Limiting-and-Excess-Reactants

Page 1: Limiting and Excess Reactants

Turkey Sandwich Example

  • Recipe Equation:2 toast slices + 2 turkey slices + 1 lettuce leaf + 1 tomato slice → 1 turkey sandwich

  • Ingredients Available:

    • 4 toast slices

    • 6 turkey slices

    • 3 lettuce leaves

    • 3 tomato slices

  • Limiting Ingredient:

    • Toast slices are the limiting ingredient as they restrict the production of sandwiches. You can only make 2 sandwiches, despite having enough turkey, lettuce, and tomato for 3.

  • Excess Ingredients:

    • Turkey slices, lettuce leaves, and tomato slices remain after making the sandwiches.

Limiting and Excess Reactants in Chemical Reactions

  • When reactants are in amounts that match the balanced equation, they are stoichiometric amounts.

  • Real chemical reactions usually have one reactant in short supply, making them the limiting reactant which stops the reaction and determines the maximum product formed.

  • Example: Burning candle scenario where wax is present in excess, but oxygen is limited when a snuffer is used.

    • Limiting Reactant Definition:

      • A reactant that is completely consumed during a chemical reaction, limiting product formation.

    • Excess Reactant Definition:

      • A reactant that remains after the reaction has occurred.

Key Terms

  • Stoichiometric Amount: The exact molar amount predicted by a balanced equation.

  • Limiting Reactant: A reactant that limits the amount of product produced.

  • Excess Reactant: A reactant that is not completely consumed and remains after the reaction.

Summary

  • This section discusses how certain ingredients (or reactants) limit the amount of product that can be made in reactions and provides basic definitions and concepts related to limiting and excess reactants.

Page 2: The Limiting Reactant Forms Less Product

Key Concepts

  • The limiting reactant does not necessarily equate to the smallest quantity of reactant present; it is the substance that produces the least amount of product in a reaction.

  • Water Formation Example:

    • For the reaction 2 H2 + O2 → 2 H2O:

      • If 2 mol of H2 is available, and O2 is in excess, 2 mol of water is produced.

      • Conversely, if O2 is available in excess, and only 2 mol of H2 is present, 4 mol of water is produced.

  • Identifying Limiting Reactant:

    • Clearly identify limiting and excess reactants because it impacts the yield of products.

Investigation Activity

  • Furniture Company Example

    1. Parts in Stock:

      • 36 frames

      • 128 legs

      • 256 leg braces

      • 100 hardware packages

      • 1000 assembly manuals

    2. Tasks:

      • Determine complete chair kits that can be made.

      • Identify limiting items and items in excess.

      • Calculate the remaining quantities of excess items.

  • Reinforcing Questions:

    1. Why Chair Frames are Not Limiting:

      • The presence of other parts limits production, not merely the smallest quantity.

    2. Impact of Excess Items:

      • Items in excess do not directly affect the quantity of products made.

Page 3: Identifying the Limiting Reactant Problem

Ammonia Production Example

  • Chemical Equation:N2(g) + 3 H2(g) → 2 NH3(g)

  • Given Quantities:

    • 4.20 g of nitrogen gas

    • 0.750 g of hydrogen gas

Strategy to Identify Limiting Reactant

  1. Molar Mass Calculation:

    • Molar mass of N2 = 28.02 g/mol

    • Molar mass of H2 = 2.02 g/mol

  2. Converting Mass to Moles:

    • n(N2) = 4.20 g / 28.02 g/mol = 0.14989 mol

    • n(H2) = 0.750 g / 2.02 g/mol = 0.37129 mol

  3. Calculating Product Formation:

    • n(NH3) from N2 = 0.14989 mol x (2 mol NH3 / 1 mol N2) = 0.300 mol

    • n(NH3) from H2 = 0.37129 mol x (2 mol NH3 / 3 mol H2) = 0.248 mol

  4. Determine Limiting Reactant:

    • Since H2 produces less ammonia, H2 is the limiting reactant despite being present in greater quantity.

Ratio Check

  • Confirm that the mole ratio (0.15:0.37) is less than the stoichiometric requirement (3:1), corroborating H2 as the limiting reactant.

Page 4: Practice Problems

  • Listings and explanations for chemical scenarios to identify limiting and excess reactants across various reactions with specified masses and reactants.

    • Examples include reactions of sulfuric acid with calcium fluoride, reactions producing esters, and various forms of precipitation reactions.

Page 5: Using the Limiting Reactant to Find Amount of Product

Thermite Reaction Example

  • Balanced Equation:2Al(s) + Fe2O3(s) → Al2O3(s) + 2Fe(l)

  • Given Masses:

    • 113.00 g of aluminum

    • 279.50 g of iron(III) oxide

Steps to Calculate Mass of Molten Iron Produced

  1. Calculate Molar Mass:

    • M(Al) = 26.98 g/mol

    • M(Fe2O3) = 159.70 g/mol

    • M(Fe) = 55.85 g/mol

  2. Converting Mass to Moles:

    • n(Al) = 113.00 g / 26.98 g/mol = 4.18829 mol

    • n(Fe2O3) = 279.50 g / 159.70 g/mol = 1.75016 mol

  3. Iron Formation Calculation:

    • n(Fe) from Al = 4.18829 mol x (2 mol Fe / 2 mol Al) = 4.18829 mol

    • n(Fe) from Fe2O3 = 1.75016 mol x (2 mol Fe / 1 mol Fe2O3) = 3.50032 mol

  4. Finding Limiting Reactant:

    • Fe2O3 limits iron production.

  5. Mass of Iron Produced:

    • m = n × M = 3.50032 mol × 55.85 g/mol = 195.49 g

Page 6: Additional Practice Problems

  • Lists of additional practice questions emphasizing the identification of limiting reactants in various scenarios.

Page 7: Applications of Stoichiometry and Limiting Reactants

Real-Life Applications

  • Stoichiometry is essential beyond laboratories; it impacts cleaning solutions, pharmaceuticals, agricultural products, and more.

  • Examples:

    • Correct proportions in cleaning products for safety and effectiveness.

    • Medication doses calculated to achieve desired effects.

    • Agricultural chemicals mixed to avoid environmental damage.

Research Activity

  • Choose a product involving stoichiometry and investigate:

    • Purpose of the product

    • Required calculations for safe usage

    • Consequences of mixing errors

Page 8: Section Summary

Review Questions

  • Questions bridging the understanding of limiting/excess reactants with practical examples and calculations to reinforce concepts learned.

  • Explores concepts of stoichiometric calculations and limits encountered in chemical processes.