10/29/2014
Introduction to Intermolecular Forces
How the forces between molecules
influence physical and chemical properties
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Today:
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◦ Intermolecular bonds vs.
intramolecular bonds
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◦ Coulomb’s Law
◦ London Dispersion Forces
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◦ Dipole-Dipole Forces
◦ Hydrogen Bonding
◦ How do these forces influence: 
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Boiling Points
Melting Points
Solubilities
Visoscities
Surface Tension
Problem Set 5 DUE tonight
at 11 pm
Problem Set 6 DUE next
Monday, Nov. 3rd at 11 pm
Midterm Exam 2 Study
Guide on CANVAS
REMINDER: Poster
Presentation Topics
DUE next Monday
Valence Shell Electron Pair Repulsion (VSEPR) Theory:
Molecules and polyatomic ions take on particular shapes based on
the number of electron regions located around a central atom:
• Valence Shell (VS…): Consider ONLY the valence electrons:
• Lone pair electrons & bonding electrons are all treated the
same:
• A single bond counts the same as a double or triple bond
• Lone pairs are considered the same as bonding electrons
• Electron Pair Replusions (…EPR): electrons are negative charges
that naturally REPEL from other electrons. These repulsive forces
between electrons determines molecular shape.
TRIGONAL PLANAR Molecular Geometry
Electron Dot
Structure
Total # of
ELECTRON
Regions
# of
BONDING
Regions
# of
LONE PAIRS
3
3
0
120°
Trigonal planar
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10/29/2014
TRIGONAL PLANAR Electron Geometry
BENT MOLECULAR GEOMETRY
Electron Dot
Structure
Total # of
ELECTRON
Regions
# of
BONDING
Regions
# of
LONE PAIRS
3
2
1
The Total # of Electron Regions
determines the Electron
Geometry:
3 REGIONS = TRIGONAL
PLANAR Electron Geometry
TRIGONAL PLANAR Electron Geometry
TRIGONAL PLANAR MOLECULAR
GEOMETRY
When lone pairs are present on the central atom, the
molecular geometry is DIFFERENT from the electron
geometry.
• Molecular Geometry only considers the
positions of the atoms
TRIGONAL PLANAR Electron Geometry
BENT MOLECULAR GEOMETRY
4 REGIONS = TETRAHEDRAL Electron Geometry
Electron Dot
Structure
Total # of
ELECTRON
Regions
# of
BONDING
Regions
# of
LONE PAIRS
4
4
0
109.5°
Tetrahedral
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4 REGIONS = TETRAHEDRAL Electron Geometry
ONE LONE PAIR = TRIGONAL PYRAMIDAL MOLECULAR GEOMETRY
Electron Dot
Structure
Total # of
ELECTRON
Regions
# of
BONDING
Regions
# of
LONE PAIRS
4
3
1
109.5°
Tetrahedral
Trigonal Pyramid
4 REGIONS = TETRAHEDRAL Electron Geometry
TWO LONE PAIRS = BENT MOLECULAR GEOMETRY
Electron Dot
Structure
Total # of
ELECTRON
Regions
# of
BONDING
Regions
# of
LONE PAIRS
4
2
2
109.5°
Tetrahedral
Bent
Is Carbon Tetrachloride POLAR or NONPOLAR?
Tetrahedral
ΔEN:
3.0-2.5 = 0.5
1. First write the Lewis dot structure.
2. Use a table of electronegativities to calculate ΔEN for all bonds
present.
• If ΔEN is less than or equal to 0.4, the bonds are NONPOLAR.
STOP HERE. The molecule must also be NONPOLAR.
• If ΔEN is greater than 0.4, polar bonds are present & the
molecule MIGHT be polar (depending on molecular shape).
3. Next, use the dot structure to count the number of electron
regions & determine the MOLECULAR GEOMETRY.
• If the molecule is ASYMMETRIC, it is POLAR.
• If the molecule is SYMMETRIC, it is NONPOLAR.
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10/29/2014
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
• 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)
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10/29/2014
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-Dipole Force:
STRONGER than London Dispersion Forces
δ+
HCl:
HF:
NaCl:
δ-
Boiling Point Difference in Electronegativity
-85.0 oC
3.0 – 2.1 = 0.9
Increasing
19.5 oC
4.0 – 2.1 = 1.9
Charge
o
801 C
3.0 – 0.9 = 2.1
Separation
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Intermolecular Forces
•
Intermolecular 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
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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10/29/2014
Hydrogen Bonding Network in Kevlar
δ+
δ-
δ-
δ+
δδ-
δ-
δ+
δ+
δ-
δ-
δ+
δ+
δ-
δ+
δδ+
δδ-
δ+
δ+
δ+
δ-
δ+
Hydrogen Bonding Network in Kevlar
δ-
δ+
Hydrogen
Carbon
Nitrogen
Oxygen
Animation produced with Camtasia & Jmol Model from ChemIT
Hydrogen Bonding:
Transpiration in Plants
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10/29/2014
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
Solubility: “Like dissolves Like”
Substances of similar
polarity mix together.
Substances of different
polarity don’t easily mix.
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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