CHEMICAL BONDING Set 6

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CHEMICAL BONDINGSet 6

• Cocaine

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Credits

• Thank you to Mr. Neil Rapp who provided the bulk
  of this powerpoint on his website
  www.chemistrygeek.com
• Other information comes from Zumdahl, Steven, and
  Susan Zumdahl. Chemistry. Boston Houghton
  Mifflin, 2003.

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Bond Polarity

• HCl is POLAR because it has a positive end and a
  negative end. (difference in electronegativity)

Cl has a greater share in bonding electrons than
does H.
Cl has slight negative charge (-d) and H has
slight positive charge ( d)
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Molecular Polarity

• REMEMBER Polarity refers to a separation of
  charges.
• Entire molecules can be polar or nonpolar, too!
• Polar
• Nonpolar

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Determining Molecular Polarity

1. Draw the Lewis dot structure of the molecule.
2. Find the polarity of each bond within the
   molecule.
3. If ALL the bonds are nonpolar, the molecule is
   nonpolar.
4. If one or more of the bonds is polar, determine
   the symmetry of the molecule.

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Molecular Symmetry

• Molecular symmetry is NOT the same as geometrical
  symmetry!
• Molecular symmetry occurs when all items (bonds
  and/or lone pairs) on the central atom are
  identical.
• NOTE It matters what is attached, not how it is
  attached (single, double, triple bonds count the
  same!)

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Symmetry

• If a molecule is symmetrical, the molecule is
  NONPOLAR regardless of what types of bonds it
  contains.
• If a molecule is not symmetrical AND it has at
  least one polar bond, the molecule is POLAR.
• Lone pairs ? H atoms (not symmetrical)
• All atoms are F (symmetrical)

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Intermolecular Forces

• Intermolecular forces forces of attraction
  between molecules (like international between
  countries)
• Intramolecular forces forces of attraction
  within a molecule, i.e. a bond

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Types of Intermolecular Forces (IMFS)

• Dipole-dipole forces occurs between polar
  molecules partial positive end of molecule A
  attracts partial negative end of molecule B

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Types of IMFs

• Hydrogen bonding a special subset of
  dipole-dipole interactions occurs between H
  atom of molecule A and F,O,N or S atom of
  Molecule B

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Types of IMFs

• Dipole-induced dipole forces attraction between
  a polar molecule and nonpolar molecule the
  polar molecule creates a temporary dipole in the
  nonpolar molecule.

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Types of IMFs

• London dispersion forces (or dipsersion forces or
  van der Waals forces or induced dipole-induced
  dipole forces) attractions between two nonpolar
  molecules

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Effects of IMFs

• Like Dissolves Like
• Polar dissolves Polar
• Nonpolar dissolves Nonpolar
• Other properties affected melting points,
  boiling points

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Properties of Three Molecular Compounds
Compound Molecular Formula Molar Mass (g) Boiling Point (C)
Water H2O 18.0 100
Methane CH4 16.0 -33.4
Ammonia NH3 17.0 -164

• Hydrogen bonds explain why water is a liquid at
  room temperature, while compounds of comparable
  mass are gases.
• The difference between methane and water is easy
  to explain because methane is nonpolar, the
  only forces holding the molecules together are
  relatively weak dispersion forces.
• Ammonia and water is not as obvious molecules
  of both can form hydrogen bonds, but the ammonia
  is a gas which indicates its IMFs are not as
  strong. This is because the EN difference between
  N-H is a lot less than the EN difference between
  O-H.

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IMFs and Dissolving

• This is why oil and water will not mix! Oil is
  nonpolar, and water is polar.
• The two will repel each other, and so you can not
  dissolve one in the other

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HOMEWORK

• Explain what determines a substances state at a
  given temperature.
• Compare and contrast intermolecular forces and
  describe intramolecular forces.
• Evaluate which of the molecules listed below can
  form hydrogen bonds. For which of the molecules
  would dispersion forces be the only
  intermolecular force? Give reasons for you
  answers.
• H2 b) H2S c) HCl d) HF

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MORE HOMEWORK

• 4) In a methane molecule (CH4), there are four
  single covalent bonds. In an octane molecule
  (C8H18), there are 25 single covalent bonds. How
  does the number of bonds affect the dispersion
  forces in samples of methane and octane? Which
  compound is a gas at room temperature? Which is a
  liquid?