Chemical Reactions and Stoichiometry
The Chemical Reactions and Stoichiometry domain tests balancing equations, mole conversions, limiting reactants, percent yield, solution stoichiometry, and reaction classification. Expect items that require choosing the correct mole ratio and identifying the reactant that controls product formation.
Core rules and concepts
- A balanced equation conserves atoms and charge; coefficients give mole ratios.
- Moles connect mass, particles, and gas volume through molar mass, Avogadro’s number, and molar volume when applicable.
- The limiting reactant is consumed first and determines theoretical yield; the excess reactant remains.
- Percent yield = actual yield / theoretical yield × 100%.
- For solutions, moles = molarity × volume in liters; net ionic equations remove spectator ions.
- Common reaction types include synthesis, decomposition, single replacement, double replacement, combustion, and acid-base neutralization.
Worked example
For 2H₂ + O₂ → 2H₂O, start with 10.0 g H₂ (4.96 mol) and 10.0 g O₂ (0.3125 mol). The equation requires 2 mol H₂ per 1 mol O₂, so 0.3125 mol O₂ needs 0.625 mol H₂. Hydrogen is in excess; O₂ is limiting. O₂ produces 2 × 0.3125 = 0.625 mol H₂O, or 0.625 × 18.02 = 11.3 g. Comparing grams directly would be wrong because the mole ratio, not the mass ratio, controls the reaction.
Traps that cost marks
- Using a gram ratio instead of a mole ratio.
- Choosing the reactant with the smaller mass as limiting.
- Forgetting to convert solution volume to liters.
- Reporting theoretical yield as actual yield.
- Keeping spectator ions in a net ionic equation.