Intermolecular Forces and Physical Properties: Study Notes

Intermolecular Forces & Physical Properties

Introduction

Physical properties of substances are determined by the nature and strength of the forces that exist between molecules, known as intermolecular forces. These forces influence melting point, boiling point, vapor pressure, viscosity, and surface tension.

Types of Intermolecular Forces

• London Dispersion Forces: Present in all molecules, especially nonpolar ones. Weakest type.

• Dipole-Dipole Forces: Occur between polar molecules due to permanent dipoles.

• Hydrogen Bonding: Strongest type, occurs when H is bonded to N, O, or F.

Direct Relationships

Stronger intermolecular forces lead to higher values of certain physical properties:

• Melting Point: The temperature at which a solid becomes a liquid. Higher intermolecular forces result in higher melting points.

• Boiling Point: The temperature at which a liquid becomes a gas. Stronger forces mean higher boiling points.

• Surface Tension: The energy required to increase the surface area of a liquid. Stronger forces increase surface tension.

• Viscosity: Resistance to flow. Stronger forces increase viscosity.

Example Table: Relationship Between Intermolecular Forces and Physical Properties

Property	Effect of Stronger Intermolecular Forces
Melting Point	Increases
Boiling Point	Increases
Surface Tension	Increases
Viscosity	Increases
Vapor Pressure	Decreases

Indirect Relationships

Some properties decrease as intermolecular forces increase:

• Vapor Pressure: The pressure exerted by a vapor in equilibrium with its liquid phase. Stronger intermolecular forces result in lower vapor pressure.

Practice Questions & Examples

• Example: Which compound has the highest melting point? Answer: CaCl2 (ionic compound, strong forces)

• Example: Which substance has the highest vapor pressure? Answer: H2S (weakest intermolecular forces)

• Practice: Which has the lowest boiling point? Answer: CH4 (nonpolar, weak forces)

• Practice: Which molecules cause lowest viscosity? Answer: Small, nonpolar molecules

• Practice: Which has the highest surface tension? Answer: H2O (hydrogen bonding)

Key Equations

• Vapor Pressure and Temperature (Clausius-Clapeyron Equation): $\ln P = -\frac{\Delta H_{vap}}{R} \left( \frac{1}{T} \right) + C$

Summary Table: Intermolecular Forces and Example Compounds

Type of Force	Example Compound	Relative Strength
London Dispersion	CH4	Weak
Dipole-Dipole	H2S	Moderate
Hydrogen Bonding	H2O	Strong
Ionic	CaCl2	Very Strong

Additional info: These concepts are essential for understanding the behavior of liquids and solids, and predicting physical properties based on molecular structure.