Chapter 11
intermolecular forces- attractive forces between
all molecules and atoms
-these forces hold phases of matter together
-strength of intermolecular forces determines the
phase of a substance at a given temperature
-intermolecular forces are weaker than bonding
forces b/c of distance between molecules
-it takes a much higher temp to break a bond
than to overcome intermolecular forces
Properties of Phases of Matter
Phase Density
Shape
Volume
Inter.
Forces
Solid
definite
definite
strong
definite
moderate
high
Liquid moderate indefinite
Gas
low
indefinite indefinite
weak
Changes Between Phases
A
C
B
Which best depicts vapor
emitted from a pot of boiling
water?
BOX A
Types of Intermolecular Forces
1) dispersion force (London force)
-caused by motion of e-weakest of all molecular interactions
-the more e-, the stronger the dispersion force
-ex- helium
-higher molar mass = greater dispersion forces =
higher boiling points
-molecular shape also plays a part in the strength
of dispersion forces
-page 467
2) dipole-dipole forces
-exist in all polar molecules
-polar molecules have permanent dipoles that
interact with the permanent dipoles of
neighboring molecules
*+ end of polar molecule is attracted to – end of
its neighbor
-this attraction is the dipole-dipole force
-all molecules (polar and non-polar) have
dispersion forces
-only polar also have dipole-dipole forces
-this extra force raises melting and boiling points
compared to nonpolar molecules with similar
molar masses
Which of the following have dipole-dipole
forces?
1) CO2
-has polar bonds, but is nonpolar b/c it is linear
 no dipole-dipole forces
2) CH2Cℓ2
-has polar bonds and net dipole even though
tetrahedral  has dipole-dipole forces
3) CH4
-has “slightly” polar bonds, but tetrahedral  no
dipole-dipole forces
-polarity of molecules helps determine
miscibility (ability to mix) between liquids
-polar is miscible with other polar molecules,
nonpolar with nonpolar
-“like dissolves like”
3) hydrogen bonding
-occurs when polar molecules have hydrogen
atoms bonded directly to very
electronegative atoms
-causes strong interaction between H and other
atoms in other molecules
-strongest of the first three intermolecular forces
-substances with hydrogen bonding have higher
melting and boiling points
-water exhibits very strong H-bonding and this
explains its behavior
-these two compounds have the same formula,
but different structures which results in one
having strong H-bonding and one not
Which one of the following compounds has a
higher boiling point and why?
-all have similar molar masses- similar
dispersion forces
-all are polar- all have dipole-dipole forces
**hydrogen peroxide b/c it has H-bonding,
harder to break apart
4) ion-dipole force
-occurs when an ionic compound is mixed with a
polar compound
-strongest of all the intermolecular forces
-common in aqueous solutions
-ex- salt and water
Summary of Intermolecular Forces
Results of Intermolecular Forces
1) surface tension
-inward force or pull that tends to minimize the
surface area of a liquid
-molecules with increased intermolecular forces
have increased surface tension
-water has an extremely high surface tension b/c
of the strong H-bonding
ex- forming spherical droplets, things being able
to float on water
-it is difficult to break the H-bonds
-will decrease by adding a surfactant (soap)
2) viscosity
-the resistance of a liquid to flow
-increases with greater intermolecular forces b/c
molecules cannot flow as easily
-also depends on molecular shape- higher in
longer molecules b/c they can become
entangled
-temp plays a part- the higher the temp, the lower
the viscosity
3) capillary action
-ability of liquid to flow against gravity up a
narrow tube
cohesive forces- attraction between molecules of
the same substance
adhesive forces- attraction between molecules of
different substances
-if adhesive are greater than cohesive then the
liquid will be drawn up
-if cohesive are greater than adhesive than liquid
does not rise
-page 476
Vaporization
-going from liquid to gas
-need adequate energy to break free from liquid
-the higher the temp, the more the liquid will
vaporize
-some escaped gas molecules with low energy
will plunge back into molecule
-this is called:
Condensation
-gas to liquid
-opposite of vaporization
volatile- liquids that vaporize easily
ex- nail polish remover
nonvolatile- liquids that do not vaporize easily
ex- motor oil
Rate of vaporization will increase with:
-increasing temp
-increasing surface area
-decreasing intermolecular forces
heat of vaporization (∆Hvap)
-amount of heat required to vaporize one mole of
a liquid
- ∆Hvap is always positive b/c energy is
absorbed
heat of condensation (∆Hcond)
-amount of heat required to condense one mole
of a gas to a liquid
-same magnitude as ∆Hvap, but opposite sign b/c
energy is given off
vapor pressure- pressure above the surface of
the liquid, causes vaporization
-will be lower with stronger intermolecular
forces
-if a container is sealed, the liquid will still
vaporize and condense, just not into the
atmosphere
-when rate of condensation equals rate of
vaporization the liquid has reached dynamic
equilibrium
boiling point- temp at which vapor pressure
equals external pressure
normal boiling point- temp at which vapor
pressure equals 1atm
-at lower pressure, water boils at lower temp
-will take longer to cook foods at locations with
higher altitudes
*Ex- Mt. Whitney, CA water boils at 87°C
-once boiling, the liquid will not rise above
boiling point until all of the water has been
converted to steam
critical temp- the temp at which a liquid cannot
exist
supercritical fluid- has properties of both liquids
and gases
critical pressure- pressure needed to bring the
liquid back at the critical temp
**can be used as a solvent to extract things- CO2
extracts caffeine from coffee beans
**can be easily removed by lowering the
pressure below the critical pressure and it will
evaporate away
heat of sublimation (∆Hsub)
-goes from solid to gas without passing through
liquid phase
ex- frozen foods
deposition- gas to solid skipping liquid phase
melting point- temp at which solid turns to
liquid
-melting is also called fusion
freezing point- temp at which liquid turns to
solid
heat of fusion (∆Hfus)
-amount of heat required to melt one mole of a
solid
- endothermic= +
heat of solidification (∆Hsolid)
-amount of heat required to solidify one mole of
a liquid
-exothermic= -same magnitude as ∆Hfus, but opposite sign b/c
energy is given off
-page 488
How much heat is required to melt 32.30g of
acetone at its melting point?
32.3g C3H6O x 1 mole x 5.69kJ = 3.17kJ
1
58g
1 mole
-look at heat curve page 489
*there is no temp change during conversion
Phase Diagrams page 491-493
-a map of the phase of a substance as a function
of pressure (y-axis) and temp (x-axis)
Features of a phase diagram
regions- represents conditions where solid,
liquid or gas is stable
lines- each line or curve represents a set of temp
and pressures at which the substance is at
equilibrium between the two phases on either
side of the line
triple point- unique set of conditions where all
three phases exist in equilibrium
critical point- where supercritical fluids exist