Chapter 10
Liquids, Solids, and Phase
Change
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Why Do Solids and Liquids Exist?
• Intermolecular Forces
gases – Little to no attraction between
molecules
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Why Do Solids and Liquids Exist?
• Intermolecular Forces
liquids – Fairly strong forces between a
few molecules
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Why Do Solids and Liquids Exist?
• Intermolecular Forces
solids – Strong forces prevent large
amount of motion between molecules/atoms
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What makes a molecule polar?
• Polar molecules are said to possess a
permanent dipole moment.
• Dipole moment measured in units of
Debye (Q x r ) [charge x distance]
1 D = 3.336 x 10-30 coulomb meters
Prof. Peter Debye
Noble Prize 1936
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Dr. Prushan, enough physics, what
does it mean?
• OK… Let’s go back to Lewis Structures
and VSEPR (Yes, there was a reason you needed this)
• Examples:
Draw Lewis Structures and Predict the
Geometry of : NH3, H2O, CH4,
CH3Cl,CHCl3 CO2, CCl4
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Examples
H N H
H
H
NH3
O
O C O
H
H2O
Trigonal Pyramidal
CO2
Bent
Linear
Tetrahedral
H
Cl
H
H C H
H
C
C
CH4
H
H
H
CH3Cl
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Cl
Cl
CHCl3
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Cl
Cl
Cl
C
Cl
CCl4
Cl
Polar or Not Polar That is the Question?
H N H
H
H
NH3
O
H
H2O
Trigonal Pyramidal
Bent
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O C O
CO2
Linear
Polar or Not Polar That is the Question?
H N H
H
H
NH3
O C O
CO2
Bent
m = 1.47 D
d+
H
H2O
Trigonal Pyramidal
d-
O
Linear
m = 1.85 D
m=0D
dd+
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d+
d-
Polar or Not Polar That is the Question?
H
Cl
H
H C H
H
C
C
CH4
H
H
H
CH3Cl
Cl
Cl
Cl
CHCl3
d-
Cl
C
Cl
CCl4
d+
d-
d+
m=0D
Cl
m = 1.87 D
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m = 1.90 D
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m=0D
Cl
Types of Intermolecular Forces
• 4 main types of intermolecular forces
(1) London (Dispersion) Forces
(2) Dipole-Dipole Forces
(3) Ion-Dipole Forces
(4) Hydrogen Bonds
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Increasing
Strength
London (Dispersion) Forces
Weakest of the IM forces. All molecules have
Dispersion forces
Result of interactions between instantaneous dipole
moments.
Prof. Fritz London
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Dipole-Dipole Forces
• Result of permanent dipole moments in
polar molecules
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Ion-Dipole Forces
The force which dissolves ionic compounds
A result of interactions between ions and solvent
molecules
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Hydrogen Bonds
• Strongest of the intermolecular forces
• NOT a chemical bond
• MUST contain hydrogen covalently
bonded to F, Cl, O or N.
• Essential for Life!
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Hydrogen Bonds
If the boiling point of CO2 is , and CS2 is
Molecules with larger molecular weights
have higher boiling points.
So why is H2O a liquid, but H2S is a gas?
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BONDS… Hydrogen Bonds
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Why are Snow Flakes Hexagons?
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Hydrogen Bonds and Life
• Life Depends of H-bonds
• The “Stuff” of Life relies on them to stay
together
Proteins, Enzymes
Nucleic Acids (DNA, RNA)
Sugar-phosphate Backbond and
Base Pairs
Amino Acids
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Amino acids bond together to make polypeptides
The polypeptides fold to make the secondary structure of proteins
H
O
H
H
C
R
N
C
H
O
H
N
H
O
C
H
N
N
H
C
O
H
R
R
O
N
O
R
R
H
C
N
H
O
N
C
H
H
C
O
H
C
R
C
N
H
How and Why does the alpha structure form??
Linus Pauling Discovered How, in 1948
While in bed with a cold…. Lets let him describe how…
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R
C
H
Now let’s try that…
Come on down and get a polypeptide
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Hydrogen Bonds are also important
in DNA
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NH 2
N
Adenine
Which goes with which ???
N
N
H
N
A–T
O
Thymine
NH
N
H
G–C
O
Right??? BUT WHY????
O
N
NH
Guanine
N
H
N
NH 2
Come on down and get some base pairs
NH 2
N
N
H
O
Cytosine
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The Guanine-Cytosine (GC) Base Pair
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The Guanine-Cytosine (GC) Base Pair
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Effect of Intermolecular forces on melting and
boiling points of molecular covalent substances
Why does HF have the highest BP?
Molecular wts.
132
20
77
81
32
16
128
37
Molecular wts.
Increase only due to increase in MW,
Since all are non-polar
BONDS…
CHM 112 Summer 2007
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Hydrogen Bonds