Types of Intermolecular Force (HSC SSCE Chemistry): Revision Notes

Types of Intermolecular Force

Introduction to intermolecular forces

Intermolecular forces play a crucial role in determining the physical properties of covalent molecular substances. These forces between molecules are particularly important for understanding melting points, boiling points, and whether compounds exist as solids, liquids, or gases at room temperature.

Each type varies in strength and occurs under different molecular conditions. Understanding these forces helps explain why different substances behave differently under similar conditions.

Dipole-dipole forces

What are dipole-dipole forces?

Dipole-dipole forces occur between polar molecules. Because polar molecules have distinct positive and negative regions (called dipoles), they can arrange themselves so that opposite charges attract. The positive end of one molecule is drawn towards the negative end of another molecule, creating an attractive force.

These attractive electrostatic forces between polar molecules are known as dipole-dipole forces. They represent a stronger type of intermolecular attraction than dispersion forces alone.

Example: comparing hydrogen sulphide and methane

Consider hydrogen sulphide ($\text{H}_2\text{S}$) and methane ($\text{CH}_4$):

• Hydrogen sulphide is polar and experiences dipole-dipole forces
• Methane is non-polar and does not have dipole-dipole forces

The presence of dipole-dipole forces in hydrogen sulphide makes its intermolecular forces stronger than those in methane. This difference is reflected in their boiling points:

• Hydrogen sulphide: $-60°\text{C}$
• Methane: $-162°\text{C}$

The higher boiling point of hydrogen sulphide indicates stronger intermolecular forces that require more energy to overcome.

Dispersion forces

Understanding dispersion forces

Dispersion forces exist in all molecules, whether polar or non-polar. They arise from the continuous random motion of electrons within molecules.

At any given moment, the electron cloud around atoms might not be perfectly symmetrical. This creates a temporary dipole - a fleeting separation of charge within the molecule. This temporary dipole can then induce a similar dipole in neighbouring molecules, causing a momentary attraction between them.

Although each individual interaction lasts only a fraction of a second, billions of these temporary attractions form and break constantly. Together, these countless fleeting interactions create a net attractive force between molecules.

Factors affecting dispersion force strength

The strength of dispersion forces depends primarily on the number of electrons in a molecule:

• More electrons → stronger instantaneous dipoles → stronger dispersion forces
• Larger molecules → more electrons → stronger dispersion forces

Key characteristics of dispersion forces

Important points to remember about dispersion forces:

• They are present in all molecules (polar and non-polar)
• They are relatively weak in small molecules like methane ($\text{CH}_4$)
• In non-polar compounds, dispersion forces are the only type of intermolecular force present
• When combined with dipole-dipole forces, they create stronger overall intermolecular attractions

Hydrogen bonding

Evidence for hydrogen bonding

When examining the boiling points of hydrides from Groups 14, 15, and 16, an interesting pattern emerges. For most groups, boiling point increases with period number (as expected from increasing molecular size and electron count). However, water ($\text{H}_2\text{O}$) and ammonia ($\text{NH}_3$) show unexpectedly high boiling points compared to other hydrides in their groups.

This anomaly suggests the presence of an additional, stronger type of intermolecular force beyond simple dipole-dipole interactions and dispersion forces.

What is hydrogen bonding?

Why hydrogen bonding is exceptionally strong

Hydrogen bonding is particularly strong due to two key factors:

1. High electronegativity of O, N, and F

Oxygen, nitrogen, and fluorine are the three most electronegative elements. When bonded to hydrogen, they strongly pull bonding electrons towards themselves. This creates:

• A significant negative charge ($\delta^-$) on the O, N, or F atom
• A significant positive charge ($\delta^+$) on the H atom

2. Small size of hydrogen

The hydrogen atom is exceptionally small - its nucleus is just a bare proton. This allows two molecules to approach very closely, meaning the positively charged H atom can get near the negatively charged O, N, or F atom on an adjacent molecule. This close approach creates a strong electrostatic attraction.

Strength of hydrogen bonds

Hydrogen bonding in water and ammonia

Water and ammonia both exhibit extensive hydrogen bonding networks. In these structures:

• Solid lines represent covalent bonds within molecules
• Dotted lines represent hydrogen bonds between molecules
• Each molecule can form multiple hydrogen bonds with neighbouring molecules

For water:

• Each oxygen atom has two lone pairs of electrons
• Each water molecule can form hydrogen bonds with up to four other water molecules
• This extensive hydrogen bonding network explains water's unusually high boiling point ($100°\text{C}$)

For ammonia:

• Each nitrogen atom has one lone pair of electrons
• Ammonia molecules form hydrogen bonding networks
• This explains ammonia's higher than expected boiling point compared to other Group 15 hydrides

Comparing the three types of intermolecular forces

Summary of intermolecular force types

The three types of intermolecular forces in order of increasing strength are:

1. Dispersion forces - Present in all molecules; weakest type
2. Dipole-dipole forces - Present in polar molecules; intermediate strength
3. Hydrogen bonding - Present when H is bonded to F, O, or N; strongest intermolecular force

Relative strengths

The table below shows the approximate relative strengths of these forces in small molecules, with a typical covalent bond strength set at 100 for comparison:

Type of intermolecular force	Relative strength
Hydrogen bond	10
Dipole-dipole interactions plus dispersion forces	1 to 3
Dispersion forces alone	0.1 to 1

Combinations of forces

It's important to understand that molecules can experience multiple types of intermolecular forces simultaneously:

• Non-polar molecules: Experience only dispersion forces
• Polar molecules (without H-F, H-O, or H-N bonds): Experience both dipole-dipole forces and dispersion forces
• Molecules with hydrogen bonding: Experience hydrogen bonding, dipole-dipole forces, and dispersion forces

The total intermolecular force strength is the combination of all applicable force types. This explains why polar molecules generally have higher boiling points than non-polar molecules of similar size, and why molecules capable of hydrogen bonding have even higher boiling points.

Remember!