INTERMOLECULAR FORCES

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INTERMOLECULAR FORCES

• Chap. 13 Day 2

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Type of Bond determines Forces

• Generally strongest
• Interparticle
• Ionic Bond
• Ion-ion force
• Metalic bond
• Electronic force

• Generally weaker
• intermolecular forces
• Covalent bonds
• Polar molecules
• Dipole forces
• Nonpolar molecules
• London
• Disperson
• Induced dipole forces

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Melting Point
Energy needed over come the intermolecular forces
between molecules in the solid phase
Temperature at which molecules in a solid gain
enough potential energy to change position
(become L)
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Boiling Point
Energy needed over come the intermolecular forces
between molecules in the liquid phase
Temperature at which molecules in a liquid gain
enough potential energy to change position
(become gas)
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POLAR vs NONPOLAR
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Molecular Polarity

• Molecules will be polar if
• a) bonds are polar
• AND
• the molecule is NOT
• symmetric

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Polar or Nonpolar?

• Compare CO2 and H2O. Which one is polar?

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Polar or Nonpolar?

• Consider AB3 molecules BF3, Cl2CO, and NH3.

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Molecular Polarity, BF3
B atom is positive and F atoms are negative.
BF bonds in BF3 are polar.
But molecule is symmetrical and NOT polar
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Molecular Polarity, HBF2
B atom is positive but H F atoms are negative.
BF and BH bonds in HBF2 are polar. But molecule
is NOT symmetrical and is polar.
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Is Methane, CH4, Polar?

• Methane is symmetrical
• and is NOT polar.

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Substituted Ethylene

• CF bonds are MUCH more polar than CH bonds.
• Because both CF bonds are on same side of
  molecule, molecule is POLAR.

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NONPOLAR Molecules
What causes the differences in intermolecular
forces between molecules?
Why do the intermolecular forces vary between
nonpolar molecules?
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Solid
Liquid
WHY?
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Bigger Molecules interact more
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DISPERSION FORCE
Nonpolar Molecules
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POLAR Molecules
Why do the intermolecular forces vary between
polar molecules?
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Molecular Polarity

• Moleculessuch as HCl and H2O can be POLAR (or
  dipolar).
• They have a DIPOLE MOMENT. The polar HCl molecule
  will turn to align with an electric field.

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DIPOLE FORCE
Polar Molecules
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More polar stronger force
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Why different?
liquid
gas
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?
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SOME POLAR Molecules
Have special Dipole force with Hydrogen
HYDROGEN FORCE or HYDROGEN BOND
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HYDOGEN BOND

• NEED TO BE POLAR
• H is attached to N, F, or O

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ALL molecules have LONDON DISPERSION force
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Graphing Intermolecular Forces

• See handout
• 4 different Families
• Boiling point vs period

400 k
Boiling point
100 k
2
period
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