Lecture on Intermolecular Forces

Introduction

• Focus: Intermolecular forces
• Topics covered:
  • Ion-ion interactions
  • Ion-dipole interactions
  • Dipole-dipole interactions, including hydrogen bonds
  • Difference between inter- and intramolecular forces
  • London dispersion forces
  • Van der Waal forces
• Examples of compounds and their interactions

Ion-Ion Interactions

• Example: Sodium ion (Na⁺) and Chloride ion (Cl⁻)
  • Opposite charges attract creating electrostatic force
  • Proportional to charge and inversely related to distance
• Comparison: Calcium (Ca²⁺) and Oxide (O²⁻) vs Sodium Chloride
  • Higher charge = greater interaction
  • As charge increases, electrostatic force increases
  • As size of ions increases, electrostatic force decreases
• Lattice Energy:
  • Proportional to charge magnitude
  • Inversely related to distance
  • Not squared in lattice energy equation
• Example Questions: Melting Point
  • Aluminum Nitride (AlN) vs Magnesium Oxide (MgO)
  • Higher lattice energy = higher melting point
  • Charge magnitude comparison: AlN (3x3=9) vs MgO (2x2=4)

Ion-Dipole Interactions

• Definition: Interaction between an ion and a polar molecule
• Ion vs Dipole:
  • Ion: Unequal number of protons and electrons
  • Dipole: Substance with two charges, one positive, one negative
• Example: Sodium cation (Na⁺) with water (polar molecule)
  • Oxygen's partial negative charge interacts with Na⁺
  • Hydrogen's partial positive charge interacts with chloride ion
• Dissolving Example: Sodium Chloride in water
  • Oxygen surrounds Na⁺, hydrogen surrounds Cl⁻

Dipole-Dipole Interactions

• Between two polar molecules
• Examples:
  • Carbon Monoxide (CO): Oxygen and Carbon attract
  • HBr molecules: Hydrogen attracts to Bromine
• Hydrogen Bonds:
  • Special dipole-dipole interaction
  • Between hydrogen and N, O, or F
  • Intermolecular, not intramolecular

London Dispersion Forces (LDF)

• Also known as Van der Waals Forces
• Present in all molecules, predominant in nonpolar molecules
• Temporary Dipoles:
  • Momentary distribution shifts in electron cloud
  • Induced dipoles occur due to nearby atoms/molecules
• Weakest force among intermolecular forces

Strength of Intermolecular Forces

1. Ion-ion interaction
2. Ion-dipole interaction
3. Hydrogen bond
4. Dipole-dipole interaction
5. London dispersion forces (weakest)

Compound Analysis Examples

• Magnesium Oxide (MgO): Ion-ion interaction
• Potassium Chloride and Water (KCl and H₂O): Ion-dipole interaction
• Methane (CH₄): Nonpolar, LDF only
• Carbon Dioxide (CO₂): Nonpolar, LDF only
• Sulfur Dioxide (SO₂): Polar, dipole-dipole interaction
• Hydrofluoric Acid (HF): Hydrogen bond
• Methanol and Lithium Chloride (CH₃OH and LiCl): Ion-dipole interaction

Boiling Point Comparisons

• Factors: Strength of intermolecular forces, molecule size
• Order of Boiling Points:
  • Iodine > Bromine > Chlorine > Fluorine
  • Methanol > Methane
  • Propanol > Methanol
• Polarity vs Solubility:
  • Methanol more soluble in water than propanol
  • Longer hydrocarbon chains decrease solubility

Structural Influence on Boiling Points

• Pentane vs Neopentane:
  • Straight chain (pentane) has higher boiling point than branched (neopentane)
  • Surface area affects intermolecular interactions

Molecular Comparisons

• Water (H₂O), Hydrogen Sulfide (H₂S), Hydrogen Selenide (H₂Se):
  • Water with hydrogen bonds has highest boiling point
  • Size affects boiling point when comparing H₂S and H₂Se
  • Larger size = more LDF

Summary

• Understand the various types of intermolecular forces and their strengths
• Recognize the influence of molecular structure and size on properties like boiling point and solubility
• Identify intermolecular forces in different compounds using examples