Forces
The strengths of
intermolecular
forces vary greatly
depending on the
substance, but
they are generally
much weaker that
intramolecular
forces.
http://www.chem.ufl.edu/~itl/2045/matter/FG11_002.GIF
Boiling and Melting Point
The boiling point of liquids and
melting point of solids are
dependent on intermolecular forces
The higher the temperature at the
boiling/melting point, the stronger the
forces
Ion – Dipole Forces
Exist between an ion and the partial
charge on the end of a polar molecule
Polar molecules are dipoles
Ion-dipole forces are important for
solutions of ionic substances in polar
liquids (NaCl in water)
Dipole – Dipole Forces
Exist between neutral polar molecules
They are effective only when the
molecules are very close together and
are generally weaker than ion-dipole
forces
The strength of these forces tends to
increase with increasing polarity of
the molecules involved
London Dispersion Forces
Exist between ALL molecules
The molecules create an instantaneous
dipole that causes the non-polar
molecules to attract or repel
Like dipole-dipole, these forces are
only significant when the molecules
are very close together
Tends to increase with increasing
molecular weight
London Dispersion Forces
Polarizability is the ease with which
the electrons in a molecule can be
distorted (the “squashiness” of the
electron cloud)
More polarizable molecules have
stronger London dispersion forces
London Dispersion Forces
Pentane bp=309.4 K
http://genchml
ab.union.edu/c
hem101_110_i
ntermolecular_
forces/npentane.jpg
Neopentane bp=282.7 K
http://uploa
d.wikimedia.
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a/commons/
b/b4/Neope
ntane-3Dballs.png
The shape of the
molecules also
affects the strength
of the dispersion
force
The larger the
surface area of the
molecule, the
stronger its
dispersion force
Comparing Intermolecular Forces
When the molecule have similar molecular
weights and shapes, dispersion forces are
basically equal. In this case, the
differences are due to dipole-dipole
attractions, with the most polar molecules
having the strongest attractions.
When molecules vary greatly in their
molecular weights, dispersion forces tend
to be the decisive forces.
Hydrogen Bonding
Hydrogen bonding is a special type of
intermolecular force that exists
between the hydrogen atom in a
polar bond and a fluorine, oxygen, or
nitrogen atom.
H-F, H-O, H-N
It is the strongest of the
intermolecular forces
Distinguishing Between Forces
http://itl.chem.ufl.edu/2041_f97/matter/FG11_012.GIF
Phase Changes
http://www.alterniawhatif.com/HPS%20Project/Phase%20Chan
ges_files/phase_change.jpg
Heating Curves
Heating curves incorporate the phase
diagram but also show the energy required
for the phase change
http://library.thinkquest.org/C006669/media/Chem/img/Gra
phs/HeatCool.gif
Critical Temperature and Pressure
The critical temperate is the highest
temperature at which a substance can
exist as a liquid.
Critical pressure is the pressure
required to liquefy the substance at
this critical temperature
The greater the intermolecular forces,
the higher the critical temperature
and the more easily is liquefies
Phase Diagrams
D
D
B
A
C
http://ltl.tkk.fi/research/theory/TypicalPD.gif
Phase Diagrams
A phase diagram is a visual used to
explain the conditions under which
equilibria exist between the different
states of matter
The line from A to B is the liquid vapor-pressure
curve
It ends at the critical point (the critical
temperature and pressure of the substance)
Beyond this point, the liquid and gas phases are
indistinguishable (supercritical fluid)
Phase Diagrams
The line from A to C represents the
vapor pressure of the solid as it
sublimes at different temperatures
The line from A to D represents the
change in melting point of the solid
with increasing pressure
Point A is known as the triple point.
All 3 phases are at equilibrium at this
temperature and pressure
Phase Diagrams of H2O and CO2
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The phase diagram of carbon dioxide (right) follows
the typical behavior, with its melting point
increasing with increasing pressure
The phase diagram of water (left) shows that the
melting point decreases with increasing pressure