Lecture on Intermolecular Forces

Overview

• Focus on Intermolecular Forces (IMFs)
  • Ion-Ion Interactions
  • Ion-Dipole Interactions
  • Dipole-Dipole Interactions (including Hydrogen Bonds)
  • London Dispersion Forces and Van der Waals Forces
• Difference between Inter- and Intramolecular Forces
• Examples and identification of interactions in compounds

Ion-Ion Interactions

• Example: Sodium ion (Na⁺) and Chloride ion (Cl⁻)
  • Opposite charges attract via electrostatic forces
  • Electrostatic force ∝ charge magnitude and inversely ∝ distance
• Higher charge → Greater interaction strength
  • Example: Ca²⁺ and O²⁻ have stronger interactions than Na⁺ and Cl⁻
  • Lattice energy related to charge magnitude and distance (not squared in equation)

Ion-Dipole Interactions

• Definition of Ions and Dipoles
  • Ion: Unequal protons and electrons (e.g., Al³⁺)
  • Dipole: Molecule with two opposite charges (e.g., CO)
• Ion-Dipole Example: Sodium cation and water (H₂O)
  • Water’s polarity: Oxygen (partial negative), Hydrogen (partial positive)
  • Interaction between ion (e.g., Na⁺) and polar molecule (e.g., H₂O)

Dipole-Dipole Interactions

• Interaction between two polar molecules (e.g., CO molecules)
• Hydrogen Bonds as a Special Type
  • Occurs between H and N, O, or F
  • Example in water (H₂O): Between different H₂O molecules
  • Hydrogen bond is intermolecular, while covalent bond is intramolecular

London Dispersion Forces (Van der Waals Forces)

• Present in all molecules, significant in nonpolar molecules
• Arise from temporary induced dipoles (e.g., neon atoms)
• Weakest type of intermolecular force

Strength of Intermolecular Forces

1. Ion-Ion Interaction
2. Ion-Dipole Interaction
3. Hydrogen Bonds
4. Dipole-Dipole Interactions
5. London Dispersion Forces (weakest)

Examples and Problem Solving

• Examples:
  • Magnesium Oxide: Ion-Ion Interaction
  • Potassium Chloride and Water: Ion-Dipole Interaction
  • Methane: London Dispersion Forces (nonpolar)
  • Carbon Dioxide: London Dispersion Forces (nonpolar)
  • Sulfur Dioxide: Dipole-Dipole Interaction
  • Hydrofluoric Acid: Hydrogen Bonds
• Boiling Point Comparisons:
  • Iodine (I₂) vs. Bromine (Br₂): Iodine has higher boiling point due to size
  • CH₃OH vs. CH₄: Methanol has higher boiling point due to hydrogen bonds
  • Propanol vs. Methanol: Propanol has higher boiling point due to size
• Solubility and Volatility:
  • Methanol more soluble in water than propanol
  • Neopentane more volatile than pentane due to branching
  • H₂O vs. H₂S vs. H₂Se: Water highest boiling point due to hydrogen bonds

Compound Ranking by Boiling Point

• HF, HBr, HI, HCl
  • HF highest due to hydrogen bonds
  • Ranked by size: HF > HI > HBr > HCl

Summary

• Understanding intermolecular forces aids in predicting physical properties such as boiling points, melting points, and solubility.