AP Chemistry Unit 3: Properties of Substances and Mixtures

Introduction

• Presented by Jeremy Krug
• Overview of Unit 3: Properties of Substances and Mixtures
• Resources available at Ultimate Review Packet

Intermolecular Forces

• London Dispersion Forces

  • Present in all molecules, often the weakest
  • Can be stronger in large molecules
  • Only force in nonpolar molecules
  • Polarizability increases with more electrons

• Dipole-Dipole Forces

  • Present in polar molecules
  • Positive pole attracts negative pole of neighbors
  • Usually stronger than dispersion forces

• Hydrogen Bonding

  • Occurs with hydrogen atoms bonded to O, F, or N
  • Especially strong intermolecular force

• Ion-Dipole Forces

  • Between polar molecules and ions
  • Example: Water with ions in an ionic compound
  • Strong enough to dissolve compounds like sodium and fluoride in water

Types of Solids and Properties

• Ionic Solids

  • High melting points, brittle, conduct electricity in solution

• Covalent Network Solids

  • Extremely strong, e.g., diamond, silicon dioxide

• Molecular Solids

  • Individual molecules, weak forces, low melting points, e.g., sugar

• Metallic Solids

  • Pure metals and alloys, malleable, ductile, good conductors

• Crystalline vs. Amorphous Solids

  • True solids are crystalline
  • Amorphous: non-crystalline, e.g., plastics

States of Matter

• Solids: Close particles, vibrational motion only
• Liquids: Particles farther apart, can flow
• Gases: Independent motion, far apart, compressible

Ideal Gas Law

• Formula: PV = nRT
  • P: Pressure (atm)
  • V: Volume (L)
  • n: Moles of gas
  • T: Temperature (Kelvin)
  • R: Universal Gas Constant (0.08206 L atm/mol K)
• Partial Pressures
  • Total pressure is the sum of partial pressures
  • Partial pressure = mole fraction × total pressure

Temperature and Kinetic Energy

• Temperature measures average kinetic energy
• At higher temperatures, more particles move faster
• Boltzmann distribution illustrates range of molecular speeds

Gas Behavior

• Ideal gases: No intermolecular attractions, no volume
• Real gases approximate ideal conditions at high temperatures, low pressures

Mixtures

• Heterogeneous Mixtures: Visible separate components

• Homogeneous Mixtures (Solutions): Uniform distribution

• Molarity

  • M = moles of solute/liters of solution

Separation Techniques

• Distillation: Based on boiling points
• Chromatography: Based on adherence to a column
  • Stationary phase vs. mobile phase

Solubility

• "Like dissolves like"
  • Polar dissolves in polar, nonpolar in nonpolar

Electromagnetic Spectrum and Molecule Interactions

• Ultraviolet/Visible Light: Electron transitions
• Infrared Radiation: Molecular vibrations
• Microwave Radiation: Molecular rotations
• Light acts as waves and photons

Calculations

• Photon Energy: E = hν, where h is Planck’s constant (6.626 x 10⁻³⁴ J·s)
• Speed of Light: c = λν, c ≈ 3 × 10⁸ m/s

Spectrophotometry

• Beer-Lambert Law: A = εbc

  • A: Absorbance
  • ε: Molar absorptivity
  • b: Path length
  • c: Concentration

• Calibration curves: Used to find unknown concentrations

• Next review: Unit 4
• Encourage continued learning in chemistry