Chemical Reactions
Unit 4 of AP Chemistry, worth 7–9% of the exam. 14 questions below, each with the working. Every answer was checked by a second pass before it was published.
Physical and chemical change, net ionic equations, representations of reactions, stoichiometry, titrations, types of reactions, acid-base and redox reactions.
How this unit is tested
Start by fixing the vocabulary: every reaction question on this unit hinges on being able to tell physical change from chemical change, and molecular equations from complete and net ionic equations. Practice converting a molecular equation to a full ionic equation and then crossing out spectator ions — this is one of the most heavily tested skills in Unit 4, and the AP exam expects correct phases (s, l, g, aq) and correctly balanced charge, not just mass.
Next, drill reaction classification (synthesis, decomposition, single replacement, double replacement/precipitation, combustion, acid–base, redox) until you can look at an equation and name its type in seconds, because this identification is often the first step of a longer free-response question. For redox specifically, get fast and reliable at assigning oxidation numbers using the standard rule hierarchy, then use the changes in oxidation number to name what was oxidized, what was reduced, and which species is the oxidizing vs. reducing agent — students frequently reverse oxidizing/reducing agent, so practice this direction deliberately.
For stoichiometry, always start from a correctly balanced equation and convert everything to moles before doing any ratio work; identify the limiting reactant whenever two amounts are given, and only then compute theoretical yield, percent yield, or solution concentrations. Titration problems are just stoichiometry in a solution context: use moles = molarity × volume (in liters), respect the mole ratio from the balanced equation (it is not always 1:1), and use the equivalence point condition that moles of acid's transferable H⁺ equal moles of base's transferable OH⁻ (or accepted protons).
Finally, practice reading particulate (molecular-level) diagrams and matching them to a written or ionic equation, since the AP exam frequently asks you to connect a microscopic representation to a macroscopic reaction type or to identify spectator ions/precipitates directly from a picture.
What you have to know
Law of Conservation of Mass
In any chemical reaction, the total mass of the reactants equals the total mass of the products; atoms are neither created nor destroyed, only rearranged.
Net Ionic Equation Rule
To write a net ionic equation, write the complete ionic equation by dissociating all strong electrolytes into ions, then cancel any ion that appears unchanged on both sides (a spectator ion); only species that actually react remain.
Oxidation Number Assignment
Free elements are 0; monatomic ions equal their charge; oxygen is usually -2 (-1 in peroxides); hydrogen is usually +1 (-1 with metals); the sum of oxidation numbers equals the overall charge of the species.
Redox Definitions
Oxidation is a loss of electrons (increase in oxidation number); reduction is a gain of electrons (decrease in oxidation number). The oxidizing agent is the species reduced; the reducing agent is the species oxidized.
Titration Equivalence Point Condition
At the equivalence point of an acid–base titration, moles of H+ delivered by the acid equal moles of OH- (or accepted protons) delivered by the base; for a monoprotic acid and monobasic base this gives M(acid)×V(acid) = M(base)×V(base).
Mole Ratio Stoichiometry
Given a balanced chemical equation, the coefficients give the mole ratio between any two species; convert given quantities to moles, apply the ratio, then convert to the desired unit, always checking for a limiting reactant when two reactant amounts are given.
14 practice questions
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A piece of dry ice (solid CO2) is left on a table and disappears, leaving behind CO2 gas. Is this a physical or chemical change, and why?
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Answer. Physical change
Sublimation only changes the state of CO2 from solid to gas; the chemical identity of the substance (CO2) is unchanged, so no new substance is formed. -
Which observation is the strongest evidence that a chemical change has occurred?
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Answer. Formation of a gas that was not present before, accompanied by a temperature change
Gas evolution with an accompanying energy change indicates new bonds are forming and breaking, unlike a simple change of state or dissolving, which are physical. -
Write the net ionic equation for the reaction between aqueous silver nitrate and aqueous sodium chloride.
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Answer. Ag+(aq) + Cl-(aq) → AgCl(s)
The molecular equation is AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq). Dissociating all strong electrolytes and canceling the spectator ions Na+ and NO3- leaves only the ions that form the precipitate. -
In the reaction Pb(NO3)2(aq) + 2 KI(aq) → PbI2(s) + 2 KNO3(aq), identify the spectator ion(s).
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Answer. K+ and NO3-
K+ and NO3- appear in identical form on both sides of the complete ionic equation and do not participate in forming the precipitate PbI2, so they are removed to write the net ionic equation. -
Classify the reaction CaCO3(s) → CaO(s) + CO2(g).
