Intermolecular forces

Report
Chemistry 11
Resource: 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
solid
liquid
gas
Volume/
shape
Density
Compressibility Motion
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 appearance
 melting point
 boiling 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 forces
Types of intermolecular forces:
1. dipole-dipole
2. dipole-induced dipole
3. (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 forces
Melting points of similar nonpolar compounds
Compound
CH4
CF4
Melting point (C)
-182.5
-150.0
CCl4
CBr4
CI4
-23.0
90.0
171.0
What trend do you notice? What is
responsible for this?
(London) dispersion forces
Intermolecular forces
Identify the type of intermolecular forces
that exist between:




HBr and H2S
Cl2 and CBr4
I2 and NO3
NH3 and C6H6
Intermolecular forces
Identify 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 forces
Relationship between intermolecular
forces and physical properties
Property
Melting point
Boiling point
Viscosity
Surface tension
Phase at room
temperature
Effect of increased
intermolecular forces
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

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