Attractive Forces Between
Molecules

Which phase has particles more strongly
attracted?

Solid , Liquid, Gas?
Intermolecular Forces of Attraction
Forces of attraction between molecules.
Called “Van der Waals Forces”
They arise from weak “electrostatic attractions”
between particles.
Shows electrostatic
attractions between
opposite charged
areas on molecules
Forces Between Polar Molecules

Dipole-Dipole Attractions:
Occurs between polar molecules
 Polar Molecule = “Dipole”

 asymmetrical
molecule with polar bonds
Forces Between Polar Molecules

Hydrogen Bonding

Dipole-Dipole Attraction but stronger

Occurs between molecules that have hydrogen
atoms bonded to very small, highly
electronegative atoms like F, O or N

Creates a very polar bond and an “extra polar”
molecule.

Results in unusually high MP/BP temps.
Remember:
 Hydrogen Bonding is “FON”!!


Hydrogen bonds are responsible for:

Ice Floating on Water
 Ice
is ordered with an open structure to optimize
H-bonding.
 Therefore, ice is less dense than water.

Also it is responsible for water’s
Surface tension
 How it beads

Forces Between Nonpolar Molecules

Dispersion Force (“weak force”)

Only attractive forces between nonpolar
molecules and noble gas atoms.

Weakest of all intermolecular forces.


Instantaneous or “momentary” dipoles are formed
as electrons move around the atoms.
Also called an “induced dipole” force.

For nonpolar gases to become liquids:

Gas particles are so far apart, weak dispersion
forces cannot be felt.

Molecules must be close and moving slowly so
that the momentary dipoles can be created

High pressure/low temp. conditions

Dispersion Forces have different strengths.

The more total electrons in a molecule, the
greater the force can get.

Ex:
Cl2 has 34 electrons (it’s a gas)
Br2 has 70 electrons (it’s a liquid)
I2 has 106 electrons (it’s a solid)
Why Do Ionics Dissolve in Water?

Molecule-Ion Attractions:
Attraction between polar solvent molecules
and ions in an ionic crystal.
 Allows polar solvent (like water) to “pull” the
ions into solution, creating “hydrated” ions.

Remember
which side of
water is
negative!
http://www.chem.iastate.edu/group/Greenbowe/sections/projectfolder/flashfile
s/thermochem/solutionSalt.html
What Effects do these Forces Have?
Melting and Boiling Point Temperature:
 Stronger forces make it harder to separate
molecules and undergo a phase change.
H-Bonding = highest MP/BP
 Dispersion Forces = lowest MP/BP

Solubility: solutes must form
attractions to solvent to dissolve in it.
Polar solvents attract polar solutes
 Nonpolar solutes only dissolve in nonpolar
solvents.
 Polar solvents can attract ions

Viscosity : a measure of resistance
of a fluid to flow.

Stronger IMF = more viscosity

Since there are cohesive forces between the molecules
of liquid, like intermolecular forces, these forces create
an "internal friction" which reduces the rate of flow of that
fluid, so when a substance has greater IMF, these
frictional forces are stronger. Therefore, it has more
resistance toward moving.

http://www.youtube.com/watch?v=f6spBkVeQ4w
Vapor Pressure:
 Pressure exerted by collisions of vapor
particles above a liquid

The weaker the attractions
between molecules in a liquid,
the easier to become a gas.
Therefore:
 Nonpolars with weaker attractions have higher
VP than polars
Vapor Pressure

Measured in a closed system at a specific
temperature.
http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/vaporv3.swf
Vapor Pressure

If the temperature of a liquid increases,
what happens to VP?
http://group.chem.iastate.edu/Greenbowe/sections/projectfolder/flashfil
es/propOfSoln/vp3.html
Vapor Pressure
Boiling Point:
 occurs when VP equals the atmospheric
pressure pushing down on the liquid.


Normal Boiling Point:

The temp. a liquid boils at standard pressure
1
atm
 101.3 KPa
 760 mmHg (torr)
For water it is 100°C.

Can you boil water at different
Temperatures?

Yup!

It depends more on the amount of air
pressure pushing down on the liquid.
http://www.youtube.com/watch?v=8lyqFkFsH28&safe=active
Using Reference Table H
Significant Figures
Click for Powerpoint:
 http://www.slideshare.net/crespiryan/signifi
cant-figures-550789

Sig Figs Practice:
 http://www.sciencegeek.net/Chemistry/tate
rs/Unit0Sigfigs.htm

Accuracy vs. Precision

https://www.youtube.com/watch?v=8Cl5C
eiT7hU