Limiting Reactants: Which Ingredient Runs Out First?
50 min · HS-PS1-7
Objective
Students will identify the limiting reactant in a balanced chemical reaction and calculate the theoretical yield of product using mole ratios.
Put a s'mores recipe on the board: 2 graham crackers, 1 marshmallow, and 1 chocolate square make 1 s'more. Tell students you have 10 graham crackers, 4 marshmallows, and 7 chocolate squares. Ask how many complete s'mores they can make and what is left over. The ingredient you run out of first is the limiting reactant.
Optional video:search YouTube for“limiting reactant sandwich analogy”
Preview it yourself before class to make sure it fits and is classroom-appropriate.
- 10m
From recipe to reaction
Connect the s'mores recipe to a balanced equation. Write N₂ + 3H₂ → 2NH₃ on the board. The coefficients are a recipe: 1 mole of N₂ needs 3 moles of H₂. If there is not enough H₂, the leftover N₂ has nothing to react with. Emphasize that mass is conserved, so every atom on the left appears on the right.
- 12m
Comparing mole ratios
Work a full example. Given 4 mol N₂ and 9 mol H₂, ask which runs out first. Using the 1:3 mole ratio, 4 mol N₂ would need 12 mol H₂, but only 9 mol is available, so H₂ is the limiting reactant. Then calculate theoretical yield: 9 mol H₂ × (2 mol NH₃ / 3 mol H₂) = 6 mol NH₃. Model the setup so units cancel every time.
- 15m
Baking soda and vinegar limiting-reactant labLab
Students react baking soda with vinegar: NaHCO₃ + CH₃COOH → CH₃COONa + H₂O + CO₂. Groups hold the mass of baking soda fixed and add increasing volumes of vinegar, recording the mass lost as CO₂ escapes. When adding more vinegar stops increasing the mass lost, baking soda has become the limiting reactant.
Optional video:search YouTube for“baking soda and vinegar reaction”
Preview it yourself before class to make sure it fits and is classroom-appropriate.
Materials
- Baking soda (NaHCO₃)
- White vinegar (CH₃COOH)
- Balance
- Plastic cups
- Weigh boats
- Goggles
Example outputs
- A graph that rises, then flattens once baking soda becomes limiting.
- A written claim: past 40 mL of vinegar, baking soda is the limiting reactant because more vinegar produced no additional CO₂.
No-equipment fallback
Give groups a data table from a completed run and have them graph mass of CO₂ released versus volume of vinegar added, then mark the point where the limiting reactant changes from vinegar to baking soda.
For N₂ + 3H₂ → 2NH₃, you have 5 mol N₂ and 12 mol H₂. Which is the limiting reactant?
multiple choiceH₂. 5 mol N₂ would need 15 mol H₂, but only 12 mol is available.Using the amounts above, calculate the theoretical yield of NH₃ in moles.
calculation12 mol H₂ × (2 mol NH₃ / 3 mol H₂) = 8 mol NH₃.Explain in one sentence why the leftover reactant is called the excess reactant.
short answerIt is the reactant that remains after the limiting reactant is used up, so the reaction stops before it is fully consumed.
- Limiting reactant
- The reactant that runs out first and stops the reaction, setting the maximum amount of product.
- Excess reactant
- The reactant left over after the reaction stops.
- Mole ratio
- The ratio of moles of one substance to another, taken from the coefficients in the balanced equation.
- Theoretical yield
- The maximum amount of product that can form, calculated from the limiting reactant.
- Stoichiometry
- The math that relates the amounts of reactants and products in a chemical reaction.
- Students assume the reactant with the smaller mass is always the limiting reactant, ignoring the mole ratio.
- Students compare moles of reactants directly without using the coefficients from the balanced equation.
- Students think the limiting reactant is whatever is written first in the equation.
- Baking soda
- White vinegar
- Balance
- Plastic cups
- Weigh boats
- Goggles
- Calculator