AP Chemistry Unit 3: Intermolecular Forces & Properties

TLDR

• Understand London dispersion, dipole–dipole, hydrogen bonding, and ion–dipole forces.

• Stronger IMFs → higher boiling point, melting point, viscosity, and surface tension.

• Weaker IMFs → higher vapor pressure and faster evaporation.

• Master boiling point trends, solubility rules, phase diagrams, and Clausius–Clapeyron.

• ✅ Download the full Unit 3 Cheat Sheet (PDF)

Why This Unit Matters

Many AP Chemistry questions ask you to compare substances or justify why one boils, evaporates, dissolves, or condenses faster than another.
The secret: it all comes down to IMFs.

Intermolecular forces explain real-world behavior — why oil and water don’t mix, why water has unusually high boiling point, why ionic compounds dissolve in water, and even why ice floats.
Master this unit and you gain a powerful lens for predicting physical properties throughout the entire course.

1. The Four Types of Intermolecular Forces

London Dispersion Forces (LDF)

• Present in all molecules and atoms.

• Weakest IMF; strength increases with molar mass and surface area.

• Larger electron clouds → easier to polarize → stronger LDFs.
  Examples: I₂ > Br₂ > Cl₂ > F₂ in IMF strength.

Dipole–Dipole Forces

• Occur between polar molecules with permanent dipoles.

• Stronger dipole moment → stronger interaction.

• You must know how to determine molecular polarity from shape.

Hydrogen Bonding

• A special, very strong dipole interaction.

• Occurs when H is bonded to N, O, or F.

• Responsible for water’s high boiling point and ice’s structure.

Ion–Dipole Interactions

• Between ions and polar molecules (ex. Na⁺–H₂O).

• Key for understanding why ionic compounds dissolve in water.

Mnemonic: “London → Dipole → H-bond → Ionic” (weak to strong).

2. Forces & Properties

Stronger intermolecular forces lead to:

• Higher boiling point

• Higher melting point

• Higher viscosity

• Higher surface tension

• Lower vapor pressure

Weaker IMFs → liquids evaporate more easily.

Mnemonic: “IMFs climb, vapor pressure dives.”

3. Solubility & Miscibility

• “Like dissolves like”:

  • Polar dissolves polar

  • Nonpolar dissolves nonpolar

• Ion–dipole interactions explain why salts dissolve in water.

• Network solids (diamond, SiO₂) do not dissolve because they have covalent bonds, not IMFs.

4. Vapor Pressure, Boiling Point & Phase Changes

• Vapor pressure: pressure exerted by molecules escaping liquid surface.

• High IMFs → fewer molecules escape → low vapor pressure.

• Boiling point = when vapor pressure equals external pressure.

Key Equation (Clausius–Clapeyron):
ln(P₁/P₂) = −(ΔHvap / R)(1/T₁ − 1/T₂)
Used for comparing vapor pressures at different temperatures.

Phase Change Energy:

• Heating within a phase: q = mCΔT

• Phase change: q = nΔHvap or q = nΔHfus

5. Phase Diagrams

• Show relationship between P and T for solid, liquid, gas.

• Triple point: all 3 phases coexist.

• Critical point: above this, liquid and gas become indistinguishable.

• Water’s solid–liquid line slopes backward → ice is less dense than liquid water.

Mnemonic: “Triple touch, Critical climb.”

6. Solids & Crystal Types

Ionic solids: high melting, brittle, conductive when molten.
Molecular solids: low melting, weak IMFs.
Metallic solids: malleable, ductile, conductive.
Network covalent: extremely high melting (C, SiO₂).

Strength order: Network > Ionic > Metallic > Molecular.

7. Chromatography & IMF Application

Chromatography separates based on polarity and IMF strength.

• If stationary phase is polar → polar molecules stick more, travel slower.

• Rf = distance solute / distance solvent front.

High Rf = weak IMF with stationary phase.

Common Pitfalls

• Confusing intermolecular forces (between molecules) and intramolecular bonds (within a molecule).

• Thinking hydrogen bonding occurs with any H — it must be H–N, H–O, or H–F.

• Assuming strong IMFs → high vapor pressure (it’s the opposite).

• Forgetting that network solids have covalent bonds, not IMFs.

• Misidentifying molecular polarity because of incorrect geometry.

Tutor Tip

Before answering any property question, ask:
What IMFs does this molecule have, and how strong are they?
This single step accurately predicts boiling point, melting point, vapor pressure, solubility, and even chromatography behavior.

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