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Answer. Decomposition reaction
A single compound breaks down into two or more simpler substances, which is the defining pattern of a decomposition reaction (this one also happens to be driven by heat). -
Classify the reaction Zn(s) + 2 HCl(aq) → ZnCl2(aq) + H2(g), and state which type of reaction it also represents.
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Answer. Single replacement reaction; it is also a redox reaction
A free element (Zn) replaces H in the compound HCl. Because Zn loses electrons (0 → +2) and H gains electrons (+1 → 0), electron transfer occurs, making it both a single replacement and a redox reaction. -
What is the oxidation number of manganese in the permanganate ion, MnO4-?
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Answer. +7
Oxygen is assigned -2 each, giving -8 total for four oxygens. Since the overall ion charge is -1, Mn must be +7 to balance: Mn + (-8) = -1, so Mn = +7. -
In the reaction Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s), which species is the oxidizing agent?
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Answer. Cu2+
Cu2+ is reduced (gains 2 electrons, +2 → 0), and the species that is reduced is by definition the oxidizing agent, since it removes electrons from Zn. -
In the reaction 2 Fe3+(aq) + Sn2+(aq) → 2 Fe2+(aq) + Sn4+(aq), which species is reduced?
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Answer. Fe3+
Fe3+ gains one electron to become Fe2+ (oxidation number decreases from +3 to +2), which is the definition of reduction; Sn2+ is oxidized to Sn4+. -
Write the complete ionic equation for BaCl2(aq) + Na2SO4(aq) → BaSO4(s) + 2 NaCl(aq).
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Answer. Ba2+(aq) + 2 Cl-(aq) + 2 Na+(aq) + SO4^2-(aq) → BaSO4(s) + 2 Na+(aq) + 2 Cl-(aq)
All soluble strong electrolytes (BaCl2 and Na2SO4 as reactants, NaCl as product) are written as dissociated ions, while the insoluble BaSO4 stays as a solid formula, since it does not dissociate. -
In the reaction HC2H3O2(aq) + NaOH(aq) → NaC2H3O2(aq) + H2O(l), which reactant is the Bronsted-Lowry acid?
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Answer. HC2H3O2 (acetic acid)
A Bronsted-Lowry acid is a proton (H+) donor. HC2H3O2 donates a proton to OH- to form water, while NaOH's hydroxide ion accepts the proton, making it the base. -
For the reaction N2(g) + 3 H2(g) → 2 NH3(g), if 2.0 mol N2 is mixed with 3.0 mol H2, which reactant is limiting?
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Answer. H2 is the limiting reactant
Complete reaction of 2.0 mol N2 requires 2.0 × 3 = 6.0 mol H2, but only 3.0 mol H2 is available, so H2 runs out first and limits the amount of NH3 produced. -
A reaction has a theoretical yield of 45.0 g of product, but only 38.2 g is actually recovered. What is the percent yield?
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Answer. 84.9%
Percent yield = (actual yield / theoretical yield) × 100% = (38.2 g / 45.0 g) × 100% ≈ 84.9%. -
A 25.00 mL sample of unknown H2SO4 is titrated with 0.200 M NaOH via H2SO4(aq) + 2 NaOH(aq) → Na2SO4(aq) + 2 H2O(l), requiring 30.00 mL of NaOH to reach the equivalence point. What is the molarity of the H2SO4 solution?
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Answer. 0.120 M
Moles NaOH = 0.200 M × 0.03000 L = 6.00×10⁻³ mol. Using the 1:2 mole ratio, moles H2SO4 = 3.00×10⁻³ mol. Dividing by the acid's volume (0.02500 L) gives 0.120 M H2SO4.
What people get wrong
- Assuming every titration ratio is 1:1 — check the balanced equation first; polyprotic acids or bases with more than one reactive OH- change the ratio, as in H2SO4 + 2 NaOH.
- Forgetting to convert volumes from mL to L before using M = mol/L, which produces an answer off by a factor of 1000.
- Writing a net ionic equation without first checking solubility rules, so a spectator ion is left in (or a species that should dissociate is kept molecular) — always confirm which compounds are strong electrolytes before canceling ions.
- Confusing the oxidizing agent with the species that is oxidized — the oxidizing agent is the substance that gets reduced (gains electrons), even though it 'causes' oxidation in the other species.
- Skipping the limiting reactant check when two reactant quantities are given, and basing yield calculations only on one reactant, which gives a wrong theoretical yield.
- Calling any color change or bubble formation proof of chemical change without considering context — dissolving a colored solid can look like a color change but may be purely physical (e.g., simple dissolution without reaction).
Drill this unit until it sticks
These questions come back on a schedule built from what you get wrong, alongside the rest of AP Chemistry. Free, and no account needed to start.