i ntermolecular forces
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Chemistry 11 Resource: Chang’s Chemistry Chapter 9. I ntermolecular forces. Objectives. Predict whether or not a molecule is polar from its molecular shape and bond polarities. Objectives. - PowerPoint PPT PresentationTRANSCRIPT
INTERMOLECULAR FORCES
Chemistry 11Resource: Chang’s Chemistry Chapter 9
Objectives Predict whether or not a molecule is polar
from its molecular shape and bond polarities.
Objectives Describe the types of intermolecular forces
(attractions between molecules that have temporary dipoles, permanent dipoles or hydrogen bonding) and explain how they arise from the structural features of molecules.
Describe and explain how intermolecular forces affect boiling points of substances.
Polarity Recall the compound HF. How did we predict / know that this was
a polar molecule? Demonstration of symbols.
We can also predict the polarity of a molecule based on its geometry.
Polarity A dipole moment is a quantitative
measure of the polarity of a bond. Molecules without dipole moments are
nonpolar. Molecules with dipole moments are
polar.
Does HF have dipole moments? HCl? H2? O2?
Polarity A dipole moment is a quantitative
measure of the polarity of a bond. Molecules without dipole moments are
nonpolar. Molecules with dipole moments are
polar.
Does HF have dipole moments? HCl? H2? O2?
Polarity Consider the compound BF3. Based on electronegativity, the B-F
bond polar?
Now consider the geometry of BF3.
Is the molecule (as a whole) polar?
Polarity The difference in electronegativities of B-F
is 2.0, so YES, the bond is polar.
The geometry, however is very symmetrical, so NO, the molecule is not polar.
It is important to consider both the electronegativities of the atoms and geometry of the molecule to determine polarity.
Polarity Construct the following molecules and
label them. Are they polar or not? HBr H2S Cl2 CBr4
I2
NO3
NH3
C6H6
Kinetic molecular theory Compare solids, liquids, and gases in
terms of volume, shape, density, compressibility, and molecular motion.
State Volume/ shape
Density Compressibility Motion
solidliquidgas
Intermolecular forces What does intermolecular mean? What does intramolecular mean? Which one is involved in bonding?
Intermolecular forces Intermolecular forces are the attractive
forces between molecules. Also called van der Waals forces.
Are there intermolecular forces between molecules in:solids?liquids?gases?
Why are intermolecular forces important?
Intermolecular forces Molecular geometry and intermolecular
forces are responsible for the gross properties of matter, such as:physical appearancemelting pointboiling point
Intermolecular forces It takes about 41 kJ to vaporize one
mole of water and about 930 kJ to break the two O-H bonds in 1 mole of water.
What does this imply about the strengths of inter- and intramolecular forces?
Intermolecular forces Generally, intermolecular forces are
much weaker than intramolecular forces.
Which physical property (physical appearance, boiling point, or melting point) is a better measure of the strength of intermolecular forces?
Intermolecular forces Boiling point is the best indication of the
intermolecular forces in a substance. Evaporation involves almost completely
overcoming the attractive forces between molecules.
If the boiling point is high, what does it tell you about the intermolecular forces?
Intermolecular forcesTypes of intermolecular forces:1. dipole-dipole2. dipole-induced dipole3. (London) dispersion forces
Intermolecular forces Attractive forces between polar
molecules (molecules with dipole moments).
How do you think the polar molecules would line up?
How would an ion interact with a polar molecule?
What kind of force is responsible?
Dipole-dipole
Intermolecular forces There is a specific dipole-dipole
interaction called the hydrogen bond. This is a misnomer because it is not
actually a bond. It is the interaction between the
hydrogen atom in a polar bond and an electronegative atom.
Dipole-dipole
Intermolecular forces What would happen if you place an ion
near a nonpolar molecule?
Induced dipole - dipole
Intermolecular forces Although a nonpolar molecule may not
possess dipole moments, dipoles can be induced.
Ions and polar molecules can induce dipoles in nonpolar molecules.
The separation of positive and negative charges in a nonpolar molecule is due to the proximity or a polar molecule.
Induced dipole - dipole
Intermolecular forces Does this mean all substances with
ions/polar molecules will have induced dipole – dipole forces?
What factors might affect the likelihood of an induced dipole?
Induced dipole - dipole
Intermolecular forces Because electrons are always moving, it
is possible that a dipole can exist in an atom / nonpolar molecule for an instant.
This is called a temporary dipole. A temporary dipole can induce dipoles in
the surrounding atoms / molecules.
(London) dispersion forces
Intermolecular forces In gases, these temporary dipoles do
not have much impact. At low temperatures, however, they can
cause nonpolar substances to condense.
Why is this so?
(London) dispersion forces
Intermolecular forcesMelting points of similar nonpolar compounds
What trend do you notice? What is responsible for this?(London) dispersion forces
Compound Melting point (C)CH4 -182.5
CF4 -150.0
CCl4 -23.0
CBr4 90.0
CI4 171.0
Intermolecular forcesIdentify the type of intermolecular forces
that exist between: HBr and H2S Cl2 and CBr4
I2 and NO3
NH3 and C6H6
Intermolecular forcesIdentify the type of intermolecular forces
that exist between: HBr and H2S – dipole-dipole Cl2 and CBr4 – dispersion I2 and NO3 – ion-induced and dispersion NH3 and C6H6 – dipole-induced dipole and
dispersion
Intermolecular forcesRelationship between intermolecular
forces and physical propertiesProperty Effect of increased
intermolecular forcesMelting pointBoiling point
ViscositySurface tensionPhase at room
temperature
Homework Water is a very common (and yet very unique)
substance on Earth. What are the properties of water? Explain its
properties using what we’ve learned about molecular geometry and intermolecular forces.
Quiz next class on molecular geometry and intermolecular forces.
Long test on Wednesday, 25 February.Coverage: Atomic theory and bonding