Limiting and Excess Reactants, Theoretical and Percent Yield

Key Concepts

• Limiting Reactant: The reactant that runs out first in a reaction, limiting the amount of product formed.
• Excess Reactant: The reactant that remains after the reaction.
• Theoretical Yield: The maximum amount of product that can be formed from a given amount of reactants.
• Percent Yield: The ratio of the actual yield to the theoretical yield, expressed as a percentage.

Example 1: Combustion of Propane

• Reaction: 2 moles of propane (C₃H₈) react with 8 moles of O₂.
• Balanced Equation:
  • C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
• Steps to Balance:
  1. Balance carbon atoms.
  2. Balance hydrogen atoms.
  3. Balance oxygen atoms.
• Finding Limiting Reactant:
  • Divide moles by coefficient in balanced equation.
  • Propane: 2/1 = 2
  • Oxygen: 8/5 = 1.6 (O₂ is the limiting reactant)
• Theoretical Yield Calculation:
  • Convert moles of limiting reactant to moles of product using the molar ratio.
  • Maximum moles of CO₂ from O₂: 4.8 moles.
• Percent Yield:
  • Actual yield: 4.5 moles of CO₂
  • Percent yield = (Actual yield / Theoretical yield) × 100 = 93.75%
• Calculating Excess Reactant Leftover:
  • Total moles of excess reactant - moles that reacted = moles leftover.

Example 2: Combustion of Benzene

• Reaction: 50g of benzene (C₆H₆) with 160g of O₂.
• Balanced Equation:
  • C₆H₆ + 15/2O₂ → CO₂ + H₂O
  • Doubling: 2C₆H₆ + 15O₂ → 12CO₂ + 6H₂O
• Theoretical Yield Calculation:
  • Convert mass of reactants to mass of product using molar mass and ratios.
  • Benzene gives theoretical yield of 34.6g water.
  • Oxygen gives theoretical yield of 36g water.
  • Limiting reactant is benzene.
• Percent Yield:
  • Actual yield: 30g
  • Theoretical yield: 34.6g
  • Percent yield = (30/34.6) × 100 = 86.7%
• Calculating Excess Reactant Leftover:
  • Convert grams of limiting reactant to grams of excess reactant to find how much reacts.
  • Subtract reacted amount from total excess reactant to find leftover.

General Tips

• Always balance your chemical equations first before doing calculations.
• Understand and apply the concept of mole ratios from the balanced equation.
• Use dimensional analysis for converting units (grams to moles, etc.).
• Know that the limiting reactant determines the maximum possible yield of product.
• Percent yield gives insight into the efficiency of a reaction.