Study guide: Conserving atoms and predicting reaction amounts
Use balanced symbolic models, mass boundaries, and stoichiometric ratios.
Ideas to explain in your own words
- Count each type of atom on both sides of a chemical equation.
- Explain apparent mass changes by tracking material across a boundary.
- Scale a balanced equation without changing its formulas.
- Count complete reaction sets and leftover particles.
Watch your reasoning
- 2 H₂ + O₂ → 2 H₂O has four H and two O atoms on each side. The other choices have unequal hydrogen or oxygen counts; changing a coefficient affects every atom in that formula.
- 75 − 69 = 6 g crossed out under the complete no-entry model. This is a boundary transfer, not destruction of matter.
- The balanced ratio H₂O:H₂ is 2:2 = 1:1, so 4 mol H₂ produces 4 mol H₂O.
- N₂ limits the packet to two sets. They use 6 H₂ and make 4 NH₃, leaving 9 − 6 = 3 H₂.
A worked review example
Check whether an equation conserves atoms: In 2 H₂ + O₂, hydrogen count is 2 × 2 = 4 and oxygen count is 2. In 2 H₂O, hydrogen count is 2 × 2 = 4 and oxygen count is 2 × 1 = 2. Both element counts match. Check Mass with a System Boundary: Closed total stays 160 g. Open ledger: 152 g remaining + 8 g transferred out = 160 g. The apparent 8 g loss is material outside the measured boundary. Use Coefficients as Mole Ratios: 6 mol H₂ × (1 mol O₂ / 2 mol H₂) = 3 mol O₂. The H₂O:H₂ ratio is 2:2, giving 6 mol H₂O under the completion and sufficient-reactant assumptions. Find the Limiting Reactant in a Particle Model: Three sets form 6 NH₃ and leave 1 N₂. Before: 8 N atoms and 18 H atoms. After: 6 N and 18 H in NH₃ plus 2 N in leftover N₂, preserving both element counts.
Make a practice plan
- Try the supplied lesson problems without looking at the answers.
- Compare your method with the worked examples and explain any correction.
- Use the new review cases, then complete the transfer task on paper or in your own document.