Types of Intermolecular Forces

Overview

• Intermolecular Forces: Electrostatic forces that hold molecules together, weaker than covalent bonds.
• Affect bulk properties like melting and boiling points.
• Depend on the interactions between charged species: attractive and repulsive components.

Types of Intermolecular Forces

Dispersion Forces (London Forces)

• Occur in nonpolar molecules due to temporary fluctuations creating instantaneous dipoles.
• Weak forces falling off rapidly with distance (energy proportional to 1/r^6).
• Larger atoms/molecules (more polarizable) exhibit stronger forces.
• Example: Argon, Xenon can be liquefied due to these forces.

Dipole-Dipole Interactions

• Occur between polar molecules with permanent dipole moments.
• Molecules align such that negative end of one is near positive end of another.
• Weaker than ionic bonds; energy falls off rapidly with distance (1/r^3).
• Example: HCl is a gas because it's held by dipole-dipole interactions.

Hydrogen Bonding

• Strong dipole-dipole interaction between molecules with H bonded to N, O, or F.
• H-bond donor: Molecule with hydrogen attached to electronegative atom.
• H-bond acceptor: Adjacent molecule with a lone pair on electronegative atom.
• Responsible for high boiling point of water and its unique properties like ice being less dense than liquid water.

Ion-Dipole Interactions

• Occur when ionic compounds dissolve in polar substances like water.
• Stronger than dipole-dipole interactions.

Importance of Molecular Shape and Mass

• Molecular shape affects the surface area available for intermolecular forces.
• Boiling points generally increase with larger molar masses due to stronger London forces.
• Shape influences how molecules interact (e.g., linear vs. spherical molecules).

Examples and Exercises

• Arrangement of boiling points based on polarity and molar mass.
• Exercises: Predicting boiling points and identifying types of intermolecular forces.

Summary

• van der Waals Forces: Collective term for dispersion and dipole-dipole forces.
• Transitions between states (solid, liquid, gas) involve changes in intermolecular interactions, not intramolecular.
• Hydrogen bonds are crucial for properties of water and biological molecules.
• Understanding intermolecular forces helps explain physical properties and phase changes.