Intermolecular Forces - Chemistry 2e | OpenStax

Learning Objectives

• Describe types of intermolecular forces: dispersion forces, dipole-dipole attractions, hydrogen bonding.
• Identify intermolecular forces based on molecular structure.
• Relate intermolecular forces to physical state changes and associated temperatures.

Kinetic Molecular Theory & Phases

• Solid: Particles packed in regular pattern, vibrate but don't move.
• Liquid: Particles close, irregular arrangement, move past each other.
• Gas: Particles far apart, no regular arrangement, move independently.
• Phases depend on intermolecular forces (IMFs) and kinetic energies (KE).
• Stronger IMFs hold particles close; higher KE overcomes IMFs, increasing particle distance.

Intermolecular vs Intramolecular Forces

• Intramolecular forces: Within molecules (e.g., covalent bonds).
• Intermolecular forces: Between molecules (determine physical properties).

Types of Intermolecular Forces

1. Dispersion Forces (London Forces)

• Present in all condensed phases.
• Result from temporary dipoles due to asymmetric electron distribution.
• Increase with larger and heavier atoms/molecules due to greater polarizability.
• Example: Halogens (F2, Cl2, Br2, I2) show increasing boiling/melting points due to stronger dispersion forces.

2. Dipole-Dipole Attractions

• Occur in polar molecules with permanent dipoles.
• Example: HCl (polar) has higher boiling point than F2 (nonpolar) due to dipole-dipole attractions.

3. Hydrogen Bonding

• Strong dipole-dipole attraction in molecules with H bonded to F, O, or N.
• Stronger than normal dipole-dipole and dispersion forces.
• Example: Water (H2O) has high boiling point due to hydrogen bonds.
• Observed in DNA, where hydrogen bonds stabilize the double helix.

Factors Affecting Intermolecular Forces

• Molecular Shape: Affects surface area for intermolecular contact; more contact = stronger dispersion forces.
  • Example: n-pentane (elongated) has stronger dispersion forces than neopentane (compact).
• Polarizability: Ease of electron cloud distortion; more polarizable = stronger dispersion forces.

Real-world Examples

• Geckos use dispersion forces to stick to surfaces through millions of setae with spatulae that interact with surfaces via van der Waals forces.

Summary

• Intermolecular forces play a crucial role in determining the physical properties and phase transitions of substances.
• Understanding these forces helps explain the behaviors of different materials and their interactions.

References

• OpenStax Chemistry 2e, Chapter 10.1 Intermolecular Forces. Access for free at OpenStax