AP

3.5 | Limiting Reactants

Limiting Reactants

  • Definition: Reactants that are not present in precise stoichiometric amounts during a chemical reaction, determining the extent of the reaction.

Example: Combustion Reaction in Power Plants

  • Reaction: CH₄ + 2 O₂ → CO₂ + 2 H₂O

  • Power plants operate with excess O₂ to maximize energy from hydrocarbon fuel and minimize harmful byproducts (e.g., carbon monoxide).

  • The quantity of CH₄ introduced dictates the amount of CO₂ and H₂O produced, as well as the energy released.

Quantitative Calculations for Reactions

  • Learn to identify limiting reactants and calculate:

    • Amounts (in grams or moles) of reactants consumed

    • Amounts of products formed

    • Percent yield from actual yield and quantities of each reactant

Case Study: Sandwich Making as a Chemical Equation

  • Analogy: 2 BD + C H → 2 BD2CH (making sandwiches)

  • Given:

    • 10 slices of bread (BD)

    • 7 slices of cheese (C H)

  • Result: Maximum of 5 sandwiches can be made, leaving 2 slices of cheese leftover.

  • Conclusion: Bread is the limiting reactant, restricting the total number of sandwiches.

Application in Chemical Reactions

Example: Water Formation from H₂ and O₂

  • Reaction: 2 H₂ + O₂ → 2 H₂O

  • Starting amounts:

    • 10 moles H₂

    • 7 moles O₂

  • Calculation of O₂ needed:

    • For 10 moles H₂: 5 moles O₂ are required (using stoichiometric ratio 2:1).

    • 7 moles O₂ available means 2 moles O₂ remain after H₂ is consumed.

  • Conclusion: H₂ is the limiting reactant, stopping the reaction upon its depletion.

Importance of Limiting Reactants

  • Determine quantities of reactants consumed and amounts of products formed.

  • Some reactions purposefully use an excess of one reactant for efficiency (e.g., combustion reactions in air with excess O₂).

Theoretical Yield and Percent Yield

  • Theoretical Yield: Maximum amount of product calculated when the limiting reactant is completely consumed.

  • Actual Yield: Amount of product obtained, often less than the theoretical yield due to:

    • Incomplete reactions

    • Side reactions

    • Loss of product during recovery

Percent Yield Formula

  • Percent Yield = (Actual Yield / Theoretical Yield) × 100%

Summary of Concepts

  • Limiting Reactant: Restricts the reaction extent, defines product quantities.

  • Excess Reactant: Remains after the limiting reactant is consumed.

  • Theoretical vs. Actual Yield: Theoretical yield is calculated; actual yield is what is obtained in practice. Percent yield indicates reaction efficiency.