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.