AP Chemistry Unit 4 review: Chemical Reactions
Unit 4 represents reactions with equations, particle diagrams and measured amounts. It is 7–9% of multiple choice. Expect to identify reacting species, balance atoms and charge, and use mole ratios to calculate limiting reactants, yields and titration results.
Unit 4 on the May 2027 exam
| 2027 exam | |
|---|---|
| Weight | 7–9% of the multiple-choice section |
| Exam date | Thursday, May 6, 2027, afternoon session |
| Multiple choice | 60 questions in 90 minutes, worth 50% of the exam score |
| Free response | 7 questions in 105 minutes, worth 50%: three long questions (10 points each) and four short questions (4 points each). |
| Calculator | Both sections allow scientific or approved graphing calculators. Four-function calculators are allowed but not recommended. Bluebook includes both Desmos scientific and graphing calculators. |
| Class time | About 14–15 class periods in College Board's pacing guide |
| 2026 results | 15% of students scored a 5 on the whole exam, and about 76% scored 3 or higher |
Sources: College Board's course and exam description, exam page, 2027 exam dates and score distributions.
What Unit 4 covers
College Board's course description splits the unit into 9 topics. The second column is what you should be able to do for each one.
| Topic | What you should be able to do |
|---|---|
| 4.1 Introduction for Reactions | Use observations to propose a reaction and identify evidence for chemical change. |
| 4.2 Net Ionic Equations | Write complete and net ionic equations while retaining weak electrolytes, solids and liquids intact. |
| 4.3 Representations of Reactions | Translate between balanced equations and particle diagrams with conserved atoms. |
| 4.4 Physical and Chemical Changes | Distinguish changes in particle arrangement from changes in chemical identity. |
| 4.5 Stoichiometry | Calculate limiting reactants, excess reactants, theoretical yield and percent yield. |
| 4.6 Introduction to Titration | Use a balanced reaction and titrant volume to find the amount of an analyte. |
| 4.7 Types of Chemical Reactions | Identify precipitation, acid–base and oxidation–reduction reactions from the species involved. |
| 4.8 Introduction to Acid-Base Reactions | Identify proton donors and acceptors and represent their reaction. |
| 4.9 Oxidation-Reduction (Redox) Reactions | Assign oxidation numbers and balance electron transfer in redox reactions. |
A stoichiometry route that works beyond 1:1 reactions
Balance first. Coefficients compare numbers of particles and moles, not grams.
| Task | Method | Check |
|---|---|---|
| Reactant mass → product mass | Mass / molar mass × coefficient ratio × product molar mass | Units should end in grams of product. |
| Limiting reactant | Compare available moles / coefficient for each reactant | The smallest value limits reaction extent. |
| Excess left over | Initial moles − moles consumed | Use the limiting reactant to calculate consumption. |
| Titration | M × volume in liters, then apply mole ratio | At equivalence, amounts match coefficients, not necessarily each other. |
| Percent yield | Actual yield / theoretical yield × 100% | Use the same units for both masses. |
| Net ionic equation | Split strong aqueous electrolytes; cancel unchanged ions | Atoms and total charge must balance. |
| Redox | Match electrons lost and gained | The oxidizing agent is reduced. |
Common mistakes
Each of these loses points on questions you otherwise understand.
- Choosing the limiting reactant by smaller mass. Convert to moles and compare with coefficients. The CED identifies mass-to-mass comparisons as an error on printed p. 75.
- Assuming every reaction is 1:1. A balanced equation supplies the mole ratio for both synthesis and titration. CED p. 75.
- Splitting a precipitate or weak acid into ions in a net ionic equation. Keep those species intact. CED topic 4.2.
- Changing subscripts to balance an equation. Change coefficients; changing a subscript changes the substance. CED topics 4.2–4.3.
6 practice questions with answers
Written by Ashore for this page, in the style of the multiple-choice section. Try each one before you open the answer.
