IMF, Solids, and Liquids
Intermolecular forces (between molecules) vs. Intramolecular forces (within
molecules).
Intermolecular forces are also known as Van der Waal forces.
• The strength of the intermolecular forces can be used to determine whether a
covalent compound exists as a solid, liquid, or gas under standard conditions.
• The strength of the intermolecular forces can also be used to compare melting
and boiling points. The more strongly the molecules are attracted to each other,
the higher the boiling and melting points
• Solids have the strongest intermolecular forces of attraction between their
particles.
• The intermolecular forces of attraction between the molecules of liquids are not
as strong as those found between the particles of a solid.
• Gases have the weakest intermolecular forces of attraction between their
particles.
Weakest to Strongest IMF:
1. London Dispersion Forces: creation of a temporary, or instantaneous, dipole
moment between nonpolar atoms and molecules (only effective when molecules are
close)
• London dispersion forces exist in all covalent molecules, however; they are the
most noticeable between nonpolar molecules and the nonbonding atoms of noble
gases.
• London dispersion forces arise from the motion of valence electrons.
• From the probability distributions of orbitals, it is concluded that the electrons
are evenly distributed around the nucleus. However, at any one instant, the
electron cloud may become distorted as the electrons shift to an unequal
distribution.
• Example: oxygen gas
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2. Dipole-Dipole Forces: Polar molecules attracting each other; Creates a
permanent dipole.
• Dipole-dipole forces of attraction exist between polar molecules.
• Polar molecules contain uneven distributions of charge.
• The negative dipole of one molecule is attracted to the positive dipole of
another molecule.
• Example: hydrochloric acid
3. Hydrogen Bonds:
• Hydrogen Bonding is a special type of dipole-dipole force. Since no electrons
are shared or transferred, hydrogen bonding is not a chemical bond.
• Hydrogen bonding exists between where the very electronegative elements of
nitrogen, oxygen and fluorine are covalently bonded to hydrogen. Hydrogen
bonding occurs between hydrogen and the unbonded electron pairs of nearby N,
O, F molecules
• Examples:
• Hydrogen bonding is about ten times stronger than ordinary dipole-dipole
forces.
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The chart below can help you identify the types of intermolecular forces of attraction
exhibited by a substance. Reminder: London Dispersion Forces are exhibited by all
covalent molecules.
Practice:
1. List the intermolecular forces of attraction in order of increasing strength.
2. What type of intermolecular forces of attraction would be exhibited by each of
the following substances? Justify your answer. (Hint: Draw the Lewis
Structure for the molecule in order to help you determine the polarity of the
molecule.)
a. NH3
c. HI
b. CO2
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d. BeH2
3. Two factors that affect boiling point are the mass of the compound (molar
mass) and the strength of the intermolecular forces of attraction. The
stronger the intermolecular forces of attraction the higher the boiling point.
Examine the table below:
Boiling Points of Halogens
Name
Formula
fluorine
F2
Physical State at
Room
Temperature
gas
chlorine
Cl2
bromine
iodine
Molar Mass
(g/mol)
Boiling Point
(K, at 1 atm)
38.0
85.0
gas
70.9
239.1
Br2
liquid
159.8
331.9
I2
solid
253.8
457.4
1. What relationship exists between the mass of the halogens and the boiling point?
2. Arrange the halogens in order of increasing intermolecular strength of attraction.
Justify your answer.
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