General Chemistry 2: Intermolecular Forces

Overview

• Introduction to intermolecular forces, phase changes, phase diagrams, and liquids.
• Distinction between chemical bonds and intermolecular attractions.

Chemical Bonds vs. Intermolecular Forces

• Chemical Bond: Strong force holding atoms within a molecule (e.g., covalent bonds in H₂O).
• Intermolecular Force: Weaker attractions between molecules causing aggregation (e.g., water molecules forming a liquid).

Types of Bonds

• Ionic Bonds: Electrostatic attractions between oppositely charged particles (e.g., NaCl).
• Covalent Bonds: Electrons shared between atoms; can be nonpolar or polar.
• Metallic Bonds: Metallic cations in a sea of delocalized electrons.

Intermolecular Forces in This Chapter

1. Dipole-Dipole Forces

   • Occur between polar molecules with permanent dipoles.
   • Example: HCl, where H is partial positive and Cl is partial negative.
   • Strength increases with increasing polarity.

2. Hydrogen Bonds

   • Strong attraction between polarized hydrogen on one molecule and a highly electronegative atom (F, O, N) on another.
   • Not a true bond, but an intermolecular force.
   • Responsible for high boiling/melting points and properties of water.
   • Key role in DNA structure.

3. London Dispersion Forces

   • Present in nonpolar substances due to temporary induced dipoles.
   • Increase with increasing molecular size and polarizability.

Identifying Intermolecular Forces

• Determine presence of H bonds (HF, HO, HN).
• Assess polarity of the molecule.
• Use steps to identify the forces present and relevant exam problems.

Phase Changes

• Endothermic: Melting, vaporization, sublimation.
• Exothermic: Freezing, condensation, deposition.
• Heat of Fusion/Vaporization: Energy required for phase changes.
• Heating curves show temperature vs. heat absorbed.

Vapor Pressure and Boiling Point

• Vapor: Gas phase of a substance that can be condensed.
• Vapor Pressure: Pressure exerted by a vapor in equilibrium with its liquid.
• Stronger intermolecular forces lead to lower vapor pressure.
• Boiling point: Temperature where vapor pressure equals external pressure.

Practice Problems

• Worked examples of identifying intermolecular forces and calculating phase change heat.

Phase Diagrams

• Diagram showing stable states at various temperatures and pressures.
• Fusion Curve: Solid-liquid equilibrium.
• Sublimation Curve: Solid-gas equilibrium.
• Vaporization Curve: Liquid-gas equilibrium.
• Triple Point: All three phases coexist.
• Critical Point: Beyond which a substance is a supercritical fluid.

Properties of Liquids

1. Surface Tension

   • Energy to increase surface area of liquid.
   • Stronger intermolecular forces result in higher surface tension.

2. Viscosity

   • Resistance to flow; affected by intermolecular forces, molecular size, and temperature.

3. Capillary Action

   • Liquid rising in a narrow tube due to cohesive and adhesive forces.