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Intramolecular vs. Intermolecular Forces
Intramolecular Forces are bonds.
Types of intermolecular forces
Determining relative boiling points
H
N
ot
H
O
H
es
O
Intermolecular Forces 5.1
Intermolecular
Forces are forces of
attraction between
molecules.
'L
ec
tu
re
H
Ion-Dipole
ts
Na+(aq) and Cl-(aq)
–
+
+
–
–
+
–
–
+
+
–
δ+
δ-
Na+
δ+
+
–
δ+ δδ+
δ+
δ-
δ+
δ-
Cl-
δ-
δ+
δ+
δ+
δ+
δ+
δ+
δ-
δδ+
δ+
δ+
δ+
δ+
δ-
δ+
of
–
δ-
δ+
δ+ δ+
δ+ δδ-
St
+
ud
en
• The forces of attraction between an ion
and a polar molecule.
C
op
y
+
Hydrogen Bonds
A type of Dipole - Dipole
pl
e
Dipole - Dipole
Sa
m
The attractive forces between the negative
end of one polar molecule and the positive
end of another polar molecule.
+
• Occurs between a Hydrogen that is covalently
bonded to Fluorine, Oxygen, or Nitrogen and
another F, O, or N with at least one lone pair
A1 – H
-
+
-
:A2 –
F, O, or N
• Five to ten times stronger than other dipole-dipole
attractions
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Hydrogen Bonds Between
Water Molecules
Hydrogen Bonds
Why are H-Bonds so strong?
.. ..
H
.. ..
O
H
'L
ec
tu
re
H
N
ot
H
es
O
1) F – H, O – H, and N – H bonds are very polar.
2) These atoms are very small, so the partial
charges caused by the difference in
electronegativity is highly concentrated.
3) The lone pair(s) on F, O, or N increases the
already partially negative charge on these atoms,
thereby creating a stronger attraction for the
slightly positive hydrogen.
Hydrogen Bonds Between
Water and Methanol
H
H
H
O
H C H
..
H O:
H
H
H C H
H
H
C
H
H
:O:
C
..
N
N:
H
C
H
C
H
H
:O:
H
Boiling Point (221oC)
C
op
y
of
H
H
St
H
..
..
..
:O
ts
O
Forms two H-Bonds with itself
ud
en
.. ..
Both form H-Bonds
Hydrogen Bonds in Acetamide
Sa
m
This structure
does not form
H-Bonds.
Hydrogen is bonded to Carbon.
Hydrogen needs to bond with
F, O, or, N to form an H-bond.
..
H C O:
H C H
H
Hydrogen Bonds
Boiling Temperature at 1 atm (oC)
pl
e
Ethanal does not form
Hydrogen Bonds
+100
H2O
NH3
Only molecules with
H-bonds on this
graph
H2Te Group 6
H2Se
H 2S
5
Group
SbH3
PH3
AsH3
CH4
SiH4
2
3
GeH4
4
Group
SnH4
-200
4
5
Period
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Ion Induced Dipole
Dipole Induced Dipole
–
+
+
–
+
+
Ion
Polar Molecule
Non-Polar Species
N
ot
Non-Polar Species
Electron density is greater
here for brief spurts
It continually flipflops between being
polar and non-polar
+
–
–
+
Polar Molecule
Polarized
+
Polarized Molecule
'L
ec
tu
Ion
–
re
+
–
es
Electrons are attracted
to the positive charge
London Dispersion Forces
ud
en
• These forces exist between all species:
atoms, ions, non-polar and polar molecules.
ts
London Dispersion Forces
t = 0.2s
–
– Contribute to the overall force of attraction
between all particles.
St
• London Dispersion Forces are the only
intermolecular forces that keep assemblages
of non-polar species together.
t = 0.1s
–
+
t = 0.3s
Instantaneous
charge
distributions
are polar.
–
t = 0.4s
C
op
y
of
Averaged
over time,
this atom is
non-polar.
–
Larger species with more electrons
are more polarizable.
+
-
+
-
+
t = 0.3s
-
+
-
+
-
+
-
-
Br2
Boiling Point (K)
-
Sa
m
+
I2
460
t = 0.1s
Cl2
He
+
0
pl
e
London Dispersion Forces
0
F2
60
120
180
240
300
Molar Mass (g/mol)
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Intermolecular Forces of Attraction
ion-ion
ion-dipole
H-bonds
dipole-dipole
ion-induced dipole
dipole-induced dipole
London dispersion
'L
ec
tu
re
• This is not always true when moving from left to right
across a period, as atomic radius decreases.
es
Forces of Attraction Increase
• They have a weaker hold on their outer electrons.
• When moving down a group, or constructing
molecules with more atoms, the resulting species
has more electrons and is larger.
– The more electrons a species has, the more
polarizable it is.
N
ot
Larger species with more electrons
are more polarizable.
Ex2)Which species has the higher boiling point?
Identify the intermolecular forces acting on each.
Identify the intermolecular forces acting on each.
or
C3H8
Both are Non-Polar
H H ..
H C C O.. H
H H
ud
en
C2H6
ts
Ex1)Which species has the higher boiling point?
C3H8
St
The larger molecule has the higher BP, as
larger molecules are more polarizable.
Only London dispersion forces act on these
molecules.
BPC H = - 42oC
6
3
8
C
op
y
2
of
BPC H = - 89oC
Ex3)Which species has the higher boiling point?
pl
e
Identify the intermolecular forces acting on each.
H H H
H C C C OH
H OH H
Identify the intermolecular forces acting on each.
CH4
CI4
Sa
m
H ..
H C O
.. H
H
Ex4)Which has the higher melting point?
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Ex5)Which species has the higher boiling point?
Identify the intermolecular forces acting on each.
es
H2 O
Sa
m
pl
e
C
op
y
of
St
ud
en
ts
'L
ec
tu
re
N
ot
NaCl
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