📚Study Guide: Intermolecular Forces and Properties
Unit 3: Intermolecular Forces and Properties
The properties of matter--boiling point, viscosity, surface tension, vapor pressure, and solubility--are determined not by the strength of intramolecular bonds but by the intermolecular forces (IMFs) between molecules. This unit requires students to identify and rank London dispersion forces, dipole-dipole interactions, hydrogen bonding, and ion-dipole forces, and to connect these forces to macroscopic properties. The liquid state is explored in depth, including vapor pressure curves, phase diagrams, and heating/cooling curves. Students must understand that vapor pressure increases with temperature and decreases with stronger IMFs, and be able to read phase diagrams to identify melting point, boiling point, critical point, and triple point. Solutions and mixtures are also covered extensively, including the distinction between electrolytes and nonelectrolytes, the factors affecting solubility, and colligative properties. The AP exam frequently presents scenarios where students must rank substances by boiling point, predict solubility in polar versus nonpolar solvents, or calculate molality and molarity. Understanding the ideal gas law and deviations from ideal behavior, as well as the kinetic molecular theory, rounds out this unit. These concepts are essential for predicting and explaining the behavior of substances in the laboratory and in nature.
Key Concepts
- Types of Intermolecular Forces: London dispersion forces (LDF) exist in all molecules; strength increases with molar mass and surface area. Dipole-dipole forces exist in polar molecules. Hydrogen bonding is a strong dipole-dipole force between H bonded to N, O, or F and another N, O, or F. Ion-dipole forces exist between ions and polar molecules.
- Properties of Liquids and Solids: Vapor pressure decreases as IMF strength increases. Boiling point increases as IMF strength increases. Surface tension and viscosity also increase with stronger IMFs.
- Phase Diagrams: Show the phases of a substance at different temperatures and pressures. The triple point is where all three phases coexist in equilibrium. The critical point marks the end of the liquid-gas boundary; beyond this, the substance is a supercritical fluid.
- Solutions and Solubility: "Like dissolves like": polar solvents dissolve polar/ionic solutes; nonpolar solvents dissolve nonpolar solutes. Solubility of gases in liquids decreases with increasing temperature (Henry's Law: C = kP). Solubility of most solids increases with temperature.
- Colligative Properties: Properties that depend on the number of solute particles, not their identity. Vapor pressure lowering (Raoult's Law), boiling point elevation (delta Tb = i Kb m), freezing point depression (delta Tf = i Kf m), and osmotic pressure (pi = i M R T).
Vocabulary
- Vapor Pressure: The pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases at a given temperature.
- Surfactant: A substance that reduces surface tension by disrupting hydrogen bonding at the surface of a liquid.
- Supercritical Fluid: A state of matter above the critical temperature and pressure, where distinct liquid and gas phases do not exist.
- Electrolyte: A substance that dissociates into ions when dissolved in water, producing a solution that conducts electricity.
- Colligative Property: A property of solutions that depends only on the ratio of the number of solute particles to solvent molecules, not on the nature of the chemical species.
- van't Hoff Factor (i): The ratio of moles of particles in solution to moles of solute dissolved; accounts for dissociation (e.g., i = 2 for NaCl, i = 3 for CaCl2).
Essential Formulas
- P V = n R T (Ideal Gas Law)
- P_total = P1 + P2 + P3 + ... (Dalton's Law of Partial Pressures)
- Molarity (M) = moles of solute / liters of solution
- Molality (m) = moles of solute / kilograms of solvent
- Percent by mass = (mass of solute / mass of solution) x 100
- Raoult's Law: P_solution = X_solvent x P degrees_solvent
- delta Tb = i Kb m (Boiling point elevation)
- delta Tf = i Kf m (Freezing point depression)
- pi = i M R T (Osmotic pressure)
- C = k P (Henry's Law)
- R = 0.0821 L atm/(mol K) or 8.314 J/(mol K)
Common Mistakes
- Confusing Intramolecular and Intermolecular Forces: Breaking IMFs (boiling) requires far less energy than breaking covalent bonds. Do not confuse the two when explaining phase changes.
- Assuming All Hydrogen Bonds Are Equal: Hydrogen bonding only occurs when H is directly bonded to N, O, or F AND there is a lone pair on N, O, or F in another molecule.
- Forgetting the van't Hoff Factor: In colligative property calculations, multiply by i. For ionic compounds, i equals the number of ions produced upon dissociation.
- Ignoring Non-Ideal Behavior: Real gases deviate from ideal behavior at high pressure and low temperature due to finite molecular volume and intermolecular attractions.
AP Exam Strategies
- Rank Boiling Points by IMF Strength: When asked to rank substances by boiling point, first identify the strongest IMF present in each, then consider molar mass for LDF comparisons.
- Draw Phase Change Graphs: On heating curves, label the plateau regions as phase changes (melting, boiling) where temperature is constant despite heat input.
- Calculate Colligative Properties Stepwise: First calculate molality, then apply the appropriate equation. Remember to include the van't Hoff factor.
- Explain with Particle-Level Diagrams: When explaining vapor pressure or solubility, describe the behavior of molecules at the particle level--escaping into the gas phase or being surrounded by solvent molecules.
Real-World Applications
- Antifreeze: Ethylene glycol lowers the freezing point and raises the boiling point of water in car radiators via colligative properties.
- Water's Anomalous Properties: Hydrogen bonding gives water high specific heat, high heat of vaporization, and lower density as ice, which is critical for aquatic ecosystems and global climate regulation.
- Reverse Osmosis: Applying pressure greater than osmotic pressure forces water through a semipermeable membrane, desalinating seawater for drinking water.