Intermolecular Forces

https://njctl.org/video/?v=7HIBk4CKt28

Intermolecular Forces
and Dipoles

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of Contents

The state of a substance at a particular temperature
and pressure depends on two major factors:

The strength of the intermolecular forces that hold molecules
together

The kinetic energy of the molecules

States of Matter & Intermolecular Forces

Molecules have the highest kinetic energy in which state?

Intermolecular Forces

Intermolecular forces are electrostatic forces of attraction or
repulsion that exists between molecules.

The attractions between molecules, intermolecular forces, are
not nearly as strong as the intramolecular attractions that hold
compounds together.

H

Cl

H

Cl

Intermolecular
attraction ( weak)
Covalent bond
(strong)

Without intermolecular forces (IMF's), all substances would
behave like ideal gases...there would be no liquids or solids.

States of Matter & Intermolecular Forces

https://njctl.org/video/?v=LrGgOUUFq3Y

Boiling represents a transition from a liquid to a gas.

To make that transition, molecules in the liquid must break free
of the intermolecular forces that bind them.

Intermolecular Forces & Boiling Points

The kinetic energy of the molecules is proportional to the
temperature: as kinetic energy rises, so does temperature.

The boiling point refers to the temperature at which the
molecules' energy overcomes the intermolecular forces binding
them together.

The higher the boiling point of a substance, the stronger the
intermolecular forces.

Intermolecular Forces & Boiling Points

Water molecules
overcome their
intermolecular forces at
100 C.

There are four types of Intermolecular Forces:

Dipole-dipole interactions

London dispersion forces (LDF's)

Hydrogen bonding

Ion-Dipole

Intermolecular Forces

The first three of those listed above are also called Van der
Waals forces.

Dipole-Dipole Interactions

A dipole is a polar molecule.
Remember what makes a molecule polar?

Bond Type

Non-Polar Covalent

Polar Covalent

Ionic

Electronegativity Difference

very small or zero

about 0.2 to 1.6

above 1.7
(between metal & non-metal)

HF is an example of a polar molecule or dipole. The fluorine end of
the molecule has higher electron density than the hydrogen end.

Dipoles

H F

We use the symbol to designate a dipole (2 poles).
The "+" end is on the more positive end of the molecule and the
arrow points towards the more negative end.

Dipole-Dipole Interactions

Molecules that have permanent
dipoles are attracted to each other.

The positive end of one is attracted to the
negative end of the other and vice-versa.

These forces are only important when the
molecules are close to each other.

+

-

+

-

+

-

+

-

+

-

+
-

The interaction between any
two opposite charges is
attractive ( red)

The interaction between
any two like charges is
repulsive (black)

Only polar molecules will have this type
of Intermolecular Force.

Dipole-Induced Dipole Interactions

When they become close together, the dipole induces an
instantaneous dipole in the nonpolar molecule as its electrons
move towards the positive pole of the dipole molecule.

Dipole-induced dipole interactions emerge when a polar and
nonpolar molecule become close.

e-
e-
e-
e-
e-e-
e-
e-
e-e-
e-

e-
e-
e-

e-

e-
-

+

Dipole

e-e-
e-e-
e-
e-
e-e-

Instantaneous
Dipole

e-e-
e-

e-
e-
e-

e-

e-

-

+

-

-

+

Dipole

Nonpolar

MOVED to here

Dipole-Induced Dipole Interactions

Dipole-induced dipole interactions are present between a polar and
nonpolar molecule. A polar molecule, or permanent dipole, can
induce a temporary, or "instantaneous" dipole.

· These forces are always attractive.

· The strength of these forces increases with the magnitude of the
dipole of the polar molecule.

· The strength also increases with the polarizability of the nonpolar
molecule.

The polarity of a molecule is measured by its dipole moment, μ.
The more polar the molecule, the greater its dipole moment.

The more polar the molecule, the stronger the attraction
between molecules, the higher the boiling point.

Dipole-Dipole Interactions

Substance

Acetonitrile, CH 3 CN 41 3.9 355
Acetaldehyde, CH 3 CHO 44 2.7 294
Methyl chloride, CH3 Cl 50 1.9 249
Dimethyl ether, CH3 OCH3
46

1.3 248
Propane, CH 3 CH 2 CH 3 44 0.1 231

Molecular
Weight
(amu)

Dipole
Moment
μ(D)

Boiling
Point
(K)

Multiple Choice

A chemical bond is

• an electrostatic force of repulsion

• an electrostatic force of attraction

• a physical connection between objects that are touching

Multiple Choice

Which of the following correctly ranks electrostatic forces from weakest to strongest?

• covalent bond, ionic bond, intermolecular forces

• ionic bond, covalent bond, intermolecular forces

• intermolecular forces, covalent bond, ionic bond

• intermolecular forces, ionic bond, covalent bond

Multiple Choice

Which of the following is pointing to an intermolecular bond?

• A

• B

• C

• D

Multiple Choice

The arrow below is pointing to a(n)

• Intramolecular bond

• Ionic bond

• Intermolecular bond

• Both Intramolecular and Ionic

Multiple Choice

Intermolecular forces are strongest in

• Solids

• Liquids

• Gases

• Vapor

Multiple Choice

A substance boils when the kinetic energy of its molecules

• overcomes the intermolecular forces bonding them together

• overcomes the intramolecular forces bonding them together

• reaches 100 Celsius

• none of the above

Multiple Choice

The interaction between water and methane would be a dipole-induced dipole interaction.

• True

• False

Multiple Choice

How many of these substances would have dipole-dipole interactions?

• 0

• 1

• 2

• 3