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  • NOYCE Presentation
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How Many Moles of CO2 Form If 22.0 Moles of C8H18 Are combusted (Burned)?

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Mole-to-Mass and Mass-to-Mass Conversions

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  • 2 C8H18(l) + 25 O2(g) → 16 CO2(g) + 18 H2O(g)
  • Problem:
  • Determine the mass (grams) of CO2 produced when 3.6 × 1015 grams of C8H18 is burned in excess oxygen gas.
  • Strategy:
    • Need a balanced reaction
    • From the balance reaction, use the stoichiometric relationship between C8H18 and CO2.
  • Mass of C8H18 → Moles of C8H18 → Stoichiometric ratio 2 C8H18 : 16 CO2 → Moles of CO2 → Mass of CO2

Problem: Determine the mass (grams) of CO2 produced when 3.6 × 1015 grams of C8H18 is burned in excess oxygen gas.

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  • 2 C8H18(l) + 25 O2(g) → 16 CO2(g) + 18 H2O(g)

Practice Problem: Stoichiometry plants produce glucose (C6H12O6) from CO2 and water. How much glucose can a plant produce from 37.8 g CO2

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Limiting Reactant, Theoretical Yield

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  • Back to the pizza recipe:
    • 1 crust + 5 oz. tomato sauce + 2 cups cheese → 1 pizza
  • Suppose you have 4 crusts, 10 cups of cheese, and 15 oz. tomato sauce. How many pizzas can you make?
  • Strategy:
  • We have enough crusts to make
  • We have enough cheese to make
  • We have enough tomato sauce to make
    • If you are given two or more quantities or reactants, then it is a limiting reagent problem

Limiting Reactant: Pizza Problem Continued

  • We have enough crusts for 4 pizzas, enough cheese for 5 pizzas, but ONLY enough tomato sauce for 3 pizzas.
    • Therefore, only 3 pizzas can be made.
    • The tomato sauce limits how many pizzas can be made.

Limiting Reactant and Theoretical Yield Connection

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  • In the pizza analogy, the tomato sauce is the limiting reactant, the reactant that makes the least amount of product.
    • The limiting reactant is also known as the limiting reagent.
  • The maximum number of pizzas that can be made depends on this ingredient, the tomato sauce.
    • In chemical reactions, this is called the theoretical yield.
  • Theoretical yield is the amount of product that can be made in a chemical reaction based on the amount of limiting reactant.
    • Example:
      • The ingredient that makes the least amount of pizza determines how many pizzas you can make (theoretical yield).

More Making Pizzas

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  • Assume that while making pizzas, a pizza is burnt or dropped on the floor and only two pizzas are available to eat.
  • The actual amount of product made in a chemical reaction is called the actual yield.
  • Actual yield is about efficiency.
    • To determine your efficiency in making pizzas, a percentage value can be calculated.
    • In chemical reactions, this is called a percent yield.

Summarizing Limiting Reactant and Theoretical Yield

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  • The limiting reactant (or limiting reagent) is the reactant that is completely consumed in a chemical reaction and limits the amount of product.
  • The reactant in excess is any reactant that occurs in a quantity greater than is required to completely react with the limiting reactant.
  • The theoretical yield is the amount of product that can be made in a chemical reaction based on the amount of limiting reactant.
  • The actual yield is the amount of product actually produced by a chemical reaction.
  • The percent yield is calculated as:
    • (actual yield /theoretical yield) × 100 = percent yield%

Chemical Reaction

  • In reactions with multiple reactants, it is likely that one of the reactants will be completely used before the others.
    • When this reactant is used up, the reaction stops and no more product is made.
  • The reactant that limits the amount of product is called the limiting reactant.
    • It is sometimes called the limiting reagent.
    • The limiting reactant gets completely consumed.
  • Reactants not completely consumed are called excess reactants.
    • The reactant in excess is any reactant that occurs in a quantity greater than is required to completely react with the limiting reactant.
  • The amount of product that can be made from the limiting reactant is called the theoretical yield.

Practice Problem: Stoichiometry—Limiting Reactant and Theoretical Yield

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  • Ammonia, NH3, can be synthesized by
    • 2NO(g) + 3H2(g) è 2NH3(g) + 2H2O(g)
  • Starting with 86.3 g NO and 25.6 g H2, find the theoretical yield of NH3

What is the percent yield of C2H2 if 62.80 g of water yields 15.38 g of C2H2 using the following equation.

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    • CaC2(s) + 2 H2O(l) → Ca(OH)2(aq) + C2H2(g)
    • A) 13.84%
    • B) 33.90%
    • C) 91.47%
    • D) 48.10%
    • E) 68.52%

Term Definition
limiting reactant the reactant that makes the least amount of product
theoretical yield the amount of product that can be made in a chemical reaction based on the amount of limiting reactant
actual yield the actual amount of product made in a chemical reaction
percent yield the efficiency percentage of how much product is made in a chemical reaction (Calculated as 100 * actual yield / theoretical yield )
excess reactants reactants not completely consumed are called