AP Chemistry Unit 3 Cheat Sheet: Intermolecular Forces

Introduction

College Board lists Unit 3, Properties of Substances and Mixtures, as the most heavily weighted unit on the AP Chemistry exam, at 18 to 22 percent. Intermolecular forces sit at its center, and they explain why one substance boils, evaporates, dissolves, or condenses faster than another. This cheat sheet walks through the four types of intermolecular forces, how they set boiling point and vapor pressure, solubility, phase diagrams, and crystal types, so you can justify your answers the way the exam asks.

Once Unit 3 feels solid, the rest of the unit-by-unit AP Chemistry cheat sheets cover the other eight units.

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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