CHM 214: INTERMOLECULAR FORCES AND PHASE CHANGES OF
MATTER
Why doesn’t everything exist as a gas?
Because intermolecular forces (IMF) or interactions keep molecules
together gas
Gas
Liquid
Solid
shape &
volume
assumes
vol &
shape of
container
assumes
shape of
container
has its
own shape
& volume
compress
easy to
compress
v. difficult to
compress
diffusion flow
molecular
order
Rapid readily
disordered
Slow readily
disordered
v. difficult
to
compress
very slow no
ordered
KINETIC ENERGY & INTERMOLECULAR FORCES

Kinetic energy (KE) prevents molecules from interacting with each
other, i.e., KE overcomes attractive intermolecular forces (IMF)
Gas:
KE
IMF
Liquid:
KE
IMF
Solid:
KE
IMF
•
KE 𝛼 T
•
Energy IN (heating): solid → liquid → gas
•
Keep this mind; we’ll be coming back to this during rest of
semester!
PHASE CHANGES & TEMPERATURE

Energy
Change:
attractions…)
∆Hsub,
∆Hfus,
∆Hvap
(preview
of
coming
Intermolecular forces
•
•
Relative strengths of IM forces can be experimentally observed:
higher boiling point (or melting point)
⇓
intermolecular forces
Electrostatic attraction↑ as intermolecular distances↓; different IMFs have varying
dependence on distance
•
•
Strength of IM forces depends on:
Q
charge on ion
μ
dipole moment
α
polarizability
polarizability (α): ease with which e– clouds become distorted
α
as number of e– ↑
α
as size (MW) ↑
Types of intermolecular forces
depends on
ion–ion
Q
ionic bond
Q, μ
ion–dipole
ions in aqueous solutions of electrolytes (Chap 13)
μ
dipole–dipole
neutral polar molecule interactions
Q, α
ion-induced dipole
ions in nonpolar solvents
μ, α
dipole-induced dipole
polar molecules in nonpolar solvents
α
dispersion
induced-dipole–induced dipole
hydrogen bonding
directional
must have H bonded to N,O,F
Electrostatic Forces
dipole-dipole
draw these Lewis structure & determine ED & molecular geometries and polarity:
excellent practice for the exam
•
for molecules of ~ equal MW & size: more polar molecules (larger μ) have IM
forces
•
Stronger IM forces ⇒ melting point (MP) or boiling point (BP); use MP or BP to
compare relative strengths of IMF (just like what we did when comparing lattice
energies [ion-ion interactions] in Lecture 8).
Dispersion forces
•
Polarizability (α): ease of electronic distortion; α increases as number of e– ↑; α
increases as size (MW) ↑; highly polarizable molecules: more subject to
dispersion forces
London dispersion forces (LDF)
•
Observation: nonpolar molecules liquefy, so there must be attractive IM forces
present, otherwise it would remain a gas
•
Electrons are in constant motion (Chap 6)
•
Dipole moment forms when there are more electrons on one side of the molecule;
when this happens, a dipole in a neighboring molecule is induced; flicker in sync
•
LDF occur with all molecules! (not ionic compounds)
LDF (cont.)
Ar ••••
•
Ar H–Cl in addition to
≡
dipole-dipole
Cl–H
dispersion forces are related to size (primary factor) & shape (secondary factor)
•
size (number of e– or MW) related to polarizability BLB Table
Dispersion interactions (cont.)
•
•
Intermolecular forces ion-dipole dipole-dipole London dispersion forces hydrogen
bonds
phase diagrams