Chemical Bonding: Intermolecular Forces

Introduction

• Focus on intermolecular forces (IMF), not intramolecular forces.
• Previously studied bonding types: covalent, ionic, polar covalent, nonpolar covalent, and metallic.

Key Definitions

• Intermolecular Force: Weak attraction between molecules, ions, or atoms of noble gases.
• Weaker than intramolecular (covalent/ionic) forces.
• Less energy is needed to separate molecules compared to breaking bonds within a molecule.

Importance of Intermolecular Forces

• Affects phase changes (solid, liquid, gas) without changing chemical properties.
• Determines the amount of energy required for phase changes.

Phase Change Examples

• Water (H₂O): Phase change from solid to gas involves breaking interatomic attractions.
• Oxygen (O₂), Neon (Ne), and Water (H₂O) simulations show the effect of heating on molecular behavior:
  • Solids become liquids as intermolecular forces weaken with heat.
  • Liquids become gases when molecules gain enough energy to overcome forces.

Energy and Temperature

• Oxygen: solid at -240°C, liquids at -209°C, gas at -187°C.
• Neon: solid at -259°C, gas at -230°C.
• Water: solid at -127°C, liquid at 0°C, gas at higher temperatures.

Boiling Points and Intermolecular Forces

• Boiling points indicate the strength of intermolecular forces.
• Example substances:
  • Water: Boiling point 100°C (strong IMF).
  • Hydrogen Chloride (HCl): Boiling point -85°C.
  • Fluorine (F₂): Boiling point -188°C.
  • Helium (He): Boiling point -269°C (weak IMF).

Types of Intermolecular Forces (Van der Waals Forces)

1. London Dispersion Forces (Momentary Dipole-Induced Dipole):
   • Present in all molecules, significant in nonpolar ones.
2. Dipole-Dipole Forces:
   • Occur in polar molecules.
3. Hydrogen Bonds:
   • Strong type of dipole-dipole force in molecules with H bonded to N, O, or F.

Conclusion

• Understanding IMF is crucial for predicting physical properties like boiling/melting points.
• Knowledge of molecular polarity and shape helps in identifying the type of IMF present.
• Further videos and notes will elaborate on individual intermolecular forces.