🧪AP Chemistry Unit 3 – Intermolecular Forces and Properties

What's This Unit All About?

• Explores the forces between molecules and how they influence properties of matter
• Focuses on understanding the different types of intermolecular forces (dipole-dipole, hydrogen bonding, London dispersion forces)
• Examines how intermolecular forces affect properties such as boiling point, melting point, viscosity, and surface tension
• Investigates the relationship between intermolecular forces and the behavior of gases, liquids, and solids
• Applies knowledge of intermolecular forces to real-world situations (water's unique properties, capillary action, adhesion, cohesion)

Key Concepts You Need to Know

• Intermolecular forces are attractions between molecules, not to be confused with intramolecular forces (bonds within molecules)
• Dipole-dipole forces occur between polar molecules due to uneven distribution of charge
  • Polar molecules have a positive end and a negative end (HCl, H2O)
• Hydrogen bonding is a strong type of dipole-dipole force that occurs when hydrogen is bonded to N, O, or F
  • Hydrogen bonding explains water's high boiling point and surface tension
• London dispersion forces are weak attractions between nonpolar molecules caused by temporary dipoles
  • Larger molecules have stronger London dispersion forces (pentane vs. methane)
• Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid phase
  • Substances with stronger intermolecular forces have lower vapor pressures

Types of Intermolecular Forces

• Dipole-dipole forces: Attractions between the positive end of one polar molecule and the negative end of another
  • Strength depends on the magnitude of the dipole moment (HCl > HF)
• Hydrogen bonding: A special case of dipole-dipole forces involving H bonded to N, O, or F
  • Responsible for water's unique properties and the structure of DNA and proteins
• London dispersion forces (induced dipole-induced dipole): Attractions between nonpolar molecules
  • Strength increases with molecular size and surface area (I2 > Br2 > Cl2)
• Ion-dipole forces: Attractions between ions and polar molecules
  • Occurs when ionic compounds dissolve in polar solvents (NaCl in water)
• Ion-induced dipole forces: Attractions between ions and nonpolar molecules
  • Weaker than ion-dipole forces (Na+ and Cl- in benzene)

Properties Affected by Intermolecular Forces

• Boiling point: Temperature at which vapor pressure equals atmospheric pressure
  • Stronger intermolecular forces lead to higher boiling points (H2O > CH3OH > CH4)
• Melting point: Temperature at which the solid and liquid phases are in equilibrium
  • Stronger intermolecular forces result in higher melting points (NaCl > HCl > Cl2)
• Viscosity: Resistance to flow
  • Stronger intermolecular forces cause higher viscosity (glycerol > water > ethanol)
• Surface tension: Tendency of a liquid to minimize its surface area
  • Stronger intermolecular forces create higher surface tension (water > ethanol > hexane)
• Solubility: Ability of a solute to dissolve in a solvent
  • "Like dissolves like" - polar solutes dissolve in polar solvents, nonpolar in nonpolar (NaCl in water, I2 in CCl4)

Real-World Applications

• Water's high specific heat capacity moderates Earth's climate and helps regulate body temperature
• Capillary action allows water and nutrients to move up through plant stems and blood through narrow vessels
• Adhesion and cohesion of water enable insects to walk on water and water to form droplets on surfaces
• Surfactants lower the surface tension of water, allowing for more effective cleaning and mixing (detergents, emulsifiers)
• Geckos can climb smooth surfaces due to van der Waals forces between their feet and the surface

Lab Experiments and Demonstrations

• Comparing the boiling points of polar and nonpolar liquids (water vs. hexane)
• Observing the effect of intermolecular forces on viscosity (glycerol vs. water vs. ethanol)
• Demonstrating surface tension using a penny and droplets of water, ethanol, and hexane
• Investigating the relationship between molecular size and London dispersion forces (melting points of I2, Br2, and Cl2)
• Exploring the solubility of polar and nonpolar solutes in various solvents (NaCl and I2 in water and hexane)

Common Mistakes and How to Avoid Them

• Confusing intermolecular forces with intramolecular forces (bonds)
  • Remember: intermolecular forces are between molecules, intramolecular forces are within molecules
• Assuming that all polar molecules have strong intermolecular forces
  • Consider the strength of the dipole moment and the presence of hydrogen bonding
• Neglecting the role of molecular size in determining the strength of London dispersion forces
  • Larger molecules have stronger London dispersion forces, even if they are nonpolar
• Forgetting to consider the effect of intermolecular forces on properties other than boiling point
  • Intermolecular forces also influence melting point, viscosity, surface tension, and solubility
• Oversimplifying the "like dissolves like" rule
  • Some polar solutes may dissolve in slightly polar solvents, and vice versa

Exam Tips and Practice Questions

• When comparing boiling points, consider the type and strength of intermolecular forces present
  • Rank the following compounds in order of increasing boiling point: CH4, CH3OH, H2O
• Use the concept of hydrogen bonding to explain the unique properties of water
  • Why does water have a higher boiling point than other hydrides (H2S, H2Se)?
• Apply your knowledge of intermolecular forces to predict the behavior of mixtures
  • Which compound is more soluble in water: NaCl or I2? Explain your reasoning.
• Practice drawing dipole moments and identifying the types of intermolecular forces present in various compounds
  • Draw the dipole moment for HF and predict the intermolecular forces present in a sample of HF.
• Remember to consider all the properties affected by intermolecular forces when analyzing a substance
  • How do intermolecular forces influence the viscosity and surface tension of a liquid?