Intermolecular Forces

Chapter 12

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The Kinetic Molecular Theory The Kinetic Molecular Theory explains the forces between molecules and

the energy that they possess. This theory has 3 basic assumptions.

Matter is composed of small particles (molecules). The measure of space that the molecules occupy (volume) is derived from the space in between the molecules and not the space the molecules contain themselves.

The molecules are in constant motion. This motion is different for the 3 states of matter.

Gas - The kinetic energy of the molecule is greater than the attractive force between them, thus they are much farther apart and move freely of each other. Lack strong forces of attraction between molecules.

Liquid - Molecules will flow or glide over one another, but stay toward the bottom of the container. Motion is a bit more random than that of a solid. Significant forces of attraction.

Solid - Molecules are held close to each other by their attractions of charge. They will bend and/or vibrate, but will stay in close proximity. Vibrate around fixed position. Significant forces of attraction.

When the molecules collide with each other, or with the walls of a container, there is no loss of energy.

Gas Liquid Solid

From: http://www.psinvention.com/kinetic.htm

A phase is a homogeneous part of the system in contact with other parts of the system but separated from them by a well-defined boundary.

2 Phases

Solid phase - ice

Liquid phase - water

12.1

Observable Properties

Takes shape and volume of its container

Low density

Very compressible

Takes shape of its container, has definite volume

High density

Difficult to compress

Has definite shape and volume

Density slightly higher than for liquid

Highly incompressible

Gas Liquid Solid

Intermolecular Forces Intramolecular Forces

Forces of attraction between molecules

Much weaker than intramolecular forces

Boiling points and melting points of a substance generally increase with the strength of intermolecular forces

Hold atoms together in a molecule

Intermolecular Forces

12.2

Intermolecular vs Intramolecular

• 41 kJ to vaporize 1 mole of water (inter)

• 930 kJ to break all O-H bonds in 1 mole of water (intra)

Generally, intermolecular forces are much weaker than intramolecular forces.

“Measure” of intermolecular force

boiling point

melting point

Hvap

Hfus

Hsub

Types of Intermolecular Forces

Dipole-dipole

Ion-dipole

Dipole-induced dipole

Ion-induced dipole

Dispersion forces (London forces) (Induced dipole-induced dipole)

Intermolecular Forces

Dipole-Dipole Forces - Attractive forces between polar molecules (molecules with dipole moments)

Orientation of Polar Molecules in a Solid

12.2

• The larger the dipole moment greater the force

Intermolecular Forces

Ion-Dipole Forces - Attractive forces between an ion and a polar molecule

12.2

Ion-Dipole Interaction

Strength of interaction is dependent on:

• charge and size of the ion • magnitude of dipole moment • size of molecule

12.2

Intermolecular ForcesDispersion Forces (London forces) (Induced dipole-induced dipole) - Attractive forces that arise as a result of temporary dipoles induced in atoms or molecules

12.2

ion-induced dipole interaction - Attractive interaction between an ion and induced dipole

dipole-induced dipole interaction - Attractive interaction between a polar molecule and induced dipole

• Attractive interaction in non-polar substances

• Also present in polar molecules

Induced dipole: dipole in a non-polar molecule or atom

• Caused by separation of + and – charges in an atom or non-polar molecule due to proximity of an ion or a polar molecule

What affects if dipole moment is induced?• Charge on ion• Strength of dipole• Polarizability of atom or molecule

Induced Dipoles Interacting With Each Other

12.2

Only type of intermolecular forces present in non-polar molecules are dispersion forces

All molecules (both polar and non-polar) have dispersion forces

**However, while polar molecules may have other types of intermolecular forces present, non-polar molecules have ONLY dispersion forces

Polarizability – ability to distort electron cloud (change shape of electron cloud)

The more electrons in a molecule the more polarizable it is (easier to distort electron cloud) larger molar mass stronger dispersion forces

The more electrons in a molecule the higher the melting point

SO

O

What type(s) of intermolecular forces exist between each of the following molecules?

HBrHBr is a polar molecule: dipole-dipole forces. There are also dispersion forces between HBr molecules.

CH4

CH4 is nonpolar: dispersion forces.

SO2SO2 is a polar molecule: dipole-dipole forces. There are also dispersion forces between SO2 molecules.

12.2

Types of Intermolecular Forces

Dipole-dipole Hydrogen bonding *** (see next slide)

Strong type of dipole-dipole interaction

Dipole-induced dipole

Dispersion forces (Induced dipole-induced

dipole)

These types of intermolecular forces are referred to as van der Waals forces

Other types of Intermolecular Forces that are NOT van der Waals forces

Ion-dipole

Hydrogen bonding Placed in a separate category since only

certain elements can take part in formation of hydrogen bonding

Hydrogen bonding

Special type of dipole-dipole interaction between hydrogen atom in a polar bond (N-H, O-H, or F-H) and an electronegative O, N, or F atom

Strong type of intermolecular attraction

However, hydrogen bond is still much weaker than covalent (intramolecular) bonds

12.2

A H … B A H … Aor

A & B are N, O, or F

Hydrogen bonding

Hydrogen Bond

12.2

Boiling points of NH3, H2O, HF are much higher than expected Why? due to hydrogen bonds

Boiling points

Usually increase as molar mass increases, for similar compounds composed of elements in the same group on periodic table Why?

More electrons in a molecule increase in dispersion forces increase in boiling point

Figure 12.6 in textbook (see next slide)

Hydrogen compounds of Group 4A (increase molar mass increase boiling point) CH4 (lightest compound) lowest boiling point SnH4 (heaviest compound) highest boiling point

Hydrogen compounds of Groups 5A, 6A, 7A (NH3, H2O, HF) do NOT follow this trend In these groups, the lightest compound heaviest

boiling point this is due to hydrogen bonding (NH3, H2O, HF have

stronger intermolecular attractions (hydrogen bonding) than other molecules in the same groups)

Decreasing molar massDecreasing boiling point

12.2