Lecture on Intermolecular Forces

Overview

• Focus on intermolecular forces
  • Ion-ion interactions
  • Ion-dipole interactions
  • Dipole-dipole interactions (including hydrogen bonds)
  • London dispersion forces (Van der Waals forces)
• Difference between intermolecular and intramolecular forces
• Examples of compounds and their interactions

Ion-Ion Interactions

• Electrostatic attraction between oppositely charged ions
• Proportional to charge; inversely proportional to distance
• Calcium oxide (Ca²⁺ and O²⁻) interactions stronger than sodium chloride (Na⁺ and Cl⁻) due to higher charge
• Lattice energy is related to magnitude of charges and distance

Ion-Dipole Interactions

• Interactions between an ion and a polar molecule
• Example: Sodium cation (Na⁺) and water (H₂O)
  • Oxygen in water has partial negative charge
  • Hydrogen has partial positive charge
  • Results in ion-dipole interaction

Dipole-Dipole Interactions

• Occur between two polar molecules
• Example: Carbon monoxide (CO)
  • Oxygen has partial negative charge
  • Carbon has partial positive charge

Hydrogen Bonds

• Specialized dipole-dipole interaction
• Occurs between hydrogen and nitrogen, oxygen, or fluorine
  • Example: Water (H₂O)
  • Strong due to high electronegativity and small atomic size
  • Intermolecular (between molecules), not intramolecular (within a molecule)

London Dispersion Forces (Van der Waals Forces)

• Present in all molecules, significant in non-polar molecules
• Result from temporary dipoles
• Weaker than typical dipole-dipole interactions
• Example: Neon atoms with temporary dipoles inducing interactions

Strength of Intermolecular Forces

1. Ion-ion interactions – strongest
2. Ion-dipole interactions
3. Hydrogen bonds
4. Dipole-dipole interactions
5. London dispersion forces – weakest

Examples and Analysis

• Magnesium Oxide (MgO): Ion-ion interactions
• Potassium Chloride (KCl) and Water: Ion-dipole interactions
• Methane (CH₄): Only London dispersion forces (non-polar)
• Carbon Dioxide (CO₂): Non-polar, London dispersion forces
• Sulfur Dioxide (SO₂): Polar, dipole-dipole interactions

Boiling Points and Molecular Size

• Stronger intermolecular forces lead to higher boiling points.
• Larger molecules have higher boiling points due to increased London dispersion forces.
• Comparisons:
  • Iodine (I₂) vs. Bromine (Br₂): Iodine has higher boiling point.
  • Methanol (CH₃OH) vs. Methane (CH₄): Methanol has higher boiling point due to hydrogen bonds.
  • Propanol (CH₃CH₂CH₂OH) vs. Methanol (CH₃OH): Propanol has higher boiling point due to larger size.

Solubility

• Polar substances dissolve better in polar solvents like water.
• Methanol is more soluble in water than propanol due to smaller non-polar region.

Structural Influence on Boiling Point

• Straight-chain alkanes have higher boiling points than branched alkanes.
• Pentane (C₅H₁₂) vs. Neopentane (isomer): Pentane has higher boiling point.

Summary

• Understanding of intermolecular forces is critical for predicting physical properties like boiling points and solubility.
• Larger atomic size and increased molecular interactions lead to stronger intermolecular forces.
• Specialized interactions like hydrogen bonding have significant impact on physical characteristics.