Chapter 3 - Intermolecular Forces

Introduction to Intermolecular Forces

Types of Forces

• Intramolecular Forces: Hold atoms within a molecule (e.g., Ionic and Covalent Bonds).

  • Types of Covalent Bonds:

    • Non-polar: Equally shared electrons.

    • Polar: Unequally shared electrons.

• Intermolecular Forces: Forces between different molecules based on bond types and shapes.

Ionic Compounds

• Do not exhibit intermolecular forces; held together by ionic bonds.

Overview of Intermolecular Forces

• Definition: Attraction between molecules influencing physical properties (state, melting/boiling points, hardness, solubility etc.).

  • determined by attraction between positive and negative ends

Types of Intermolecular Forces

• Dipole-Dipole Forces: Attraction between polar molecules.

  • the more polar the molecule, the stronger the forces between them.

• Hydrogen Bonds: Strong dipole-dipole forces

  • occurs in molecules when H is bonded to highly EN atoms, very polar bonds ( N, O,F)

  • significant partial charges bc EN difference is high

• London Dispersion Forces (LDF): Weak forces from temporary electron distributions, affecting all molecules. (polar and non-polar)

  • Factors Affecting LDF: temporary dipole on one molecule is strong enough to induce the opposite dipole - opposite dipoles attract.

  • For non polar molecules, the size determined LDF ( the larger the molecule, the more electrons it has, means larger LDF)

• Weakest to Strongest ( LDF - DDF - H-BOND)

Solubility Principle

• “Like Dissolves Like”: Polar with polar, non-polar with non-polar.

Physical Properties and IMF

• Stronger IMFs correlate with solid states and higher melting/boiling points, hard to break, high surface tension.

• *C-H BOND IS ALWAYS NON-POLAR

• down a group: larger molecules have higher boiling point bc of more LDF.

Unique Properties of Water

• High melting/boiling points, lower density as ice, and high surface tension due to hydrogen bonding, high specific heat capacity.