CHEM121 Notes - Limiting Reactants

Concept of Limiting Reactant

Definition:

The limiting reactant (LR) is the reactant that runs out first in a chemical reaction, limiting the amount of product that can be formed.

Example of Limiting Reactant with Ammonia (NH₃)

Scenario:

Calculating product formation with 5 grams of N₂ and an excess of H₂. Both reactants are needed to produce NH₃.

Outcome:

Once N₂ runs out, no more NH₃ can be produced regardless of the amount of H₂ left.

Calculation Steps:

  1. Convert grams of N₂ to moles using the molar mass.

  2. Use stoichiometry to convert moles of N₂ to moles of NH₃.

  3. Convert moles of NH₃ back to grams using its molar mass.

Key Point:

N₂ is the limiting reactant in this scenario.

Example with Pizza Ingredients

Ingredients:

4 crusts, 15 ounces of sauce, 10 cups of cheese.

Calculating Maximum Pizzas:

  • Crust: One crust = one pizza (4 pizzas).

  • Sauce: 5 ounces per pizza (Can make 3 pizzas from 15 ounces).

  • Cheese: 2 cups per pizza (Can make 5 pizzas from 10 cups).

Conclusion:

The maximum number of pizzas is 3, limited by the sauce.

Definition:

The limiting reactant (sauce) restricts the product amount.

Leftover Ingredients Calculation

To find how much of each ingredient remains after maximum pizzas are made:

  • Crust (4 initial - 3 used) = 1 leftover.

  • Sauce (15 initial - 15 used) = 0 leftover (limiting reactant).

  • Cheese (10 initial - 6 used) = 4 leftover.

Terminology:

  • Reactants in Excess: Ingredients that remain after the reaction.

  • Theoretical Yield: Maximum amount of product that can be formed. In the pizza example, it was 3 pizzas.

  • Actual Yield: The measured amount of product actually obtained. For example, if only 2 pizzas were salvaged after cooking.

  • Percent Yield Formula: Percent Yield = (Actual Yield / Theoretical Yield) x 100%.

    • Example: (2 pizzas / 3 pizzas) x 100% = 67% yield.

Applying Concepts to Chemical Reactions

Example Reaction with N₂ and H₂

Reactants: 2 molecules of N₂ and 8 molecules of H₂.

Determining Limiting Reactant:

N₂ reacts with H₂ in a ratio according to the balanced equation. After calculations, O₂ was determined to be the limiting reactant because it would run out first, preventing further production of NH₃.

Detailed Example with Chemical Reaction Calculations

Given: 21 g of N₂ and 1.5 g of H₂.Objective: Identify the limiting reactant through stoichiometry and compute leftover amounts.

Steps to Solution:

  1. Convert grams of N₂ and H₂ to moles using their respective molar masses.

  2. Calculate the theoretical yields for ammonia (NH₃) derived from each reactant.

  3. Identify the limiting reactant by determining which reactant yields the least amount of NH₃ (in this case, it may be found that H₂ is the limiting reactant).

  4. Calculate the remaining amounts for N₂ and H₂ after the reaction completion.

Key Terms:

  • Limiting Reactant: The reactant that limits the amount of product made.

  • Reactants in Excess: The reactants left over after the reaction.

  • Theoretical Yield: The maximum calculated amount of product.

  • Actual Yield: The measured amount of product after the reaction.

  • Percent Yield: A measure of the efficiency of a reaction.

Common Reasons for Discrepancies:

Loss of product, incomplete reactions, side reactions, impurities.