AP Chemistry · Handsworth Secondary 2026–27
Unit 4 · Study Guide
Dr. Ras Mulinta
Chemical Reactions & Stoichiometry
Exam-focused review
A one-page map of what Unit 4 expects on the AP exam. There is no separate Unit 4 test: but these skills show up all over the exam (and feed Units 8 and 9), so own them cold. This is a checklist, not a re-teach: if a line doesn't click, go back to that section of the notes package. A periodic table is provided.
Must be able to do
- 4.1 / 4.4 Tell a physical change from a chemical one by evidence (heat/light, gas, precipitate, colour) and by whether bonds form/break.
- 4.2 / 4.3 Write & balance equations in all three forms, molecular → complete ionic → net ionic (cancel spectators); conserve mass and charge.
- 4.7 Classify a reaction as precipitation, acid–base, or redox (combustion = redox) and justify it from the equation.
- 4.8 Identify Brønsted–Lowry acids (H⁺ donors), bases (H⁺ acceptors), and conjugate acid–base pairs.
- 4.5 Mole-ratio stoichiometry through moles, mass n = m/M, solution n = M·V, gas PV = nRT; find the limiting reactant by moles and % yield = actual/theoretical × 100%.
- 4.6 Use a titration: moles titrant → mole ratio → moles analyte → unknown concentration; distinguish equivalence point from endpoint.
- 4.9 Assign oxidation numbers, identify oxidized/reduced species, balance a simple redox reaction from half-reactions (full redox in Unit 9).
- Lab Error analysis: % error = |measured − accepted|/accepted × 100%, plus naming specific sources and their direction.
The big idea that ties it together
Atoms are only rearranged, so count them. Every skill in this unit is the same conservation law wearing a different hat: a balanced equation conserves mass and charge, a net ionic equation strips it to the atoms that actually change, and stoichiometry, limiting reactant, and titration are all just the mole ratio applied. When stuck, go back to moles.
Don't waste time on (excluded by the CED)
Rote "solubility rules" beyond all Na⁺, K⁺, NH₄⁺, NO₃⁻ salts are soluble (problems will give you the rest) · the terms "oxidizing agent" / "reducing agent" (identify what is oxidized/reduced, but the labels aren't graded) · Lewis acid–base concepts (AP sticks to Brønsted–Lowry proton transfer in aqueous solution). Know the patterns, skip the edge cases.
Quick self-check (answer in your head, then verify)
- Net ionic equation for mixing AgNO₃(aq) with NaCl(aq) (AgCl is insoluble)?
- Balance: C₃H₈ + O₂ → CO₂ + H₂O.
- N₂ + 3 H₂ → 2 NH₃. Mix 28.0 g N₂ (M = 28.02) with 5.0 g H₂ (M = 2.016). Limiting reactant, and mol NH₃ formed?
- 25.00 mL of HCl is neutralized by 30.00 mL of 0.100 M NaOH (1 : 1). Find [HCl].
- In HCO₃⁻ + H₂O ⇌ CO₃²⁻ + H₃O⁺, what is the conjugate base of HCO₃⁻?
- Oxidation number of Cr in Cr₂O₇²⁻?
- A lab measures 9.6% when the accepted value is 10.0%. Percent error?
Check yourself: 1) Ag⁺(aq) + Cl⁻(aq) → AgCl(s) 2) C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O 3) n(N₂)=1.00 mol (÷1=1.00); n(H₂)=2.48 mol (÷3=0.826) → H₂ limits → NH₃ = 2.48 × 2/3 = 1.65 mol 4) n(NaOH)=0.100×0.03000=3.00×10⁻³ mol → [HCl]=3.00×10⁻³/0.02500 = 0.120 M 5) CO₃²⁻ (HCO₃⁻ donates an H⁺) 6) 2x + 7(−2) = −2 → x = +6 7) |9.6 − 10.0|/10.0 × 100% = 4.0%.