LESSON-1.2-INTERMOLECULAR-FORCES-OF-ATTRACTION

Intermolecular Forces of Attraction

Overview

  • Intermolecular forces are attractive forces between molecules.

  • These forces determine various physical properties of substances, including boiling and melting points.

Kinetic Molecular Theory

  • Matter consists of tiny particles that carry energy and are in constant random motion.

  • The state of matter (solid, liquid, gas) depends on the distance and interaction between particles:

    • Solids: Particles are closely packed with little motion.

    • Liquids: Particles are still close but can flow and have more random motion.

    • Gases: Particles are far apart, moving freely with much random motion.

Types of Intermolecular Forces

  1. London Dispersion Forces

    • Weakest type of intermolecular force.

    • Present in all molecules; arise from temporary dipoles in nonpolar molecules.

    • Caused by fluctuations in electron distribution.

  2. Dipole-Dipole Forces

    • Occur between polar molecules.

    • Formed between the positive end of one dipole and the negative end of another.

  3. Ion-Dipole Forces

    • Occur between ions and polar molecules.

    • Responsible for the dissolution of ionic solids in polar solvents.

  4. Hydrogen Bonding

    • Special case of dipole-dipole interaction.

    • Occurs when hydrogen is bonded to highly electronegative atoms (F, O, or N).

    • Confers unique properties to substances, notably water.

Predicting Intermolecular Forces

  • Determine whether the compound is ionic or covalent:

    • Ionic Compounds: Exhibit ion-ion interactions.

    • Covalent Compounds: Interactions depend on polarity.

  • Polar Covalent Compounds: May show dipole-dipole interactions or hydrogen bonding, depending on the presence of H.

  • Nonpolar Covalent Compounds: Exhibit London dispersion forces only.

Influences on Strength of Intermolecular Forces

  • Ion-Ion Interactions: Governed by Coulomb's law, influenced by the distance between ions and their charges.

  • Ion-Dipole Interactions: Depend on the charge of the ion and polarity of the dipole.

  • London Dispersion Forces: Rely on polarizability of the molecule; larger molecules have stronger dispersion forces.

Summary of Key Points

  • Hydrogen bonding leads to higher boiling points in compounds like H2O, HF, and NH3.

  • Polarizability impacts the electron distribution and thus the strength of London dispersion forces.

Common Concepts

  • Ionic bonding involves charged particles; ion-ion interactions are very strong.

  • Dipole-dipole interactions arise between polar molecules, while hydrogen bonding is stronger.

  • All molecules experience London dispersion forces, regardless of polarity.