3/9/2014
Introduction to Intermolecular Forces
How the forces between molecules
influence physical and chemical properties

Today:
◦ Intermolecular bonds vs.
intramolecular bonds
◦ Review of Coulomb’s Law
◦ London Dispersion Forces
◦ Dipole-Dipole Forces
◦ Hydrogen Bonding
◦ How these forces influence:
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Challenge Problem Set 6
DUE tonight at 11 pm
Next Meeting
◦ Please read Chapter 5,
Sections 5.2-5.5, pp. 176-187
before Wednesday
Boiling Points
Melting Points
Solubility
Visoscity
Surface Tension
iClicker Participation Question:
Determining Molecular Polarity based on Bond Polarity & Shape
Which molecule below is the MOST POLAR?
Element Electronegativity
C
2.5
H
2.1
Cl
3.0
A.
B.
C.
D. All are equally
NONPOLAR
E. Cannot determine from information provided
Intermolecular Forces (IMFs):
More likely to
exist as a solid
Higher Viscosity
Higher
Surface Tension
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3/9/2014
INTERmolecular Forces vs. INTRAmolecular Bonds
• Intermolecular Forces: the interactions BETWEEN
neighboring molecules
• Intramolecular Bonds: the connections WITHIN
molecules that keep atoms connected together
Intermolecular forces are typically small in
comparison to the strength of intramolecular bonds.
Intermolecular Forces
•
Intramolecular forces must be partially broken to change phase
(to go from solid to liquid or liquid to gas)
Boiling Point: temperature at which
a liquid is converted into a gas
A high boiling point means it takes a lot of energy to
break the intermolecular forces between molecules
Coulomb’s Law:
q2 -
q1 +
The larger the charges,
The stronger the electrostatic interaction
+2
q1
-2
q2
The smaller the distance,
The stronger the electrostatic interaction
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3/9/2014
• Ability distort and shift the
electron distribution within
a molecule or atom.
Br—Br
δ- δ+
London Dispersion Forces (LDFs)
• Ability to distort and shift
electron distribution within
a molecule or atom.
• Scales with volume
occupied by electrons
Cl2 (g)
Br2 (l)
• LDFs increase as total
number of electrons &
molecular surface area
increase.
I2 (s)
iClicker Participation Question:
Structure-Property Relationships
Match the substances at the front of the room with the most likely
hydrocarbon structures below.
A.
1 = CH3(CH2)5CH3, 2 = C40H82, 3 = C20H42
B.
2 = CH3(CH2)5CH3, 1 = C40H82, 3 = C20H42
C.
3 = CH3(CH2)5CH3, 2 = C40H82, 1 = C20H42
D.
2 = CH3(CH2)5CH3, 3 = C40H82, 1 = C20H42
E.
All of these substances would have the same formula
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3/9/2014
Refining Crude Oil: A cornerstone of our current economy
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Light hydrocarbons with few carbon atoms boil easier (they have weaker IMFs),
and move farther up a distillation column.
HEAVY hydrocarbons with MANY carbon atoms have a harder time melting or
boiling (they have STONGER IMFs)
Plastics are long chains of
carbon atoms strung together
polyethylene
Intermolecular Forces in Polar Molecules
Dipole Moment
Increasing Electronegativity
Decreasing Electronegativity

H
2.1
δ+
δ-
Cl
3.0
Measures molecular polarity
by quantifying the degree of charge separation over the
distance between charges
Difference in Electronegativity (ΔEN) = 0.5-1.7
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Down a group:
Boiling point
increases with
increasing
molecular
weight
Water is a liquid under standard
conditions because it has STRONG
intermolecular forces (hydrogen bonds)
keeping H2O molecules held together
If H2O was LINEAR, it would be
NONPOLAR and would likely be a
GAS under standard conditions
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3/9/2014
Water is HIGHLY Polar:
Strongly interacts with charged particles
Hydrogen Bonding:
Transpiration in Plants
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3/9/2014
Hydrogen Bonding Network in Kevlar
δ+
δ-
δ-
δ+
δδ-
δ-
δ+
δ+
δ-
δ-
δ+
δ+
δ+
δδ+
δδ-
δ-
δ+
δ+
δ+
δ-
δ+
Hydrogen Bonding Network in Kevlar
δ-
δ+
Hydrogen
Carbon
Nitrogen
Oxygen
Animation produced with Camtasia & Jmol Model from ChemIT
iClicker Participation Question:
Predicting Boiling Points
Which molecule would have the highest boiling point?
A. CH4 → NONPOLAR
B. CO2 → NONPOLAR
C. NH3 → N—H Bonds: POLAR
D. N2 → NONPOLAR
E. CH3CH2CH3 → NONPOLAR
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3/9/2014
Solubility: “Like dissolves Like”
Substances of similar
polarity mix together.
Substances of different
polarity don’t easily mix.
iClicker Participation Question:
Intermolecular Forces and Solubility
Which substance would be the MOST SOLUBLE in water?
A.
B.
C.
D.
E. None of these substances
would be soluble in water
iClicker Participation Question:
Solubility as related to Molecular Polarity
Which substance would be best suited to repel water from a
surface?
A.
B.
C.
D.
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3/9/2014
Predicting Reactions with Polarity
Structure-Property Relationships
Ionic Compounds
Covalent Compounds
Strong Inter-Ionic
Forces
0.05 – 40 kJ/mol
London Forces:
Comparison
of Binding Forces
Dipole-Dipole Forces:
5 – 25 kJ/mol
Polar Molecules
Hydrogen Bonding:
10 – 40 kJ/mol
INTRAMOLECULAR
Covalent Bonds:
100 – 1100 kJ/mol
Strength of Interaction
IONIC Bonds:
400 – 4000 kJ/mol
Applying Coulomb’s Law to understand
how molecules interact and react
Non-Polar
Molecules
Hydrogen
Bonding
Dipole-Dipole
Forces
London
Forces
London
Forces
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