AP Chemistry — Unit 4: Chemical Reactions
Practice questions, answers, and key terms for Unit 4, aligned to the College Board CED.
Exam weighting: Unit 4 is 7-9% of the AP Chemistry exam.
What AP Chemistry Unit 4 covers
The College Board Course and Exam Description breaks Unit 4 into 9 topics:
- 4.1 Introduction for Reactions
- 4.2 Net Ionic Equations
- 4.3 Representations of Reactions
- 4.4 Physical and Chemical Changes
- 4.5 Stoichiometry
- 4.6 Introduction to Titration
- 4.7 Types of Chemical Reactions
- 4.8 Introduction to Acid-Base Reactions
- 4.9 Oxidation-Reduction (Redox) Reactions
AP Chemistry Unit 4 practice questions
What defines a physical change?
A change in a substance's properties or phase, not its composition. Chemical identity stays the same; phase changes are physical.
You mix two clear solutions and a bright yellow solid immediately forms at the bottom of the flask. What type of change is this, and what is the key evidence?
A chemical change — a precipitate signals new substances formed. Precipitate, color change, gas, and heat/light are the four signs.
Why must every chemical equation be balanced?
Atoms are never created or destroyed, so each element balances. This conserves mass and charge across the equation.
How does a net ionic equation differ from a complete ionic equation?
A net ionic equation omits spectator ions, showing only the species that react. Spectator ions appear unchanged on both sides.
When aqueous AgNO₃ and aqueous NaCl are mixed, AgCl(s) forms. What are the spectator ions, and why are they excluded from the net ionic equation?
Na⁺ and NO₃⁻ are spectators — they stay dissolved and unchanged. Only Ag⁺ + Cl⁻ → AgCl(s) is the real change.
What is a particulate-level representation of a chemical reaction?
A diagram of atoms, molecules, or ions before and after, using spheres. It must match the balanced equation and conserve atoms.
A particulate diagram shows 4 H₂ molecules and 2 O₂ molecules before a reaction. After complete reaction to form H₂O, what does the diagram look like?
4 H₂O molecules, with no reactants left — atoms are conserved. It matches 2H₂ + O₂ → 2H₂O; the 4 H₂ and 2 O₂ react exactly.
What distinguishes a chemical process from a physical process at the molecular level?
Chemical processes break or form chemical bonds; physical ones change only intermolecular interactions. So chemical changes make new substances; physical ones keep molecular identity.
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Key terms in Unit 4
These 12 terms show up in the Unit 4 cards. Each one links to its definition in the AP Chemistry key-term reference.
Drill all of Unit 4 with spaced repetition
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