Grade 11 Stoichiometry: Limiting Reagents

Introduction

• Important topic for Grade 11 stoichiometry
• Many students find limiting reagents confusing initially

Understanding Limiting Reagents Through Everyday Life

• Example with a cake recipe:
  • Recipe: 2 eggs + 1 teaspoon of flour = 1 cake
  • Different scenarios:
    • Scenario 1: 2 teaspoons of flour and 5 eggs
      • Needs 4 eggs to utilize all flour, so can make 1 cake.
      • Limiting reagent: Flour
    • Scenario 2: 2 teaspoons of flour and 6 eggs
      • Needs 3 teaspoons of flour for 6 eggs, not enough flour.
      • Limiting reagent: Flour

Transition to Science Concepts

• Previous lessons gave one starting material, now both are provided.
• Example with nitrogen and hydrogen:
  • Recipe: 1 nitrogen + 3 hydrogens
  • Check which one is limiting:
    • Case 1: 3 moles of nitrogen needed 9 hydrogens.
      • Only 7 hydrogens available, thus hydrogen is limiting.
    • Case 2: 19 moles of hydrogen,
      • Needs 6.33 moles of nitrogen, only 6 available.
      • Therefore, nitrogen is limiting.

Practice with Limiting Reagents

• Key concept: The reagent that runs out first limits the reaction.
• Example with 6 moles of nitrogen and 19 moles of hydrogen:
  • Nitrogen is limiting as it runs out before hydrogen.

Handling Awkward Ratios

• Example with 3:5 ratio:
  • Block one reagent and calculate necessary amounts.
  • If A is 4 moles, to react fully with B:
    • Needs 6.67 moles of B, which is sufficient.
    • Limiting reagent: A.
• Example changing roles of A and B:
  • If B is 11 moles, needs 14 moles of B.
  • Limiting reagent: B.

Conclusion

• Limiting reagent: the one that is in the least amount and restricts the reaction.
• Next lesson will build upon understanding of limiting reagents.