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					Intermolecular Forces:
 relationships between
             molecules
               Chem 11
“Inter” vs “Intra”
• Intramolecular forces hold atoms together
  in a molecule.
    • Ionic bonds, Covalent bonds, etc (everything we’ve
      talked about thus far!)


• Intermolecular forces are attractive forces
  between molecules.
    • ie. One HF molecule being attracted to a neighboring
      HF molecule
•Intermolecular forces are weak; but without
them, life as we know it would
be impossible.
•Water would not condense from vapor into
solid or liquid forms if its molecules didn't
attract each other.
•Intermolecular forces are
responsible for many properties of molecular
compounds, including crystal
structures (e. g. the shapes of snowflakes),
melting points, boiling points, heats of fusion
and vaporization, surface tension, and
densities.
•Intermolecular forces pin gigantic molecules
like enzymes, proteins, and
DNA into the shapes required for biological
activity.

  http://www.nationmaster.com/encyclopedia/Image:Myoglobin.png
Why are IM forces important?
   Intermolecular forces are responsible for
    many properties of molecular compounds,
    including crystal structures, melting points,
    boiling points and densities
   We can use IM forces to predict BP, MP, etc
Intermolecular Forces:

   1. Dipole-Dipole Interactions
   2. London Forces (dispersion forces)
   3. Hydrogen Bonding

                               Generally,
                               intermolecular
                               forces are much
                               weaker than
                               intramolecular
                               forces.
              Dipole-Dipole Forces
•Attractive forces between polar molecules
•Molecules orient themselves so that oppositely charged
ends are near one another to minimize repulsion
•Bigger difference in electronegativity = stronger DD
forces

     Orientation of Polar Molecules in a Solid
Dipole-Dipole Forces
                   Dipole = polar molecule
                   Dipoles will change/move
                    their direction so that their
                    oppositely charged ends
                    are near to one another.
                   The electrostatic
                    attraction between the
                    ends is dipole-dipole force
London (dispersion) Forces
   Weakest intermolecular
    force
   All molecules (polar
    and non-polar)
   Electron pairs are
    constantly vibrating,
    this can cause
    momentary uneven
    distribution of charge
London (dispersion) Forces (2)
   Non-polar molecules
    become polar for an
    instant and do this on a
    constant random basis
   Two factor effect               http://www.wou.edu/las/p
    strength of LF:                   hysci/ch334/lecture/inte
         Number of electrons              rmol/london.htm
         Shape of molecule
          (relative surface area,
          smaller SA = less LF)
Hydrogen Bonding
   It is the attractive force
    between the hydrogen
    attached to a highly
    electronegative atom of one
    molecule and a highly
    electronegative atom of a
    different molecule.
H bonding:
   Mostly F, N, O since they are
    highly electronegative
   Since H affectly has no electrons
    when it bonds to a highly
    electronegative atom (because
    they pull the e pair towards
    themselves) the positive proton is
    strongly attracted to negative
    charges
Biological molecules love H
bonds!
Ion Dipole Forces
   The force of attraction
    between an ion and a
    polar molecule.
   NaCl breaks up
    because the ion dipole
    with water is stronger
    than the attraction of
    Na+ to Cl-
“Like Dissolves Like”
   Ionic solutes dissolve in polar solvents
       (ex: NaCl and H2O)
   Non polar solutes dissolve in non polar
    solvents
       (ex: solid I2 and liquid Br2)

				
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