Unit 3: Intermolecular Forces and Properties

AP Chemistry: 83 practice questions with detailed explanations.

Unit Study Guide

Executive Summary

Unit 3 connects the invisible forces between particles to macroscopic behavior: boiling points, vapor pressure, gas behavior, and solubility.

Intermolecular forces (IMFs)

From weakest to strongest: London dispersion (induced dipoles — present in all species, grows with polarizability/molar mass), dipole–dipole (polar molecules), and hydrogen bonding (H bonded to N, O, or F). Ion–dipole forces dominate in salt solutions. Stronger IMFs mean higher boiling point, higher viscosity, higher surface tension, and lower vapor pressure.

Solids, liquids, gases

Solids hold fixed shape and volume (ordered lattices); liquids have fixed volume but flow; gases fill the container. Heating a liquid overcomes IMFs: during a phase change, temperature stays constant while energy goes into breaking attractions.

Ideal gas law

PV = nRT ties pressure, volume, moles, and temperature. STP is 0 °C and 1 atm, molar volume 22.4 L/mol. Rearrange for any variable; watch unit conversions (R = 0.08206 L·atm/mol·K).

Kinetic molecular theory (KMT)

Gas particles are tiny, far apart, in constant random motion, with perfectly elastic collisions and no intermolecular attractions. Average kinetic energy depends only on temperature: KE = (3/2)RT. Lighter molecules move faster at a given temperature (same KE, less mass).

Deviations from ideality

Real gases deviate most at high pressure (particles take up volume) and low temperature (attractions matter). Polar and larger molecules deviate more. Real pressure is lower than ideal near condensation.

Solutions, solubility, concentration

Solute + solvent = solution; "like dissolves like" (polar dissolves polar). Solubility usually rises with temperature for solids but falls for gases. Concentration units: molarity (mol/L), molality (mol/kg), percent by mass. Dilution: M₁V₁ = M₂V₂. Paper chromatography separates by relative attraction to mobile vs. stationary phase; distillation separates by boiling point.

Quantitative skill-set

PV = nRT and M₁V₁ = M₂V₂ are the two workhorses. Convert between concentration units through moles.

Exam traps

Hydrogen bonding exists only when H is bonded to N, O, or F. Dispersion forces exist in every substance, including polar ones. During melting/boiling, temperature is constant. The gas constant R changes units — match R to the pressure/volume units in the problem.

Top 5 Concepts to Master

  1. 1Rank IMFs: LDF < dipole–dipole < H-bond < ion–dipole; connect them to boiling point and vapor pressure.
  2. 2Apply PV = nRT with consistent units; use 22.4 L/mol only at STP.
  3. 3Justify deviations from ideality with particle volume (high P) and attractions (low T).
  4. 4Use M₁V₁ = M₂V₂ for dilutions and convert concentration units through moles.
  5. 5Predict separation method: chromatography for mixtures, distillation for liquids by boiling point.

Key Terms & Definitions

Practice with Flashcards
London dispersion forces

Induced-dipole attractions present between all particles.

Dipole–dipole forces

Attractions between permanent dipoles of polar molecules.

Hydrogen bonding

Strong dipole attraction when H is bonded to N, O, or F.

Vapor pressure

Pressure of a vapor in equilibrium with its liquid.

Boiling point

Temperature where vapor pressure equals external pressure.

Ideal gas law

PV = nRT, describing gas behavior with no IMFs or particle volume.

Kinetic molecular theory

Model of gases as point particles in elastic random motion.

Molarity

Moles of solute per liter of solution (mol/L).

Molality

Moles of solute per kilogram of solvent (mol/kg).

Solubility

Maximum solute that dissolves in a given solvent amount.

Distillation

Separation of liquids by differences in boiling point.

Chromatography

Separation by differential affinity for mobile vs. stationary phase.

Dilution

Adding solvent to lower concentration; M₁V₁ = M₂V₂.

Polarizability

Ease of distorting an electron cloud; grows with molar mass.

Common Misconceptions: Exam Traps

Only nonpolar molecules experience London dispersion forces.

Correct: Dispersion forces act between all particles; they dominate for nonpolar species.

Temperature keeps rising while a liquid boils.

Correct: During any phase change temperature is constant; energy breaks IMFs instead.

Hydrogen bonding is any bond involving hydrogen.

Correct: H must be bonded to N, O, or F and attracted to a lone pair on another N/O/F.

Bigger molar mass always means a higher boiling point.

Correct: Only when other forces are comparable — hydrogen bonding can beat dispersion forces (e.g., water vs. H₂S).

Real gases deviate most at high temperature.

Correct: Deviation is worst at high pressure and low temperature, where volume and IMFs matter.

Featured Practice Questions

Question Bank Breakdown

By difficulty

easy 29medium 38hard 16

By topic

Intermolecular Forces 18Properties of Solids 14Concentration 11Ideal Gas Law 9Solids, Liquids, and Gases 9Solubility 8Kinetic Molecular Theory 6Solutions and Mixtures 5Deviation from Ideal Gas Law 5Representations of Solutions 3Separation of Solutions and Mixtures 3

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