Matter in motion. Net ionic equations, physical vs. chemical change, stoichiometry and limiting reactants, titration, the major reaction types, and an introduction to acid-base and oxidation-reduction (redox) reactions.
Writing and balancing chemical equations, conserving both mass and charge.
Topic 4.2
Net Ionic Equations
Removing spectator ions from a full ionic equation to show only the species that actually react.
Topic 4.3
Representations of Reactions
Particulate-level diagrams that represent reactants, products, and conservation of atoms.
Topic 4.4
Physical & Chemical Changes
Distinguishing changes that alter chemical identity (chemical) from those that don't (physical).
Topic 4.5
Stoichiometry
Mole ratios from balanced equations, limiting reactants, and theoretical and percent yield.
Topic 4.6
Introduction to Titration
Using a titration's known concentration and volume to determine an unknown concentration.
Topic 4.7
Types of Chemical Reactions
Synthesis, decomposition, single replacement, double replacement, and combustion reactions.
Topic 4.8
Introduction to Acid-Base Reactions
Neutralization reactions between acids and bases, producing water and a salt.
Topic 4.9
Oxidation-Reduction (Redox) Reactions
Assigning oxidation numbers and identifying the oxidizing and reducing agents in a reaction.
About Unit 4
Unit 4 shifts the focus from individual atoms and molecules to how substances react with each other. You'll learn to write and balance chemical equations, simplify them into net ionic equations by removing spectator ions, and use stoichiometry to calculate exact quantities of reactants and products — including identifying the limiting reactant and calculating percent yield. You'll also classify reactions by type and get a first introduction to two reaction categories that get their own dedicated units later: acid-base reactions (Unit 8) and redox reactions (Unit 9).
This unit is roughly 7–9% of the AP Chem exam and takes about 13–15 class periods. Stoichiometry calculations here are foundational — they reappear constantly in Units 5 through 9, especially in kinetics, equilibrium, and electrochemistry.
The College Board ties Unit 4 to two core Big Ideas:
Big Idea SPQ
Scale, Proportion & Quantity — stoichiometry connects mole ratios to measurable mass and volume
Big Idea TRA
Transformations — every chemical reaction is a transformation that conserves mass and charge