Question 1 · 4.5
For N₂ + 3H₂ → 2NH₃, a vessel contains 0.40 mol N₂ and 0.90 mol H₂. What is the maximum NH₃ amount?
- A0.30 mol
- B0.60 mol
- C0.80 mol
- D1.30 mol
Show the answer
B. H₂ limits the reaction: 0.90/3 = 0.30 mol reaction extent, compared with 0.40/1 for N₂. NH₃ produced is 2(0.30) = 0.60 mol.
Question 2 · 4.5
A reaction has a theoretical product yield of 8.40 g and an actual yield of 6.30 g. What is the percent yield?
- A25.0%
- B66.7%
- C133%
- D75.0%
Show the answer
D. Percent yield = 6.30/8.40 × 100 = 75.0%. The theoretical amount is the denominator because it is the predicted maximum for the stated reactants.
Question 3 · 4.6
A 25.0 mL sample of a diprotic acid H₂A requires 30.0 mL of 0.200 M NaOH for complete neutralization. What is the acid concentration?
- A0.120 M
- B0.240 M
- C0.0600 M
- D0.480 M
Show the answer
A. NaOH amount is 0.0300(0.200) = 0.00600 mol. Two moles of OH⁻ neutralize one mole of H₂A, so acid amount is 0.00300 mol. Divide by 0.0250 L to obtain 0.120 M.
Question 4 · 4.2
AgNO₃(aq) reacts with NaCl(aq) to form AgCl(s). Which net ionic equation is correct?
- ANa⁺ + NO₃⁻ → NaNO₃(s)
- BAgCl(s) → Ag⁺ + Cl⁻
- CAg⁺ + Cl⁻ → AgCl(s)
- DAg⁺ + NO₃⁻ → AgNO₃(s)
Show the answer
C. Ag⁺ and Cl⁻ form the precipitate. Na⁺ and NO₃⁻ remain dissolved and cancel from the complete ionic equation. Both total charge and atom counts balance.
Question 5 · 4.9
How many moles of electrons are released when 0.0800 mol Al is oxidized to Al³⁺?
- A0.0267 mol
- B0.240 mol
- C0.0800 mol
- D0.160 mol
Show the answer
B. The oxidation half-reaction is Al → Al³⁺ + 3e⁻. Thus 0.0800 mol Al releases 3(0.0800) = 0.240 mol electrons.
Question 6 · 4.5
A 0.150 mol sample of CaCO₃ decomposes completely: CaCO₃ → CaO + CO₂. Using CO₂ = 44.0 g mol⁻¹, what mass of CO₂ forms?
- A2.93 g
- B3.30 g
- C11.0 g
- D6.60 g
Show the answer
D. The coefficient ratio is 1:1, so 0.150 mol CO₂ forms. Its mass is 0.150(44.0) = 6.60 g. The calculation uses the product molar mass after the mole conversion.
How to study Unit 4
Solve one limiting-reactant problem by calculating product from each reactant separately. Then solve it using moles divided by coefficients. Compare the results and calculate the excess remaining as a second check.
Translate one precipitation reaction into molecular, complete ionic and net ionic equations. Draw a particle diagram for the final mixture that includes the spectator ions. For titrations, write the balanced neutralization equation before multiplying molarity by volume.
Ashore's Chemistry course has a short lesson for every topic in this unit, each ending with a few questions on what it taught.
- AP Chemistry: all 9 units, exam format and scores
- AP Chemistry Unit 1: Atomic Structure and Properties
- AP Chemistry Unit 2: Compound Structure and Properties
- AP Chemistry Unit 3: Properties of Substances and Mixtures
- AP Chemistry Unit 5: Kinetics
- AP Chemistry Unit 6: Thermochemistry
- AP Chemistry Unit 7: Equilibrium
- AP Chemistry Unit 8: Acids and Bases
- AP Chemistry Unit 9: Thermodynamics and Electrochemistry
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