Condensed Phases
and
Intermolecular Forces
Let’s
look at particle diagrams
of liquids & solids and
compare them to particle
diagrams of gases
Describe & compare the relative positions and
motions of particles in each of 3 phases:
The Question:
Why
do some substances
exist as gases, some as
liquids, and some as solids
at room temp?
 part
of answer has to do with
attractive forces between
separate but neighboring
molecules
2
broad categories of attractive forces
come into play:
1.
2.
INTRAmolecular forces of attraction
INTERmolecular forces of attraction
Forces
INTERMOLECULAR
INTRAMOLECULAR
Dispersion
Covalent
Dipole-Dipole
Ionic
Hydrogen Bonding
Metallic
#1: Intramolecular Forces
 Intra
means “within”
 Intramolecular
attractive forces are
chem bonding forces
 Definition:
attractive forces that hold particles
together in chemical bonds

3 types: ionic, covalent, or metallic bonds
# 2: Intermolecular Forces (IMF)
(aka: van der Waals forces)
 Inter
means “between” or “among”
 Intermolecular
forces:
 attractive
forces between neighboring
molecules
 Intermolecular
forces are weaker than
Intramolecular forces
IMF:
 weaker
than intramolecular (bonding)
forces
≈
5% to 15% of strength of intramolecular forces
 account
 strong
 weak
for phase of matter at room temp
IMF  condensed phase (solid/liquid)
IMF  gas phase
IMF determine phase of matter
phase is determined by:
 “competition”

IMF
and

KE
between strength of
 If
IMF are strong, substance will be solid or
liquid at room temp
 strong attractive forces keep particles
close together
 If
IMF are weak, substance will be gas at
room temp
 weak attractive forces allow particles to
spread far apart & be free to move
It’s all a balancing act!
KE
IMF
[this substance = a gas at room temperature]
Intermolecular Forces vs. Kinetic Energy
IMF
KE
[this substance = a condensed phase (solid/liquid)]
 since
T is measure of average KE,
changing T can change phase
 changing
T changes average KE of
particles
T
change can allow change in phase
3 types of IMF:
1. Dispersion forces
2. Dipole-Dipole forces
3. Hydrogen bonds
1. Dispersion Forces:
●
●
weakest IMF
occur between non-polar molecules
dispersion forces & non-polar molecules
• instantaneous and momentary change
• electron cloud will fluctuate
• results from motion of electrons due to attractive
forces
Non-polar atoms/molecules
non-polar means no poles
• can’t tell one end of atom/molecule
from other end
• electrons are evenly distributed
• charge is evenly distributed
•
atom/molecule: symmetrical
Non-polar Atoms/Molecules:
 monatomic
 He,
atoms:
Ne, Ar, Kr, Xe, Rn
0
 diatomic
 H2,
elements:
N2, O2, Cl2, F2, I2, Br2
0

small symmetrical molecules:
 CO2
, CX4
0
 carbon-hydrogen
 CH4,
C2H6, C3H8
molecules:
Dispersion Forces and Size
 larger
the electron cloud, the greater the
fluctuations in charge can be
 strength of attractive dispersion forces ↑ with
increrasing molecule size
 increasing strength of dispersion forces:
 Rn
> Xe > Kr > Ar > Ne > He
 I2 > Br2 > Cl2 > F2
 C8H18 > C5H12 > C3H8 > CH4
2.
•
•
Dipole-Dipole forces:
intermediate IMF
occur between polar molecules
What do you know about charge?
 Opposites
Attract!
this
time, uneven distribution of electrons
(charge) is permanent!
 examples:
non-polar
H2
CH4
polar
HI
CH3Cl
 Polar
Molecules:
 geometry
NOT symmetrical (asymmetrical)
 uneven electron distribution

permanent separation of charge
 has
poles:
one end partly (-)
and
 one end partly (+)

neighboring molecules
orient themselves
according to their
opposing attractive
charges
3.
•
•
•
Hydrogen Bonding forces:
strongest IMF
subtype of dipole-dipole attractive forces
attractive force occurs between H in one
molecule
and
F, O or N in neighboring molecule
H-F
H-O
or
H-N
Hydrogen Bonding Force
H-O
N-H
0
this attractive force occurs between molecules with
FON!!!
Hydrogen Bonding Force
 hydrogen
bonding force is special subtype of
dipole-dipole attractive forces
 F, O, and N are all small and electronegative
 strong
electron attraction
 H has only 1 electron, so if being pulled away
H proton is almost “naked”
H
end of molecule is always positive
&
F, O, or N end is always negative
Strength of Hydrogen Bonding Force
 fluorine
most electronegative element, so
 H-F
bonds are most polar and exhibit
strongest hydrogen bonding attractive forces
 H-F
> H-O > H-N
IMF
vs.
Physical Properties

If strength of IMF  then:  If strength of IMF  then:
boiling point 
 melting point 
 heat of fusion 
 heat of vaporization 
boiling point 
 melting point 
 heat of fusion 
 heat of vaporization 


while:
while:

evaporation rate 

evaporation rate 
boiling point of N2 is 77 K (-196˚C)
IMF are very, very weak forces (dispersion)
Hydrogen bonding:
• strongest IMF
• influences physical props a great deal
IMF vs. Temp
 IMF
more important as temp is lowered
 low
temperature = low evaporation rate
 high temperature = high evaporation rate
Indicate type of IMF for each molecule:
 NH3
•
 Ar
•
 N2
•
 HCl
•
 HF
•
 Ne
•
 O2
•
 HBr
•
 CH3NH2
•
hydrogen bonding
dispersion forces
dispersion forces
dipole-dipole forces
hydrogen bonding
dispersion
dispersion
dipole-dipole
hydrogen bonding