Unit 3 Section 2: Intermolecular Forces and Physical Properties

Introduction

• Presenter: Jeremy Krug
• Topic: How intermolecular forces affect the physical properties of liquids and solids.

Vapor Pressure

• Definition: Pressure exerted by a vapor on its liquid in a closed container.
• Temperature Relationship:
  • Vapor pressure increases with temperature.
  • More evaporation at higher temperatures leads to more vapor and higher pressure.
• Intermolecular Forces:
  • Weaker intermolecular forces lead to higher vapor pressures.
  • Stronger intermolecular forces result in lower vapor pressures.

Boiling Point

• Weaker Intermolecular Forces: Lower boiling points; easier to separate molecules into the gaseous phase.
• Graph Analysis:
  • X-axis: Temperature in °C
  • Y-axis: Vapor pressure in kPa
  • Example: Estimating vapor pressure and boiling point changes through graph reading.

Solids

Ionic Solids

• Composition: Ionic compounds in a crystal lattice (e.g., sodium chloride).
• Properties:
  • Brittle due to electrostatic attractions.
  • Conduct electricity when dissolved or melted due to the presence of charged particles.

Molecular Solids

• Composition: Covalently bonded compounds (e.g., ice).
• Properties:
  • Lower melting and boiling points compared to ionic solids.

Covalent Network Solids

• Features:
  • Repeating network of covalent bonds (e.g., diamond, graphite, silicon dioxide).
  • Extremely strong due to multi-directional bonding.
• Examples:
  • Diamond: Hardest naturally occurring material, high melting point.
  • Graphite: Strong in sheets, weak between sheets, used in pencils.
  • Silicon Dioxide: Very hard, high boiling point.

Comparison of Carbon Dioxide and Silicon Dioxide

• Carbon Dioxide:
  • Low boiling point.
  • Composed of individual molecules with weak London dispersion forces.
• Silicon Dioxide:
  • High boiling point.
  • Covalent Network solid with strong multi-directional bonds.

Conclusion

• Overview of how intermolecular forces impact liquid and solid properties.
• Upcoming: Continuation to Section 3.3.