I. Intermolecular Forces (Ch. 6, p.189-193)

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I. Intermolecular Forces(Ch. 6, p.189-193)

• Chapter 6

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A. Definition of IMF

• Attractive forces between molecules.

• Much weaker than chemical bonds within
  molecules.

• a.k.a. van der Waals forces

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• Molecular Polarity is determined by
• 1) polarity of bonds - diatomic
• and
• 2) orientation of atoms arrangement
• (linear), bent, etc..
• . ..
• Examples H2O, O , ammonia NH3 N
• H H
  HHH

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Dipole-dipole forces molecular polarity

• Strongest intermolecular forces are between polar
  molecules.
• Dipole-dipole forces are the attraction between
  polar molecules.
• Negative region of one molecule attracts positive
  region of the other.
• Indicated sometimes by higher boiling points.

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B. Types of IMF

• Dipole-Dipole Forces

View animation online.
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Induced dipole

• A polar molecule can induce a dipole in a
  nonpolar molecule by momentarily attracting
  electrons.
• These forces are less or weaker than true
  dipole-dipole.

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Induced dipole

• These forces are responsible for O2, a nonpolar
  molecule to dissolve in water a polar molecule.
• H
• O- O O-
• H
• In some molecules, bond dipoles oppose one
  another causing zero polarity.
• Cl
• CCl4 Cl-C-Cl also CO2 OCO
• Cl

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HYDROGEN BONDING

• Type of bonding between a Hydrogen atom and an
  unshared pair of electrons of a strongly
  electronegative atom in another molecule
• This is responsible for the high boiling point
  and surface tension of water.
• H H O-
• O-lt-----H H

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B. Types of IMF

• Hydrogen Bonding

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B. Types of IMF

• London Dispersion Forces

View animation online.
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• London Dispersion Forces
• Weak intermolecular forces caused by constantly
  moving electrons and the creation of
  instantaneous and induced dipoles
• These are present in all atoms molecules, but,
  it is the only force in noble gases nonpolar
  molecules.
• London forces increase with atomic mass or
  molecular mass.

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B. Types of IMF