Unit 4: Chemical Reactions
Practice AP Chemistry Unit 4 — reaction types, net ionic equations, stoichiometry, limiting reagents, and titrations. Sharpen your quantitative FRQ skills.
What Unit 4 Covers in AP Chemistry
Unit 4 is one of the most quantitatively rich units in AP Chemistry. It covers the types of reactions you must recognize and balance, as well as the stoichiometric calculations that dominate the free-response section. Fluency in net ionic equations and limiting reagent analysis is essential for AP-level performance.
Core Topics in Unit 4
- Reaction types — Synthesis, decomposition, single and double displacement, combustion, acid-base (neutralization), oxidation-reduction; recognizing reaction types from reactants and products
- Net ionic equations — Writing complete and net ionic equations; identifying spectator ions; predicting precipitate formation using solubility rules
- Stoichiometry — Mole-to-mole ratios, mole-to-gram conversions, and multi-step stoichiometric calculations
- Limiting reagent and percent yield — Identifying the limiting reagent, calculating theoretical yield, and computing percent yield from experimental data
- Titrations — Acid-base titration calculations, determining unknown concentration from titrant volume, and understanding the equivalence point
Key AP Quantitative FRQ Skills
Unit 4 FRQs are often multi-step calculations. A typical question might ask you to write a net ionic equation, calculate moles of a reactant from given mass, identify the limiting reagent, and then determine the percent yield from an experimental result. Each step is scored individually, so showing clear, organized work earns partial credit even if a final answer is incorrect.
Writing Strong Net Ionic Equations
AP graders look for correctly balanced net ionic equations with correct charges and states. Common errors include omitting the state symbols (aq), (s), (l), (g) and failing to cancel spectator ions completely. Memorize solubility rules and the strong acids and bases — these determine which species remain as ions in solution.
Common Mistakes in Unit 4
- Choosing the reagent present in the smallest mass as the limiting reagent without converting to moles first
- Forgetting to account for the mole ratio from the balanced equation when identifying the limiting reagent
- Writing molecular equations instead of net ionic equations when the question specifically asks for the net ionic form
- Confusing percent yield (experimental/theoretical) with percent purity or percent composition