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
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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