Chemistry 6A Fall 2007
Dr. J. A. Mack
1.9
Br-Br
completely
molecular in
character
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Overall Polarity in molecules:
I
:
I
iodine (I2)
Notice the symmetry of the
molecule:
When divided, the top and
bottom as well as the left and
right are mirror images of one
another.
One also knows the molecule is non
polar because the bond is non polar.
CSUS Chem 6A F07 Dr. Mack
completely ionic
in character
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The polarity of the individual bonds in a molecule will determine
the overall polarity of a molecule.
All heteronuclear diatomic
molecules a non-polar.
∆EN = 2.5 − 2.5 = 0
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somewhere in
between
(polar covalent)
Cs-F
Overall Polarity in molecules:
The polarity of the individual bonds in a molecule will determine
the overall polarity of a molecule.
All homonuclear diatomic
molecules a non-polar.
H-F
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H I
Hydrogen iodide (HI)
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Notice the symmetry of the
molecule:
When divided, the top and
bottom as well as the left and
right are not mirror images of
one another.
One also knows the molecule is
polar because the bond is polar.
∆EN = 2.5 − 2.1 = 0.4
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Molecular Polarity:
Formula
Molar
mass
Boiling
point
Molecular Polarity:
Water
H2O
Methane
CH4
18.02 g/mol
16.04 g/mol
To explain the difference in boiling points, let’s look at the
shapes of the molecules:
CH4
H2O
100oC
-161
oC
Notice that water is “asymmetric” and methane is “symmetric”.
Why is there such a great difference?
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The lack of symmetry means that water is “polar” which allows for
stronger intermolecular attractive forces,
forces thus the higher boiling point.
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Molecular Shape and Molecular Polarity
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Molecular Shape and Molecular Polarity
When a molecule possesses a net dipole moment, it is polar.
Notice also that the molecule’s symmetry can be broken along
either O-H bond axis:
H2O
This side…
The individual O-H
bond polarities result
in a “net” or overall
polarity for the
molecule.
The molecule
is therefore
polar
does not look like this side!
Whenever there exists a line or plane of asymmetry,
the molecule is Polar!
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Molecular Shape and Molecular Polarity
CO2
Symmetry across all bond
axes.
non–polar
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Forces Between Particles:
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IonIon-Ion Forces: Formal charges, strongest IMF
Na+--- Cl− in salt
These are the strongest
forces.
Lead to solids with high
melting temperatures.
NaCl,
NaCl, mp = 800 oC
MgO,
MgO, mp = 2800 oC
Intermolecular forces are much weaker than the bonds that make up
compounds.
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Solvation of ions:
Ion–Dipole Intermolecular Forces (IMF):
+
The strongest IMF:
The partial charge of the dipole is attracted to the formal charge on
the ion.
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Dipole – Dipole Intermolecular Forces (IMF):
Solid Phase
When a cation exists in solution, it is surrounded by the negative dipole ends
of water molecules.
When as anion exists in solution, it is surrounded by the positive dipole ends
of water molecules.
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Dipole – Dipole Intermolecular Forces
Liquid Phase
The greater the polarity of a molecule, the stronger the dipole–
dipole interaction.
The partial charge of the dipoles are attracted to one another.
HF(g) > HCl(g) > HI (g)
The second most strong IMF
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Hydrogen Bonding: Special dipole–dipole interaction
Dipole – Dipole Intermolecular Forces
As the polarity for a given set of molecules with similar molar masses
increases, the boiling point increases.
H–bonding occurs between molecules containing N–H, O–H, or F–H groups
400
350
300
250
B oi l i ng poi nt
200
The H–atoms are “swapped” between the higher electronegative atoms
thus enhancing the attractive forces.
150
100
50
BP
Dipole
0
D i pol e M ome nt
1
molar Mass
2
3
M o l a r M a ss
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Ice, H2O(s) floats because it is less dense than water, H2O(l).
The H–bonds allow the molecules in the liquid phase to to approach closer
than normal for non H–bonding liquids.
This is why water has its maximum density at 4oC.
The water molecules
network with one another.
H–bonding in water brings about a
network of interactions which
explain phenomena such as:
capillary action
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surface tension
CSUS Chem 6A F07 Dr. Mack
why ice floats
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Boiling Points of Simple
HydrogenHydrogen-Containing
Compounds
A consequence
of hydrogen
bonding
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Induced Dipole Forces:
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Dispersion Forces: Weakest of all intermolecular forces.
The forces that hold non–polar molecules together can be explained as
follows:
How can nonnon-polar molecules such as O2 and I2 dissolve in
water?
Attractions arise from instantaneous, temporary dipoles formed due to
electron motions.
The electron cloud of a molecule can be polarized to produce a short lived
dipole that results in an attractive force.
The water dipole INDUCES a dipole in the O2 electric cloud.
Once polarized, the O2 is attracted to additional water molecules.
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Dispersion Forces:
For molecules with the same relative shape, the forces scale with
with
molar mass:
Higher molec.
molec. weight →
larger induced dipoles.
Molecule
Boiling Point (oC)
- 161.5
CH4 (methane)
- 88.6
C2H6 (ethane)
- 42.1
C3H8 (propane)
- 0.5
C4H10 (butane)
C4H10
C3H8
C2H6
Note linear relation between
bp and molar mass.
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As molar mass increases amongst non–polar molecules, bp
and mp increase due to the increased polarizability of the
molecule.
CH4
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Chapter 5: Chemical Reactions:
Reactants
Æ
Compounds
or
Products
New Compounds
Æ
Molecules
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or
Molecules
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