Transcript Good Morning!

Warm-up: Polar molecules
• What has to be true for a molecule to be
polar? (2 things)
• What is one example of a polar molecule
from your lab?
• What is one example of a nonpolar
molecule?
Good Morning!
We have 3 goals today
1. Review how you know if a molecule is
polar or not
2. Learn about the reasons for and
strengths of intermolecular forces
3. See evidence of those forces in the
boiling points of different substances
Intramolecular Forces vs.
Intermolecular Forces
•
•
•
•
Intramolecular Forces
--forces within a molecule.
--tend to be very strong.
--hold the atoms in molecules/formula
units together.
• --include: ionic bonding, covalent bonding
and metallic bonding.
• --been there…done that!
Intramolecular Forces vs.
Intermolecular Forces
•
•
•
•
Intermolecular Forces
--forces between molecules.
--tend to be weaker than intramolecular forces.
--matter has entropy (the tendency to be
disordered)
• --therefore, a force must be present to keep
the individual atoms, molecules, or ions of a
solid or liquid, in place, organized.
Intermolecular Forces are also
called Van der Waals Forces
•
•
•
•
•
There are four types:
1. London/dispersion (non-polar molecules)
2. Dipole-dipole forces (polar molecules)
3. Hydrogen bonds (special case for polar molecules)
4. Molecule-ion attractions (as the name implies,
interactions between ions and polar molecules, such as dissolving
salt in water)
London Dispersion Forces
• occurs btw molecules that are non-polar
• Due to attractive forces between e-s of
one atom and the nucleus of another
• stronger for atoms/molecules with more
electrons
Intermolecular Forces!
(this is the information from your notes)
Dispersion forces
Usually this 
Sometimes this
When they are
unevenly distributed,
there is a temporary
dipole.
This can induce
new temporary
dipoles in neighboring
molecules.
How dispersion forces are created
Temporary
RANDOM dipole





Induced dipole
Induced dipole
Induced dipoles
First random dipole can induce a neighbor to become a dipole.
And that dipole can induce the next dipole…
And then the next…and so on…
Partial positive charges are attracted to
partial negative charges!
Dispersion forces – remember
these are temporary!
• When the original
random dipole
changes back to
normal, all other
dipoles go back to
normal…
• until the next
random event.
Another Dispersion Graphic
1. Evenly distributed electrical charge
2. Uh-oh, (random event!!) uneven
distribution in one molecule (temporarily)
3. Uneven distribution in one causes
uneven distribution in the other…then
they have charged ends (dipoles) that
stick together!
4. Things return to normal, (random
event, again…) and we start the process
over
Dipole-Dipole Forces
• due to attractive
forces between the
positive end of one
molecule and the
negative end of
another
• occurs between
polar molecules
(molecules have a
permanent dipole)
Dipole-Dipole Forces
• Stronger than
dispersion forces –
molecules are
permanently polar,
so they always
want to stick
together!
Hydrogen Bonding
• A specific and EXTRA STRONG dipoledipole interaction…
• between Hydrogen and three other elements
with very high electronegativities and small
radii (Oxygen, Nitrogen and Fluorine)
• explains high boiling point of water
these atoms are electronegative enough, to
make a BIG enough dipole…
…to count as a different kind of force
Hydrogen bonding
H-bonding animation
• http://bcs.whfreeman.com/thelifewire/conte
nt/chp02/02020.html
Molecule-ion Attractions
• ionic compounds dissolve in water and
other polar liquids because of attraction
between the dipoles and the ions
Molecule-Ion attractions
Please take out a piece of paper
• Divide it into 4 sections, label each with
one kind of intermolecular force
• A’s  draw pictures for Dispersion forces
and dipole-dipole forces
• B’s  draw pictures for Hydrogen bonding
and Molecule-ion interactions
• In 2 minutes turn and share with your
neighbor
Now please take out…
• “Intermolecular bonding and boiling points”
• Use the data to complete each graph, then
start working on the questions
• When you have finished the questions 
decide which type of intermolecular force
is present for EACH compound listed
• There are 8 total compounds…
I leave you with this!
Both graphs are similar