Chemical Reactions and Stoichiometry
What this domain tests
Chemical Reactions and Stoichiometry tests balanced equations, reaction types, mole conversions, limiting reactants, percent yield, and solution stoichiometry. Expect multi-step calculations that move from mass or volume to moles and back, with attention to units and significant figures.
Balancing and reaction types
- Balance by adjusting coefficients only; subscripts define the substance. Check both atoms and charge.
- Synthesis: A + B → AB. Decomposition: AB → A + B. Single replacement uses the activity series. Double replacement uses solubility rules and often forms a precipitate. Combustion of a hydrocarbon gives CO2 + H2O.
- Net ionic equations cancel spectator ions; verify that atoms and charge balance.
Mole relationships
- 1 mol = 6.022 × 10²³ particles; n = m/M; molarity M = mol solute ÷ L solution; at STP an ideal gas occupies 22.4 L/mol.
- Stoichiometry path: mass → moles → mole ratio → moles → target mass, volume, or particles.
- Limiting reactant: divide each reactant's moles by its coefficient; the smallest quotient limits. The excess reactant remains.
- Percent yield = actual yield ÷ theoretical yield × 100.
- Empirical formula from percent composition: assume 100 g, convert to moles, divide by the smallest quotient; molecular formula = empirical formula × (molar mass ÷ empirical formula mass).
Worked example
10.0 g Al reacts with 10.0 g Cl2: 2Al + 3Cl2 → 2AlCl3. n(Al) = 10.0 ÷ 26.98 = 0.371 mol; n(Cl2) = 10.0 ÷ 70.90 = 0.141 mol. Cl2 requires 0.141 × (2/3) = 0.0940 mol Al, which is available, so Cl2 is limiting. n(AlCl3) = 0.141 × (2/3) = 0.0940 mol; mass = 0.0940 × 133.34 = 12.5 g. Aluminum remains in excess.
Traps that cost marks
- Using mass ratios instead of mole ratios.
- Skipping the balance or changing subscripts to balance.
- Choosing the limiting reactant by larger amount without comparing coefficients.
- Using 35.45 g/mol for Cl2 rather than 70.90 g/mol.
- Reporting a yield above 100% as valid; impurities or wet product often explain it.
- Substituting milliliters for liters in molarity calculations.