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Polarity and Intermolecular Forces.ppt

  1. Polarity and Intermolecular Forces
  2. Review  We know how to draw Lewis structures for simple molecules and polyatomic ions.  We also know how to predict the 3-D geometry of these molecules and ions, if we apply the VSEPR Theory.  Electronegativity (EN) is an atom's tendency to attract electrons in chemical bonds.  EN increases to the right and up on the periodic table, excluding the noble gases.
  3. Bond Polarity  When two nonmetal atoms bond, they share electrons.  They may or may not share electrons evenly.  Consider the following molecules:  F2  HF
  4. Bond Polarity  H vs. F  The EN of H is 2.2.  The EN of F is 4.0.  In F2, both atoms pull with equal strength on the bonding e-.  The e- are shared evenly between them.  In HF, the F atom pulls harder than the H atom.  The e- are drawn more toward the F atom. H EN = 2.2 F EN = 4.0 F F F H
  5. Bond Polarity  EN is same for both atoms.  e- density is spread evenly around molecule.  Highest e- density occurs between atoms.  Bond is nonpolar.  EN is much higher for F than for H.  e- density is drawn toward F side.  F atom acquires partial negative charge.  H atom acquires partial positive charge.  Bond is polar. F F F H
  6. Bond Polarity  In general, a covalent bond is:  polar if it occurs between two different atoms.  nonpolar if it occurs between two identical atoms.
  7. Dipole Moments  Dipole Moment - a measure of the polarity of a bond.  Is often represented by a special arrow. F H Arrow points toward more EN atom.
  8. Polarity of Diatomic Molecules  Diatomic Molecules - molecules made of only two atoms.  If atoms are the same, molecule is nonpolar.  If atoms are diff., molecule is polar.  NOTE: Polar does not mean charged.  Is Cl2 polar or nonpolar?  Is CO polar or nonpolar?
  9. Molecules With 3 or More Atoms  A molecule with 3 or more atoms is:  Polar if its central atom has lone pairs OR  If the outer atoms are not all the same.  Nonpolar if its central atom has no lone pairs AND  All the outer atoms are identical.
  10. CO2 vs. H2O  Consider the Lewis structure of CO2: This molecule is nonpolar.
  11. CO2 vs. H2O  Consider the Lewis structure of H2O: This molecule is polar.
  12. CH4 vs. CH3Cl  Neither CH4 nor CH3Cl has any lone pairs on the central carbon atom.  Is CH4 polar or nonpolar?  Is CH3Cl polar or nonpolar? C C H H H H Cl H H H
  13. “Like Dissolves Like”  Polar molecules mix with each other.  Nonpolar molecules mix with each other.  Polar and nonpolar molecules do not easily mix.
  14. Amphipathic Molecules  Amphipathic - has a hydrophobic region and a hydrophilic region.  Hydrophobic - “water-fearing”  Nonpolar.  Hydrophilic - “water-loving”  Polar or charged.  Dish detergents contain amphipathic molecules.  Why?
  15. Grease and Water Don't Mix!
  16. Amphipathic Molecules Hydrophilic head Hydrophobic tails
  17. Amphipathic Molecules
  18. Intermolecular Forces  Intermolecular force - a force between two molecules that does not result from chemical bonding.  Dipole-dipole interaction.  Hydrogen bonding.  London force.
  19. Dipole-Dipole Interactions  Dipole - polar molecule.  Like magnets, except poles are + and  - instead of N and S.  Polar molecules generally have higher melting and boiling points than similar nonpolar molecules.  EXAMPLE: O2 (nonpolar) boils at -183ºC.  EXAMPLE: NO (polar) boils at -152ºC.  NO has a higher boiling point due to its polarity.  Still far below the boiling point of any ionic cmpd.
  20. Dipole-Dipole Interactions
  21. Hydrogen Bonding  Hydrogen bond - a stronger form of dipole-dipole interaction.  Occurs in molecules that have H atoms bonded to O, N, or F atoms.  The small size of the H atom allows these molecules to get closer together.  Closer together = stronger forces.  EXAMPLE: H2O has a boiling point of 100ºC.  EXAMPLE: H2S has a boiling point of -60ºC.  The b.p. of H2O is higher b/c of hydrogen bonding.
  22. Hydrogen Bonding Boiling Points of Several Compounds -250 -200 -150 -100 -50 0 50 100 150 H2O H2S H2Se H2Te Compound Boiling Point (ºC)
  23. London Force (Dispersion)  London force - attraction between temporary dipoles.  e- move randomly around molecules.  Nonpolar molecules become temporarily polar.  Allows for very weak attractions between nonpolar molecules.  Named for Fritz London.
  24. London Forces + -
  25. London Forces  The more e- a molecule has, the greater its London forces are.  Large molecules tend to have higher melting/boiling points than small molecules.  London forces apply to all molecules.
  26. London Forces Boiling Points of the Noble Gases 0 50 100 150 200 250 He Ne Ar Kr X e Rn Noble Gas Boiling Point (Kelvins)