ATTRACTIVE FORCES
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Three types of attractive forces: Hydrogen bonding, dipole-dipole attractions and London
Dispersion( or van der waals)
Attractive forces are intermolecular attractions as opposed to intramolecular
o Intermolecular attractions are the attractions BETWEEN molecules
 Use the example of interstate 35 connects OK and TX. Between states.
 Use the example of strings between the fingers (Jacob’s Ladder)
 The fingers/hands are the bonds and the strings are the attractions between
the molecules.
o Intramolecular attractions are the attractions within the molecule.
 Covalent bonding
HYDROGEN BONDING:
Hydrogen bonding occurs when hydrogen is covalently bonded to a very strong electronegative
atom while being weakly attracted to an unshared pair of electrons of an electronegative atom in
nearby molecule.
Special case of dipole-dipole forces.
By experiments: boiling points of compounds with H-F, H-O, and H-N bonds are abnormally high.
Intermolecular forces are abnormally strong.
“hydrogen is married but is messing around with the next door neighbor”
Occurs when H bonded to either F, O, or N.
o “phone home” = FON H
o Draw the structure of H20 and show the dotted line of attraction. Draw HF and show the
dotted line between molecules. Do the same for NH3.
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Accounts for the high boiling point of water
 Show the trends within the family of oxygen. H2O should be much lower
 Graph depicts HF and NH3 in their respective families.
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Check for understanding: Draw CH3COCH3 and ask if this is hydrogen bonding. Draw a
dotted line across from the O and to an H. Is this HBonding? No . Hydrogen has to be
bonded to a very highly electronegative and in this case it is bonded to a carbon.
Good web site: http://dwb4.unl.edu/ChemAnime/HYBOND/HYBONDD.html
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DIPOLE-DIPOLE INTERACTIONS
Dipole-dipole forces exist between neutral polar molecules.
Polar molecules need to be close together.
Weaker than ion-dipole forces.
There is a mix of attractive and repulsive dipole-dipole forces as the molecules tumble.
If two molecules have about the same mass and size, then dipole-dipole forces increase with
increasing polarity
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Good website: http://dwb4.unl.edu/ChemAnime/DIPOLED/DIPOLED.html
LONDON DISPERSION
• Weakest of all intermolecular forces.
• Be careful with this statement. As the mass increases so does the attraction and can
therefore produce stronger/higher boiling points
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As the mass increases the electron cloud increases thus increasing the attractions and
making the boiling points higher. It is NOT the mass that causes the increased
attraction. It is just how you can pick it out.
It is possible for two adjacent neutral molecules to affect each other.
The nucleus of one molecule (or atom) attracts the electrons of the adjacent molecule (or
atom).
For an instant, the electron clouds become distorted.
In that instant a dipole is formed (called an instantaneous dipole).
• Good website: http://dwb4.unl.edu/ChemAnime/LONDOND/LONDOND.html
Polarizability is the ease with which an electron cloud can be deformed.
The larger the molecule (the greater the number of electrons) the more polarizable.
London dispersion forces increase as molecular weight increases.
London dispersion forces exist between all molecules.
• Every thing has dispersions including water, ammonia etc.
London dispersion forces depend on the shape of the molecule.
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Boiling points, melting point are phase change and NOT
breaking of bonds.
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The number one mistake made by kids is the above statement.
Phase changes involve breaking of the attractive forces
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Question to ask: Why are phase changes straight lines in the 5 step diagram of phases
changes?
 All the energy is going to break the attractive forces first then the temperature
rise.
“LIKE DISSOLVES LIKE” Can now be taught.
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Polar dissolves polar, nonpolar dissolves nonpolar
o Ask questions like:
 Will NH3 dissolve in H2O
 Will water and benzene be soluble (miscible)
 Why do you not throw water on a gasoline fire?
Ask questions like the following: