CHEM105 - Spring 2025 - Unit 10 - Intermolecular Forces

Unit 10: Intermolecular Forces

Introduction to Bonds

  • Ionic Bond

    • Formation through transferred electron(s).

    • Cation (metal) loses electron(s) to anion (nonmetal).

  • Covalent Bond

    • Involves shared electron pairs.

    • Each bond consists of 2 electrons shared by overlapping orbitals.

Lewis Dot Structures

  • Steps to create Lewis Dot Structures:

    1. Count total number of valence electrons; adjust for charge.

    2. Identify the central atom (least electronegative, except H).

    3. Bond central atom with peripheral atoms.

    4. Distribute remaining electrons to achieve octets.

    5. Use double bonds or triple bonds if needed to fulfill octets.

    6. For resonance structures, apply formal charge to find the most stable form.

Electronegativity and Molecular Geometry

  • Electronegativity

    • Measure of an atom's ability to attract bonding electrons.

  • Steps to analyze molecular geometry:

    1. Draw Lewis structure.

    2. Determine the steric number (SN):

      • SN = (number of bonded atoms) + (number of lone pairs).

    3. Use SN to determine molecular geometry.

    4. Optimize structure based on lone pairs and bonding pairs.

Electron-Pair Geometry and Molecular Geometry

  • Table summarizing different configurations based on steric numbers:

    • SN = 6

      • 6 bonded, 0 lone: Octahedral

      • 5 bonded, 1 lone: Square pyramidal

      • 4 bonded, 2 lone: Square planar

    • SN = 5

      • 5 bonded, 0 lone: Trigonal bipyramidal

      • 4 bonded, 1 lone: See-saw

      • 3 bonded, 2 lone: T-shaped

    • SN = 4

      • 4 bonded, 0 lone: Tetrahedral

      • 3 bonded, 1 lone: Trigonal pyramidal

      • 2 bonded, 2 lone: Bent

    • SN = 3

      • 3 bonded, 0 lone: Trigonal planar

      • 2 bonded, 1 lone: Bent

    • SN = 2

      • 2 bonded, 0 lone: Linear

Polar & Nonpolar Molecules

  • Characteristics of Polar Molecules:

    1. Must contain polar bonds (ΔEN).

    2. Orientation results in charge separation.

  • Characteristics of Nonpolar Molecules:

    • Dipole moment is zero (e.g., H2, CCl4).

  • For polar molecules, a net dipole points toward the more electronegative atom.

Intermolecular Forces Overview

  • Intramolecular Forces:

    • Forces that keep molecules together (ionic, covalent, metallic).

  • Intermolecular Forces:

    • Forces between distinct particles leading to various interactions.

  • Types of Intermolecular Forces:

    • Ion-dipole interactions

    • Dipole-dipole interactions (including Hydrogen bonds)

    • Dispersion forces (London forces)

    • Ion-induced dipole interactions

    • Dipole-induced dipole interactions

Ion-Ion Interactions and Coulomb's Law

  • Ion-Ion Interactions:

    • Increased attractive forces with increasing charge and decreased by greater ionic radius.

  • Coulomb’s Law:

    • E ∝ (Q1 x Q2) / d

    • Energy (E) of interaction inversely related to distance and directly proportional to charges.

Intermolecular Forces in Polar Solutions

  • Ion-Dipole Interactions:

    • Occur between ions and polar molecules, resulting in hydration spheres when dissolved.

  • Dipole-Dipole Interactions:

    • Occur when two molecules with dipole moments interact, leading to alignment based on charge.

  • Hydrogen Bonding:

    • Special class of dipole-dipole due to high electronegativity involving N, O, or F.

Induced Dipoles

  • Ion-Induced Dipole:

    • Occurs when an ion creates a dipole in a nonpolar molecule.

  • Dipole-Induced Dipole:

    • When a polar molecule induces a dipole on a nonpolar molecule.

London Dispersion Forces

  • Definition:

    • Occur due to temporary dipoles in molecules.

  • Factors Affecting Strength:

    1. Size of atoms/molecules (larger = increased polarizability).

    2. Shape of molecules (more surface area = greater interaction).

Boiling Points and Molecular Forces

  • Influence of Intermolecular Forces:

    • Increased forces lead to higher boiling/melting points and increased viscosity, while lowering vapor pressure.

Physical Properties of Liquids & Solutions

  • Effects of Intermolecular Forces:

    • Surface tension and viscosity are directly related to strength of attractive forces.

  • Cohesive vs Adhesive Forces:

    • Cohesive: interactions between like particles.

    • Adhesive: interactions between unlike particles.

  • Capillary Action:

    • Rise of liquid in a narrow tube due to adhesive and cohesive forces.

Understanding Phase Diagrams and States of Matter

  • Phase Diagram Basics:

    • Represents states of matter as functions of temperature and pressure.

    • Contains critical points and describes where phase transitions occur (Triple Point vs Critical Point).

  • States of Matter:

    • Gaseous state allows complete molecular motion.

    • Liquid state allows limited motion, and solid state locks molecules in place.

Learning Objectives

  • Understand intermolecular forces and their impacts on the physical states of matter.

  • Predict physical properties based on molecular interactions and identifications in phase diagrams.