Chemical Reactions — AP Chemistry Unit 4 practice.
This unit covers reaction types, stoichiometry and net ionic equations — essential concepts for AP Chemistry. Use our interactive study games to test your understanding, or review questions in traditional format below.
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This unit covers reaction types, stoichiometry and net ionic equations — essential concepts for AP Chemistry. Use our interactive study games to test your understanding, or review questions in traditional format below.
Key Concepts Breakdown
1 Reaction Types
Students must be able to identify and predict products for synthesis, decomposition, single replacement, double replacement, and combustion reactions. On the AP exam, you will be asked to write and balance reactions given only reactants. Understanding oxidation states is essential for identifying redox reactions within these types.
Key Points
- Synthesis: A + B → AB; decomposition: AB → A + B; single replacement requires activity series to predict if reaction occurs
- Double replacement (metathesis) produces a precipitate, gas, or water — if none form, no reaction occurs
- Combustion of hydrocarbons always yields CO₂ and H₂O; incomplete combustion also produces CO
- Redox reactions involve transfer of electrons; oxidation = loss, reduction = gain (OIL RIG)
Predict the products: aqueous lead(II) nitrate + aqueous potassium iodide →
This is a double replacement reaction. The ions swap partners: Pb²⁺ pairs with I⁻ to form PbI₂, and K⁺ pairs with NO₃⁻ to form KNO₃. PbI₂ is insoluble (check solubility rules: iodides of Pb²⁺ are insoluble), so a precipitate forms and the reaction proceeds: Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq).
2 Stoichiometry
Stoichiometry uses mole ratios from a balanced equation to relate amounts of reactants and products. Students must be able to identify the limiting reagent, calculate theoretical yield, and determine percent yield. The mole is the central unit — all conversions flow through it.
Key Points
- Mole ratio comes directly from balanced equation coefficients; always balance first
- Limiting reagent: divide moles of each reactant by its coefficient; the smallest value identifies the limiting reagent
- Theoretical yield is calculated from the limiting reagent; percent yield = (actual/theoretical) × 100%
- Excess reagent amount remaining = initial moles − moles consumed by limiting reagent (converted via mole ratio)
10.0 g of H₂ reacts with 10.0 g of O₂ to form water. What is the theoretical yield of H₂O?
Balanced equation: 2H₂ + O₂ → 2H₂O. Convert to moles: H₂ = 10.0/2.02 = 4.95 mol; O₂ = 10.0/32.00 = 0.313 mol. Divide by coefficients: H₂ → 4.95/2 = 2.48; O₂ → 0.313/1 = 0.313. O₂ is limiting. Theoretical yield = 0.313 mol O₂ × (2 mol H₂O / 1 mol O₂) × 18.02 g/mol = 11.3 g H₂O.
3 Net Ionic Equations
Net ionic equations show only the species that actually change during a reaction, eliminating spectator ions. Students must know solubility rules and strong acid/base lists to correctly identify which species are dissociated (written as ions) versus intact (written as formula units). The AP exam frequently asks for the net ionic equation, not the molecular equation.
Key Points
- Strong electrolytes (strong acids, strong bases, soluble salts) are written as dissociated ions in solution
- Weak acids, weak bases, insoluble compounds, and gases are written in molecular form — never split them
- Spectator ions appear identically on both sides; cancel them to get the net ionic equation
- Charges and atoms must balance in the net ionic equation; check both before finalizing
Write the net ionic equation for mixing aqueous solutions of Na₂SO₄ and BaCl₂.
Full molecular equation: Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq). Complete ionic equation: 2Na⁺ + SO₄²⁻ + Ba²⁺ + 2Cl⁻ → BaSO₄(s) + 2Na⁺ + 2Cl⁻. Cancel spectator ions Na⁺ and Cl⁻; net ionic equation: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s).
Questions, answered.
What is Chemical Reactions?
Chemical Reactions is Unit 4 of AP Chemistry, covering reaction types, stoichiometry and net ionic equations.
How to study for AP Chemistry Unit 4?
Start with the Quick Summary above, review the Key Concepts, then test yourself with our interactive study games. Aim for 80%+ accuracy before moving on.
How many questions are in this unit?
This unit has 30+ review questions across 5 different game modes.