1. Intermolecular Forces
AP Chemistry
Intermolecular Forces
and States of Matter

2. H2O Chemical properties are related only to chemical
composition; physical properties are related to
chemical composition AND the physical state of
the substance at the time.
H2O

3. < < intermolecular forces (IMFs): forces between molecules
-- largely determine the physical properties of
molecular liquids and solids
covalent
STRENGTH OF IMFs…
<
<
GASES LIQUIDS SOLIDS
Solids w/highly-ordered
structures are crystalline.
ionic and metallic

4. Intermolecular Forces (IMFs)
-- these are much weaker than
ionic or covalent bonds
(< 15% as strong)
-- In vaporizing water, we overcome
the IMFs between water molecules,
but...
NOT the bonds holding the
molecule together. H O
Boiling (or freezing)
doesn’t affect these

5. State of matter is determined by two opposing
influences.
solid
If KE of
particles is…
and
IMFs are…
liquid
gas
then the state
of matter is…
LOW
“STRONG”
MEDIUM
MEDIUM
HIGH
WEAK
Temp. affects… KE
Pres. affects…
IMFs
(proximity)
-- BP and FP/MP depend on IMFs
strong IMFs 
high BP, high MP/FP
weak IMFs 
low BP, low MP/FP

6. Types of IMFs
When ions are present: ion-dipole forces
For neutral molecules: dipole-dipole forces
electrostatic
forces
London dispersion forces
hydrogen bonding forces
There are different bonds
between people: parent/child,
boss/employee, friends,
lovers, etc.
We’re not talking about bonds
holding an individual together.

7. Ion-Dipole Forces (IDFs)
-- exist between an ion and a partial charge on the
end of a polar molecule
(i.e., a dipole)
-- important for...
solutions of ionic substances
in polar liquids
KCl solution is given intravenously in
cases of capital punishment by lethal
injection. The KCl causes the heart
muscle cells to cease contracting.
e.g., aq. soln. of KCl…
Cl– H O
IDF K+
“momentary kissage”

8. Dipole-Dipole Forces (DDFs)
-- exist between neutral polar molecules
that are close together --
weaker than IDFs
-- as the dipole moment m
(i.e., the polarity) increases...
DDFs
Phosphine (PH3), is used in pest
control fumigation due to its high
toxicity. In the liquid phase,
phosphine is subject to DDFs.
(thus, BP and MP )

9. London Dispersion Forces (LDFs)
-- exist between all molecules, but are the ONLY forces
between nonpolar molecules that are close together --
motion of e– causes instantaneous dipoles, which
induce dipoles on nearby m’cules; LDFs with
increasing molar mass
-- polarizability: the ease with
which the charge distribution
in a molecule can be distorted
by an external magnetic field --
large atoms w/lots of e– exhibit
high polarizability

10. Hydrogen Bonding Forces
(HBFs)
3.0 Cl
4.0 F
3.5 O N
2.8 Br
2.5 C S
2.1 P
2.4 Se --
strongest IMF; special class of DDF
-- exist between a hydrogen atom
in a polar bond and an unshared
electron pair on a nearby, small,
highly electronegative ion or atom
(e.g., F, O, N) --
“bare proton” of H is attracted to
unshared e– pairs; its small size
allows close proximity/strong bond
-- important in structures
of proteins and DNA

11. Water expands 9% when it freezes.
HBFs act between water/ice molecules.
Water expands 9% when it freezes. O H
water O H O H O H O H O H
ice O H O H O H O H O H O H