Molecules: Shapes & Forces (AQA A-Level Chemistry): Revision Notes

1.5.4 Effects of Forces Between Molecules

Intermolecular forces (IMFs) significantly impact the physical properties of substances, such as boiling and melting points, density, and solubility. The strength and type of forces between molecules determine how much energy is required to change their physical state. In this section, we explore how these forces influence properties, particularly in water.

Boiling and Melting Points

The boiling and melting points of a substance are largely determined by the strength of the intermolecular forces present. Stronger forces require more energy to overcome, resulting in higher boiling and melting points.

Trends in Group 6 Hydrides

For the hydrides of group 6 elements ($H₂S, H₂Se, H₂Te$), boiling points increase as you move down the group. This is because:

• Larger atoms have more electrons, leading to stronger van der Waals forces (induced dipole–dipole interactions). These forces increase with atomic size and the number of electrons.
• As you descend the group, the hydrides' boiling points increase due to these stronger dispersion forces.

Exception: Water ($H₂O$)

Water behaves differently from the other hydrides in group 6 due to hydrogen bonding:

• Water should, based on trends, have a low boiling point like $H₂S$ and $H₂Se$.
• However, it exhibits a much higher boiling point because of the strong hydrogen bonds between its molecules.
• Hydrogen bonds require significantly more energy to break compared to van der Waals forces, explaining why water has a much higher boiling point than expected for its size.

Example Comparison

• Hydrogen sulphide ($H₂S$) has a boiling point of around -60°C, but water ($H₂O$) boils at 100°C despite being lighter.
• This is because water forms hydrogen bonds, while $H₂S$ only experiences weaker van der Waals and dipole–dipole forces.

Density of Water and Ice

One of the most unusual properties of water is that ice is less dense than liquid water. This is due to the way hydrogen bonds arrange water molecules in solid form.

Formation of Ice

• As water cools and freezes, it forms a regular 3D lattice structure where each water molecule is hydrogen-bonded to four others.
• These hydrogen bonds are relatively long, causing the water molecules to be further apart in ice than in liquid water. This creates open spaces in the structure, leading to a lower density.
• Consequences: Ice floats on water because it is less dense, a unique feature not commonly found in other substances.

Summary of Effects

Property	Intermolecular Force Involved	Example Substances	Explanation
Boiling Point	Van der Waals forces, Hydrogen bonding	$H₂O$ vs. $H₂S$	Water has higher b.p. due to hydrogen bonding.
Density	Hydrogen bonding	Water ($H₂O$)	Ice has lower density due to open hydrogen-bonded structure.

Understanding these effects helps explain why substances with stronger intermolecular forces, like hydrogen bonds, have higher melting/boiling points and why specific arrangements of molecules, as seen in water, lead to unusual density behaviours.