SAT / PSAT
SAT / PSAT Prep
History & Social Science
AP World History AP US History AP European History AP Human Geography AP US Government & Politics AP Psychology AP Macroeconomics AP Microeconomics
English
AP English Language & Composition AP English Literature & Composition
Math & Computer Science
AP Calculus AB/BC AP Precalculus AP Statistics AP Computer Science A AP Computer Science Principles
Sciences
AP Biology AP Chemistry AP Environmental Science AP Physics 1 AP Physics 2
World Languages & Arts
AP Spanish Language AP Art History AP Music Theory Start studying →
🧪 Unit 4 · Chemical Reactions 🏠 Unit Hub 🗂 Flashcards 🗺 Cheat Sheet ⭐ Essentials 🎨 Visual Review 📝 MC Practice ✍️ SAQ Practice

AP Chemistry Unit 4 Essentials

The must-know terms and big ideas for Unit 4: Chemical Reactions. Every vocabulary word and concept you need to master.

← Back to Unit 4 hub
Big Idea 1
Every chemical equation is a conservation statement
A balanced chemical equation isn't just notation — it's a guarantee that atoms (mass) and charge are conserved from reactants to products. Net ionic equations take this one step further by removing spectator ions that don't actually participate, leaving only the atoms and charges that genuinely change. Every stoichiometry calculation in this unit relies on the coefficients of a correctly balanced equation being an accurate mole ratio.
Balancing Equations Net Ionic Equations Conservation
Big Idea 2
Stoichiometry is the mole concept from Unit 1, applied to reactions
Just as the mole bridges atomic-scale counting and measurable mass in Unit 1, stoichiometry uses mole ratios from a balanced equation to bridge the amount of one substance to the amount of another in a reaction. The limiting reactant — whichever runs out first — caps how much product can form, and percent yield compares the real-world result to that theoretical maximum. Titration is simply stoichiometry run in reverse: using a known reaction and known volumes/concentrations to solve for an unknown concentration.
Stoichiometry Limiting Reactant Titration
🔍
Balanced chemical equation
A chemical equation in which the number of atoms of each element is equal on both sides, conserving mass.
Reactions
Net ionic equation
A chemical equation that shows only the ions and molecules that actually participate in a reaction, with spectator ions removed.
Net Ionic Equations
Spectator ion
An ion present in a reaction mixture that does not participate in the actual chemical change and appears unchanged on both sides of the full ionic equation.
Net Ionic Equations
Precipitate
An insoluble solid that forms when two aqueous solutions react, often signaling a double replacement reaction.
Net Ionic Equations
Physical change
A change that alters a substance's form or appearance without changing its chemical identity (e.g., melting, dissolving).
Changes
Chemical change
A change that produces a new substance with different chemical properties than the original, by breaking and forming bonds.
Changes
Stoichiometry
The calculation of quantitative relationships between reactants and products in a chemical reaction, using mole ratios from a balanced equation.
Stoichiometry
Limiting reactant
The reactant that is completely consumed first in a reaction, determining the maximum amount of product that can form.
Stoichiometry
Excess reactant
The reactant that remains left over once the limiting reactant is fully consumed.
Stoichiometry
Theoretical yield
The maximum amount of product that could form from a given amount of limiting reactant, calculated using stoichiometry.
Stoichiometry
Percent yield
The ratio of actual yield to theoretical yield, expressed as a percentage — a measure of a reaction's real-world efficiency.
Stoichiometry
Titration
A technique that uses a solution of known concentration (the titrant) added to a solution of unknown concentration until the reaction is complete, allowing the unknown concentration to be calculated.
Titration
Equivalence point
The point in a titration at which the moles of titrant added exactly match the moles needed to completely react with the unknown solution.
Titration
Synthesis reaction
A reaction in which two or more simpler substances combine to form a single, more complex product (A + B → AB).
Reaction Types
Decomposition reaction
A reaction in which a single compound breaks down into two or more simpler products (AB → A + B).
Reaction Types
Single replacement reaction
A reaction in which one element replaces another element within a compound (A + BC → AC + B).
Reaction Types
Double replacement reaction
A reaction in which the cations (or anions) of two compounds switch places, often forming a precipitate or water (AB + CD → AD + CB).
Reaction Types
Combustion reaction
A reaction in which a substance (often a hydrocarbon) reacts rapidly with oxygen, typically producing carbon dioxide and water and releasing energy.
Reaction Types
Acid-base (neutralization) reaction
A reaction between an acid and a base that produces water and a salt, with protons transferring from the acid to the base.
Acid-Base
Oxidation-reduction (redox) reaction
A reaction involving the transfer of electrons between species, in which one substance is oxidized (loses electrons) and another is reduced (gains electrons).
Redox
Oxidation number
A bookkeeping value assigned to an atom representing its charge if all bonds were treated as fully ionic; used to track electron transfer in redox reactions.
Redox
Oxidizing agent / reducing agent
The oxidizing agent is the species that gains electrons (and is itself reduced); the reducing agent is the species that loses electrons (and is itself oxidized).
Redox