Intermolecular%20Forces%20and

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Intermolecular Forces and Liquids and Solids
Chapter 12
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1. See Table 12.3 in Chang. Note that glycerol has
   the highest viscosity of the liquids shown. Look
   up the structure and provide a reason why
   glycerol is more viscous than water.

• Viscosity increases with the strength of
  intermolecular forces.
• Glycerol and water are both polar compounds
  dispersion forces and dipole-dipole forces.
• Both are capable of hydrogen bonding, a special
  kind of dipole-dipole force.
• Difference in viscosity must be explained in
  terms of the number of hydrogen bonds that can be
  formed.

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• 12.13 Arrange the following compounds in order
  of increasing boiling point RbF, CO2, CH3OH,
  CH3Br. Explain your arrangement.

Are any of the compounds ionic? If so, they will
exhibit the stronger intermolecular forces. Are
any of the compounds nonpolar? If so, they
exhibit the weakest intermolecular forces. Are
any compounds polar? If so, they will exhibit
intermediate molecular forces. Are any of the
polar compounds capable of hydrogen bonding? If
so, they will exhibit the stronger intermediate
forces.
RuF
CO2
CH3OH, CH3Br
CH3OH
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stronger
weaker
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• 12.19 These nonpolar molecules have the same
  number and type of atoms. Which one would you
  expect to have a higher boiling point? (Hint
  Molecules that can be stacked together more
  easily have greater intermolecular attraction.)

The molecule with the larger surface area has
stronger van der Waals forces for two molecules
with the same molecular formula but different
connectivities (structural isomers), the less
compact molecule has the larger surface area. A
linear isomer is less compact than a branched
isomer.
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• 12.15 Which member of each of these pairs of
  substances would you expect to have the highest
  boiling point (a) O2 or N2, (b) SO2 or CO2, (c)
  HF or HI?
• 12.17 Explain in terms of intermolecular forces
  why (a) NH3 has a higher boiling point than CH4
  and (b) KCl has a higher melting point than I2.

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Solids

• Crystallinehigh order
• (regular repeating pattern)

Amorphouslow order
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Attractions in Ionic Crystals

• In ionic crystals, ions pack themselves so as to
  maximize the attractions and minimize repulsions
  between the ions.

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Crystalline Solids

• basic repeating structural unit of a crystalline
  solid unit cell
• spheres lattice points

• At lattice points
• Atoms
• Molecules
• Ions

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12.4
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12.4
12
Shared by 8 unit cells
Shared by 2 unit cells
12.4
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12.4
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4 atoms/unit cell
1 atom/unit cell
2 atoms/unit cell
(8 x 1/8 1)
(8 x 1/8 1 2)
(8 x 1/8 6 x 1/2 4)
12.4
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• We can determine the empirical formula of an
  ionic solid by determining how many ions of each
  element fall within the unit cell.

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• What are the empirical formulas for these ionic
  solids?
• (a) Green chlorine Gray cesium
• (b) Yellow sulfur Gray zinc
• (c) Green calcium Gray fluorine

(a)
(b)
(c)
CsCl
ZnS
CaF2
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Know diagonal of cube (c) in terms of r Know
diagonal of one face (b) in terms of a If we can
relate (c) to a then, we know (c) in terms of r
12.4
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When silver crystallizes, it forms face-centered
cubic cells. The unit cell edge length is 409
pm. Calculate the density of silver.
V a3
(409 pm)3 6.83 x 10-23 cm3
4 atoms/unit cell in a face-centered cubic cell
m 4 Ag atoms
7.17 x 10-22 g
10.5 g/cm3
12.4