Lecture Notes on Intermolecular Forces

Overview

• Focus on intermolecular forces (IMFs)
• Types of interactions: ion-ion, ion-dipole, dipole-dipole, hydrogen bonds
• Differences between intermolecular vs intramolecular forces
• London dispersion forces (LDF) and van der Waals forces
• Examples of compounds and their interactions

Ion-Ion Interactions

• Opposite charges attract via electrostatic forces
• Force proportional to charge, inversely proportional to distance
• Greater charges result in stronger interactions (e.g., Ca²⁺ and O²⁻ vs Na⁺ and Cl⁻)
• Lattice energy proportional to charge, inversely related to distance
• Example: AlN has higher melting point than MgO due to greater lattice energy

Ion-Dipole Interactions

• Ion: unequal number of protons and electrons
• Dipole: molecule with positive and negative ends (e.g., carbon monoxide)
• Example: Na⁺ in water, where water's polar nature facilitates ion-dipole interaction
• Dissolving NaCl in water demonstrates ion-dipole interactions

Dipole-Dipole Interactions

• Occur between two polar molecules
• Example: interaction between two CO molecules (Oxygen = partial negative, Carbon = partial positive)
• Hydrogen bonds: special type of dipole-dipole (e.g., in water)
  • Hydrogen bonds occur between H and N, O, or F

Inter vs. Intra Molecular Forces

• Intermolecular: forces between molecules (hydrogen bond)
• Intramolecular: forces within a molecule (covalent bond)

London Dispersion Forces (LDF)

• Present in all molecules, more significant in non-polar molecules
• Caused by temporary dipoles due to electron cloud distortion
• Weaker than dipole-dipole interactions

Strength of Intermolecular Forces

• Strongest: Ion-Ion
• Ion-Dipole
• Hydrogen Bonds (specialized dipole-dipole)
• Dipole-Dipole
• Weakest: London Dispersion Forces

Examples and Applications

• Magnesium Oxide (MgO): Ion-Ion interaction
• KCl in Water: Ion-Dipole interaction
• Methane (CH₄): LDF, as it is non-polar
• Carbon Dioxide (CO₂): LDF, linear structure results in no net dipole
• Sulfur Dioxide (SO₂): Dipole-Dipole interaction due to bent shape

Boiling Points and Molecular Size

• Larger molecules with more electrons have higher London dispersion forces
• Straight-chain alkanes have higher boiling points than branched alkanes

Solubility and Polarity

• Polar substances dissolve in water; longer hydrocarbon chains decrease solubility

Boiling Point Ranking

• Methanol vs Methane: Methanol has higher boiling point due to hydrogen bonds
• Propanol vs Methanol: Larger size of propanol results in higher boiling point
• HF, HBr, HI, HCl: HF has highest boiling point due to hydrogen bonds, others ranked by size

Volatility and Vapor Pressure

• High boiling points correlate with low vapor pressures and less volatility
• Methanol more volatile than propanol due to lower boiling point

These notes cover the essential points discussed in the lecture on intermolecular forces, including types of interactions, their significance, examples, and implications on physical properties like boiling points and solubility.