AP Chemistry, AP exams, 7–9%

Unit 4: Chemical Reactions

Unit 4 covers how reactions are represented and classified, and the stoichiometry that quantifies them. Net ionic equations, particulate representations, titration calculations, and the distinction between physical and chemical change are the core skills. It also introduces acid-base and oxidation-reduction chemistry that Units 8 and 9 develop further.

Position: 4 of 9 units
Published weighting: 7–9% in the official framework
Topics covered: 6 topics in this guide

What is covered

Topics inside this unit.

These topics describe the scope students most often review for this part of AP Chemistry. Treat them as a study checklist rather than an exhaustive list of what a single exam form can sample.

Representations of reactions: Balanced molecular, complete ionic, and net ionic equations, plus particulate diagrams of the same process.
Physical and chemical changes: Distinguishing the two at the particle level and identifying evidence that a chemical change occurred.
Stoichiometry: Mole ratios, limiting reactant analysis, percent yield, and gravimetric analysis.
Titrations: Equivalence point determination, titration curve features, and the arithmetic of a titration calculation.
Types of chemical reactions: Precipitation, acid-base, and oxidation-reduction reactions and how to recognize each from a description.
Introduction to redox: Assigning oxidation numbers, identifying the oxidized and reduced species, and balancing half-reactions.

Study tips

How to practice Chemical Reactions.

Section-specific guidance that follows from how this material is usually tested.

Write net ionic equations for every precipitation and neutralization you meet; the exam asks for them constantly.
Practice limiting reactant problems in the particulate representation as well as with numbers.
Keep solubility rules on a single card and review them until identification is instant.

Where this fits

Unit 4: Chemical Reactions is 4 of 9 units in the AP Chemistry guide, and it carries a published weighting of 7–9% in the official framework. Use the full guide to balance it against everything else you have to review.

Back to the AP Chemistry guide Official exam information

Sample Chemical Reactions questions

Practice questions, not yet expert-reviewed

Original practice questions written to the published framework for this section. No expert has reviewed them, and they are not taken from any official exam. Try each one before you open the answer.

Question 1Foundational
When the combustion reaction C3H8 + O2 -> CO2 + H2O is balanced with the lowest whole-number coefficients, what is the coefficient in front of O2?
1. A3
2. B4
3. C5
4. D7
Show answer
Answer: C. 5
Balancing carbon first gives 3 CO2 and balancing hydrogen gives 4 H2O, which together require 6 plus 4, or 10 oxygen atoms on the right. Ten oxygen atoms come from 5 molecules of O2, so the coefficient is 5. Answering 7 comes from counting the 3 CO2 and 4 H2O product molecules instead of the 10 oxygen atoms inside them.
Key insight: Balance carbon, then hydrogen, then oxygen last, because oxygen appears in more than one product.
Question 2Foundational
What is the oxidation number of sulfur in the sulfate ion, SO4^2-?
1. A-2
2. B+2
3. C+4
4. D+6
Show answer
Answer: D. +6
Each oxygen is assigned -2, so four oxygens contribute -8, and the oxidation numbers must sum to the ion charge of -2. Solving -8 plus the sulfur value equals -2 gives +6 for sulfur. Answering -2 mistakes the charge on the whole ion for the oxidation number of the central atom.
Key insight: Oxidation numbers in a polyatomic ion sum to the ion's charge, not to zero.
Question 3Intermediate
Aqueous silver nitrate is mixed with aqueous sodium chloride and a white precipitate of silver chloride forms. What is the net ionic equation for this reaction?
1. AAgNO3(aq) + NaCl(aq) -> AgCl(s) + NaNO3(aq)
2. BAg+(aq) + Cl-(aq) -> AgCl(s)
3. CNa+(aq) + NO3-(aq) -> NaNO3(s)
4. DAg+(aq) + NO3-(aq) -> AgNO3(s)
Show answer
Answer: B. Ag+(aq) + Cl-(aq) -> AgCl(s)
A net ionic equation keeps only the species that change, which here are the silver and chloride ions that leave solution as an insoluble solid. Sodium and nitrate ions remain dissolved and unchanged, so they are spectator ions and are removed. The full molecular equation is the tempting distractor, but it still lists those spectators.
Key insight: Write the complete ionic equation first, then cancel anything identical on both sides.
Question 4Intermediate
For the reaction N2 + 3 H2 -> 2 NH3, a sealed vessel contains 2.0 mol of N2 and 3.0 mol of H2. Which reactant limits the amount of ammonia produced?
1. AN2
2. BNeither, because they are present in the exact stoichiometric ratio
3. CH2
4. DIt cannot be determined without the temperature
Show answer
Answer: C. H2
The balanced equation requires three moles of H2 for every mole of N2, so 2.0 mol of N2 would need 6.0 mol of H2 but only 3.0 mol is available. Hydrogen therefore runs out first and limits the reaction, leaving 1.0 mol of N2 unreacted. Choosing N2 follows from comparing raw mole counts without applying the 3 to 1 mole ratio.
Key insight: Compare moles available to moles required by the balanced equation, never the raw amounts.
Question 5Advanced
A 25.00 mL sample of hydrochloric acid requires 32.00 mL of 0.150 mol/L sodium hydroxide to reach the equivalence point. What is the concentration of the acid?
1. A0.117 mol/L
2. B0.150 mol/L
3. C0.192 mol/L
4. D0.240 mol/L
Show answer
Answer: C. 0.192 mol/L
Moles of base equal 0.03200 L times 0.150 mol/L, which is 0.00480 mol, and HCl reacts with NaOH in a 1 to 1 ratio, so the flask held 0.00480 mol of acid. Dividing by 0.02500 L gives 0.192 mol/L. The value 0.117 mol/L comes from inverting the volume ratio, which would make the acid more dilute than the base even though a smaller volume of acid supplied the same number of moles.
Key insight: At the equivalence point moles of acid equal moles of base times the mole ratio, so solve for moles before concentration.

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Unit 4: Chemical Reactions