Unit 4: Chemical Reactions
AP Chemistry: 90 practice questions with detailed explanations.
Unit Study Guide
Executive Summary
Unit 4 turns word descriptions into balanced equations and uses stoichiometry to predict how much product forms.
Reaction types and representations
Identify synthesis, decomposition, combustion, single replacement, and double replacement (precipitation) reactions. Molecular equations show all species; complete ionic equations split aqueous electrolytes into ions; net ionic equations remove spectator ions. Acid–base neutralization produces water and a salt; redox reactions transfer electrons.
Physical vs. chemical change
Chemical changes form new substances (new bonds, color/energy change); physical changes alter state or form but not composition. Dissolving is usually physical (ions retain identity); electrolysis or precipitation is chemical.
Stoichiometry
Balanced coefficients give mole ratios. Convert grams → moles → mole ratio → moles → grams. Limiting reactant: run out first, sets product yield. Percent yield = (actual/theoretical) × 100%.
Titration
Titration delivers a solution of known concentration (titrant) to reach the equivalence point, where moles of H⁺ equal moles of OH⁻ (for acid–base). Use MₐVₐ = MbVb when the stoichiometry is 1:1; otherwise convert to moles. The endpoint (indicator color change) approximates the equivalence point.
Redox
Oxidation is loss of electrons (oxidation number increases); reduction is gain (number decreases). Assign oxidation numbers by rules (free element = 0, oxygen −2 except peroxides, hydrogen +1 except hydrides). A redox reaction always pairs the two half-reactions so electrons cancel.
Acid–base reactions
Arrhenius: acids give H⁺, bases give OH⁻. Neutralization: H⁺ + OH⁻ → H₂O. Strong acids (HCl, HBr, HI, HNO₃, H₂SO₄, HClO₄) and strong bases (Group 1 and heavy Group 2 hydroxides) dissociate completely; net ionic equations for strong–strong neutralization collapse to H⁺ + OH⁻ → H₂O.
Quantitative skill-set
Grams ↔ moles conversions; limiting-reactant chains; M₁V₁ = M₂V₂; oxidation-number bookkeeping; percent yield.
Exam traps
Coefficients in a balanced equation are mole ratios, never mass ratios. Spectator ions must cancel in net ionic equations. The limiting reactant is not always the one with fewer grams — convert to moles first. Oxidation numbers change only in redox reactions.