Chemical Bonding Part III
Intermolecular Forces of Attraction (IMFA)
These are the weaker electrostatic forces of attraction that hold molecules together in the
liquid and solid states. These are between molecules. They vary in strength but are generally
weaker than bonds that join atoms in molecules, ions in ionic compounds, or metal atoms in solid
metals. We will consider these in order of weakest to strongest.
I. Van der Waals forces (also known as London dispersion forces)
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these forces are present in all molecules but are most important in nonpolar
molecules
these forces arise as a result of induced dipoles – two molecules approaching each
other and having their electron clouds shift away from their centers because of
electron cloud-electron cloud repulsions. The result is a momentary fluctuating
dipole
the greater the size of the electron cloud, the greater the magnitude of the induced
dipole
the closer the two molecules, the more effective the van der Waals forces
Consider the halogen family. Account for the change in phase as we go down the group:
F2
gas
Cl2
gas
Br2
liquid
I2
solid
Arrange the Noble gases in order of increasing boiling points:
II. Dipole-Dipole Attractions
These are the weak electrostatic attractions that occur in all polar molecules (when
opposite charges line up and form bonds).
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III. Hydrogen Bonds
These can be considered an especially strong dipole-dipole attraction. H-bonding occurs
when hydrogen shares a pair of electrons with a small-size highly electronegative element (i.e.
N, O, or F) resulting in a very polar covalent bond and a very polar molecule. H-bonding, for
example, explains the differences in the boiling points of the hydrogen-Group 16 compounds:
Compound
boiling point (oC)
IMFA
H2O
H2S
H2Se
H2Te
100
-62
-42
-2
H-bond
dipole-dipole
dipole-dipole
dipole-dipole
H-bonding is also very important in biochemistry – it is involved in DNA base pairing and
stabilizing the three-dimensional structure of proteins.
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The IMFA Race
Equal volumes of three liquids will be simultaneously thrown on the board. Based on the
molecular structure of the three liquids, you will predict and observe certain properties.
acetone
water
methanol
Chemical formula
-----------------------------------------------------------------------------------------------------------Structural formula
-----------------------------------------------------------------------------------------------------------Predicted IMFA
-----------------------------------------------------------------------------------------------------------Strength IMFA
-----------------------------------------------------------------------------------------------------------Evaporation rate
-----------------------------------------------------------------------------------------------------------Vapor pressure
-----------------------------------------------------------------------------------------------------------volatility
-----------------------------------------------------------------------------------------------------------Boiling point
-----------------------------------------------------------------------------------------------------------Questions
1. How is the vapor pressure of a liquid related to the IMFA?
2. How is the vapor pressure of a liquid related to its boiling point?
3. Which of the molecules exhibit van der Waals forces?
4. Which of the molecules exhibit H-bonding?
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Bonding and Solubility
The general rule here is that “like dissolves like”.
1.
Polar substances will dissolve other polar substances. For example, methanol (CH3OH)
will dissolve in water. Polar substances have a “polar end” in a molecule, usually an OH or N-H bond, and are termed hydrophilic (“water loving”).
2.
Polar substances will not dissolve nonpolar substances. For example, oil (nonpolar –
long hydrocarbon) will not dissolve in water.
3.
Nonpolar substances will dissolve in other nonpolar substances. For example, oil will
dissolve in hexane (C6H14). Hydrocarbons (compounds consisting of carbon and
hydrogen atoms) are nonpolar. Nonpolar substances are termed hydrophobic (‘water
fearing”).
4.
Polar substances will dissolve ionic solids. When an ionic compound is dissolved in
water, its crystal lattice is destroyed and water molecules surround each ion forming
hydrated ions (called molecule-ion attractions).
5.
Some molecules have both nonpolar (hydrophobic) ends and polar (hydrophilic) ends.
One example is ethanol, CH3CH2OH. Another is soap (discussed later in organic
chemistry)
Demonstration
A large closed test tube containing a strip of chromatography paper and iodine crystals will be
passed around the room. Answer the following:
a) What type of bonds holds the atoms in a molecule of iodine together?
b) What type of bond holds the molecules of iodine together in the solid state?
c) Explain the development of the fingerprints using iodine vapor.
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Demonstration
Aliquots of water and hexane will be added to a test tube. Draw the structural formulas of each
of these compounds and predict their polarities. Several iodine crystals will then be added to the
test tube containing the two liquids. Predict what will happen! Several crystals of copper sulfate
will then be added. Predict what will happen! Make a sketch below. Then some methanol will
be added. Predict what will happen!
H2O
C6H14
I2
CuSO4
CH3OH
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