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