Inside This Unit: The Full Breakdown
Chemical Reactions covers the types of reactions, stoichiometry, and how to predict products. This unit teaches you to balance equations, use mole ratios, and identify reaction types including synthesis, decomposition, acid-base, and redox reactions.
Why it matters
Stoichiometry and reaction prediction are fundamental skills tested heavily on the AP exam, both in multiple choice and free response. Balancing equations and performing mole calculations are required for nearly every quantitative problem.
Key concepts
- Chemical equations must be balanced to satisfy the law of conservation of mass. Coefficients represent mole ratios.
- Stoichiometry uses mole ratios from balanced equations to convert between masses, moles, and volumes of reactants and products.
- Net ionic equations show only the species that participate in a reaction; spectator ions are removed.
- Oxidation-reduction (redox) reactions involve transfer of electrons. Oxidation is loss of electrons; reduction is gain of electrons.
Balancing and Types of Reactions
Balanced chemical equations reflect the conservation of mass — atoms are neither created nor destroyed. The coefficients in a balanced equation give the mole ratios between reactants and products. Common reaction types include synthesis (A + B → AB), decomposition (AB → A + B), single replacement (A + BC → AC + B), and double replacement (AB + CD → AD + CB). Combustion reactions involve a fuel reacting with oxygen to produce CO₂ and H₂O. Precipitation reactions occur when mixing aqueous solutions produces an insoluble product, which can be predicted using solubility rules. Recognizing reaction types helps you predict products on the AP exam.
Stoichiometry and Limiting Reagents
Stoichiometry is the quantitative study of reactants and products in chemical reactions. The mole is the central unit: one mole equals 6.022 × 10²³ particles (Avogadro's number). Molar mass (g/mol) converts between grams and moles. To solve stoichiometry problems, convert the given quantity to moles, use the mole ratio from the balanced equation, then convert to the desired unit. The limiting reagent is the reactant that runs out first, determining the maximum amount of product (theoretical yield). The excess reagent is left over. Percent yield (actual yield ÷ theoretical yield × 100) measures reaction efficiency. Stoichiometry applies to all states: for gases, the molar volume at STP (22.4 L/mol) and PV = nRT are useful conversion tools.
Ionic Equations and Redox
Reactions in aqueous solution are often written as net ionic equations, which include only the species that undergo chemical change. Strong electrolytes (strong acids, strong bases, soluble salts) dissociate completely in water and are written as separated ions. Weak electrolytes and insoluble compounds remain as complete formulas. Spectator ions appear unchanged on both sides and are removed from the net ionic equation. Redox reactions involve electron transfer: the substance that loses electrons is oxidized (its oxidation number increases), and the substance that gains electrons is reduced (its oxidation number decreases). An oxidizing agent gains electrons (is reduced); a reducing agent loses electrons (is oxidized). Assigning oxidation numbers systematically helps identify what is oxidized and reduced in complex reactions.
AP exam tip
On AP Chemistry, net ionic equation questions are very common. Always start by writing the complete ionic equation (splitting all strong electrolytes into ions), then cancel spectator ions. Practice with precipitation, acid-base, and redox reactions.
Connections to other units
- Unit 5 (Kinetics): Stoichiometry determines the amounts of reactants and products, while kinetics determines the rate at which they form.
- Unit 6 (Thermodynamics): Enthalpy changes are calculated per mole of reaction as written in the balanced equation.
- Unit 9 (Applications of Thermodynamics): Electrochemistry is based on redox half-reactions and electron transfer.
