Chemical Reactions, Stoichiometry, and Thermodynamics
What Domain IV Tests
This domain combines reaction types, equation balancing, mole calculations, limiting reactants, percent yield, and thermochemistry. Expect multistep stoichiometry and energy questions that require you to track units, signs, and the physical meaning of ΔH, ΔS, and ΔG.
Core Rules and Concepts
- Balancing conserves atoms and charge. Change coefficients, never subscripts.
- Mole map: mass → moles using molar mass; moles → particles using Avogadro's number; moles → volume of gas at STP using 22.4 L/mol or the ideal gas law.
- Limiting reactant: calculate the amount of product each reactant can form; the smaller amount is theoretical yield. Percent yield = actual/theoretical × 100.
- Thermochemistry: at constant pressure, q = m c ΔT for the calorimeter contents. Exothermic reactions have ΔH < 0; endothermic have ΔH > 0. Hess's law allows summing enthalpy changes for steps.
- Spontaneity: ΔG = ΔH − TΔS. ΔG < 0 is spontaneous. A reaction with ΔH < 0 and ΔS > 0 is always spontaneous; the opposite is never spontaneous; mixed signs depend on temperature.
- Bond energy: breaking bonds requires energy; forming bonds releases energy.
Worked Example
4.0 g of H₂ reacts with 16.0 g of O₂ to form water: 2H₂ + O₂ → 2H₂O. What is the limiting reactant and the theoretical mass of water? Reasoning: moles H₂ = 4.0 g / 2.02 g/mol ≈ 1.98 mol; moles O₂ = 16.0 g / 32.00 g/mol = 0.500 mol. From the balanced equation, 1 mol O₂ requires 2 mol H₂. The available H₂ would require about 0.99 mol O₂, but only 0.500 mol O₂ is present, so O₂ is limiting. Product: 0.500 mol O₂ × (2 mol H₂O / 1 mol O₂) = 1.00 mol H₂O. Mass = 1.00 mol × 18.02 g/mol ≈ 18.0 g H₂O. H₂ is in excess.
Traps That Cost Marks
- Choosing the limiting reactant by smaller mass rather than by mole ratio.
- Using the molar mass of H or O instead of H₂ or O₂.
- Sign errors in ΔH when reversing a reaction or multiplying a step in Hess's law.
- Ignoring the sign convention: q for an exothermic reaction is negative, and the surroundings gain the same magnitude of heat.
- Assuming a reaction is spontaneous just because it is exothermic; entropy and temperature also matter.