What it covers: Matter in motion — net ionic equations, physical/chemical changes, stoichiometry, titration, reaction types, and an intro to acid-base and redox reactions.
Exam weight: About 7–9% of the AP Chemistry exam.
The big question: How do balanced chemical equations let us predict and quantify exactly how much of each substance reacts and forms?
CED topics covered: 4.1–4.9, from introduction for reactions through oxidation-reduction (redox) reactions.
Key topics at a glance
Balancing & Net Ionic Equations
Balance mass and charge. Net ionic equations remove spectator ions, leaving only the species that actually react.
Physical vs. Chemical Change
Physical = same substance, different form. Chemical = new substance formed via bond breaking/forming.
Stoichiometry
Mole ratios from balanced coefficients. Limiting reactant caps product; percent yield = actual ÷ theoretical × 100%.
Titration
Known concentration/volume of titrant reacts completely with unknown at the equivalence point — solve for unknown concentration.
Reaction Types
Synthesis, decomposition, single replacement, double replacement, combustion — five major patterns to recognize.
Acid-Base Intro
Acid + base → water + salt (neutralization). Full treatment comes in Unit 8.
Redox Intro
Electron transfer: oxidation = lose e⁻, reduction = gain e⁻. Track with oxidation numbers. Full treatment comes in Unit 9.
The key terms you must know
Net ionic equation / spectator ion — shows only the species that react; spectators are removed.
Physical change vs. chemical change — same substance vs. new substance with new properties.
Stoichiometry — using mole ratios from balanced equations to relate amounts of reactants and products.
Limiting reactant / excess reactant — whichever runs out first caps the product; the other is left over.
Theoretical yield / percent yield — maximum possible product vs. the percentage actually obtained.
Titration / equivalence point — using a known reaction to determine an unknown concentration.
Synthesis, decomposition, single/double replacement, combustion — the five major reaction type patterns.
Oxidation number — bookkeeping charge used to track electron transfer in redox reactions.
Balanced coefficients ARE the mole ratio. Every stoichiometry problem starts with a correctly balanced equation.
The limiting reactant determines everything downstream. Always calculate which reactant runs out first before calculating yield.
Net ionic equations cut the noise. Spectator ions are real, but they don't tell you what's actually happening chemically.
This unit previews two units you'll see again. Acid-base reactions return in full force in Unit 8; redox reactions return in Unit 9 with electrochemistry.
Titration is just stoichiometry in reverse. Instead of solving for product amount, you're solving for an unknown concentration.
Common exam traps
Don't forget to balance BEFORE doing stoichiometry. An unbalanced equation gives the wrong mole ratio and ruins every downstream calculation.
The limiting reactant isn't always the one with fewer grams. You must convert to moles and compare using the mole ratio from the balanced equation, not raw mass.
Percent yield can't exceed 100% in a properly conducted reaction. A reported yield over 100% usually signals an experimental or measurement error.
Not all double replacement reactions form a precipitate. Some form water (neutralization) or a gas instead — check solubility rules carefully.
An element by itself always has an oxidation number of zero. A common error is assigning a nonzero oxidation number to elements in their natural, uncombined state.