11.1 Intermolecular Forces
Keeping Matter Together
Nature’s Forces
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Phases of Matter: Terminology
Energy is required for phase
change to occur.
Solid-Liquid-Gas
Triangle
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Heating Cooling Curve
From Steam to Ice and Vice-versa
2.09 J
g°
1.84 J
g°
4.184 J
g°
0.43 cal
g°
40.67 kJ
mol
1 cal
go
540 cal
g
How much energy of 1g H2O 100°C to 0°C ?
3
0.50 cal
g°
6.01 kJ
mol
80 cal
g
540+100+80=720cal
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Intermolecular Forces
At the molecular level:
Molecules or matter is held together by attractive force
“glue” called intermolecular forces
SOLID
4
LIQUID
GAS
Energy added (K.E. increase)
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Keeping Matter together
Intramolecular Forces Force which keeps integrity of
molecule together, i.e., bonds or
electrostatic bonding.
Intermolecular Forces Attractive force between molecules.
Responsible for keeping matter in
solid or liquid phase
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The Forces be with You
2 Basic types of Intramolecular Force
Ion - ion - Electrostatic attraction
Covalent Bonds - Mutual sharing of electrons
4 Basic types of Intermolecular Force*
1. Ion - dipole : Ion is attracted to polar molecule
2. dipole - dipole: Polar molecules attracted to each
other.
3. dipole - induce dipole: Polar molecules attracted
to nonpolar molecules.
4. induce dipole -induce dipole (Van der Waal’s forces
– also called London dispersion forces) nonpolar
molecules attraction for each other due to electron
cloud distortion.
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* plus one
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Relative Strength
Interaction
ion- ion
Covalent Bonds
ion-dipole (I-D)
dipole - dipole (D-D)
dipole - induce dipole (D-ID)
Van der Waal
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• H-Bond (10 - 40 kJ/mol
Example
Na+ ClH-H
Na+ H2O
ICl ICl
HCl O2
N2 N2
Energy
400 -4000 kJ/mol
150-1100 kJ/mol
40-600 kJ/mol
5-25 kJ/mol
2-10 kJ/mol
0.05 - 40 kJ/mol
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Ion - Ion
Covalent Bonds
Ion - Ion: Electrostatic attraction between ions
Na
Na +
11 p
12 n
11 p
12 n
11 p
12 n
F
F-
9p
10 n
9p
10 n
9p
10 n
NaF
Bond Energy: = 926 kJ/mol
Covalent Bonds: Bond between atoms as a result of
electrons sharing.
F
9p
10 n
8
F
9p
10 n
F2
9p
10 n
Bond Energy: = 159 kJ/mol
9p
10 n
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Ion - Dipole
Ion - Dipole: Charge and size dependent.
Most important for larger charge and small ionic radius.
Cation
Li+
Na+
K+
RB+
Cs+
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Ion Radius
90
116
152
166
181
DHHyd (kJ/mol)
-515
-405
-321
-296
-268
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Dipole - Dipole
Dipole - Dipole:
A permanent attractive
intermolecular force resulting from the
interaction of the positive end of one molecule
with the negative end of another.
Occurs between identical or different polar
molecules.
NonPolar
M(g/mol) bp (°C)
N2
28
-196
SiH4 32
-112
GeH4 77
-90
Br2
160
59
10
Polar
M (g/mol)
CO 28
PH3 34
AsH3 78
ICl 162
bp(°C)
-192
-88
-62
97
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Induce dipole - induced dipole:
Vander Waal’s forces
Van der Waal (Induced dipole-Induced dipole):
Intermolecular force responsible for keeping nonpolar molecules
(species) together.
Polarisability - The ease of which an e- cloud can be
distorted. Larger the atomic size, the greater the number of
electrons, the greater the polarizability.
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Boiling Point of the Halogens and Noble Gases
Halogen
B.pt (K)
Noble Gas
B.pt (K)
F2
Cl2
Br2
I2
He
Ne
Ar
Kr
4.6
27.3
87.5
120.9
Xe
166.1
85.1
238.6
332.0
457.6
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Boiling point versus polarisability
Graphs for noble gases
and for series of nonpolar
molecules; both show a
family smooth increase of
boiling point with atomic
weight (larger degree of
polarisability) due to
increasing Van der Waal
forces
0°C
-100°C
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SiH4
CH4
-200°C
He
-250°C
How about H2Te, (-20°C)
H2Se, H2S, H2O
GeH4
SnH4
Xe
Kr
Ar
Ne
Molar Mass
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Boiling point Hydrogen compounds
Graphs for family of
hydrogen containing
compounds and their
boiling point. In
general there is an
increase in the boiling
point except for H2O,
HF and NH3. Why?
There must be some
other force that
operate on these
compounds which
increases their
intermolecular forces.
100°C
0°C
H2O
HF
NH3
H2S
HCl
PH3
SiH4
-100°C
H2Te
SbH3
HI
H2Se
AsH3
HBr
SnH4
GeG4
CH4
Molar Mass (Period)
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A Special Type of Bonding
H-Bonding
H-Bonding: A special glue above and beyond dipoledipole intermolecular forces.
H-bonding is a strong type of intermolecular force (bond)
between hydrogen and very electronegative elements ( 4
- 30 kJ/mol).
N-H
O-H
F-H sometimes (Cl-H)
Bichemical structural Integrity.
Water possesses H-bond: Responsible for water’s unique
properties.
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Example: H-bonding
Which of the following substances exhibits H-bonding?
Draw the H bonds between two molecules of the substances
where appropriate.
a) C2H6
No
b) CH3OH
Yes
c) H3CCONH2
Yes
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d) H3CCOOH
Yes
e) H3CCH2OH
Yes
f) H3CCOCH3
No
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Biological Integrity
H-bonding is responsible for the structural
integrity of Biological molecules.
• Protein structures
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• DNA and RNA
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H2O: Nature of Water
Water is a liquid at
room temperature as a
direct consequence of
hydrogen bonding
between adjacent water
molecules.
(Most other molecules with
comparable Molar mass is a
gas at room temperature)
Pure water is a liquid
between 0°C and
100°C.
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Example
Identify the dominant intermolecular forces for each
of the following substances, and select the substance
with the higher boiling point in each pair;
a) MgCl2 or PCl3
ion-dipole
VdW
Higher Bpt
dipole-dipole
VdW
b) CH3OH or CH3CH2OH
H-bond
dipole-dipole
VdW
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H-bond
dipole-dipole
VdW
Higher Bpt
Higher MWt.
b) H3CNH2 or CH3F
H-bond
dipole-dipole
VdW
Higher Bpt
dipole-dipole
VdW
e) Hexane or cyclohexane
VdW
VdW
Higher Bpt
More surface area
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Overview: Recognizing Intermolecular Forces
Flowchart for recognizing the major types of intermolecular
forces. Van der Waal’s forces occur in all instances. The
strength of other forces generally increases proceeding from left
to right
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Summary of Nature’s Forces
Bonding forces are
relatively strong because
they involve larger
charges that are closer
together. Ionic (4004000 kJ/mol) Metallic
(75-1000 kJ/mol)
Intermolecular forces are
relatively weak because
they typically involve
smaller charges that are
farther apart. H-bond
(10-40 kJ/mol) LDF
(0.05 - 40 kJ/mol)
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