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Understanding Percent Yield in Chemical Reactions

Learn to calculate percent yield, identify reasons for yield discrepancies, and determine limiting reactants in reactions. Understand theoretical vs. actual yield. Practice stoichiometry calculations.

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Understanding Percent Yield in Chemical Reactions

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  1. Objectives • Define the terms theoretical yield and actual yield. • Calculate percent yield • Identify reasons that a percent yield may not by 100%

  2. Percent Yield • Theoretical Yield • The maximum amount of a given product that can be formed when the reactants are completely consumed. (as calculated in a stoichiometry calculation) • The actual yield (amount produced) of a reaction is usually less than the maximum expected (theoretical yield).

  3. Percent Yield Consider the following problem: A 2.50 g sample of Sodium bicarbonate was reacted with excess hydrochloric acid. What is the expected yield of NaCl and what is the percent yield if the actual yield was 1.56 g of NaCl. Remember, the balanced reaction is: NaHCO3 + HCl  NaCl + CO2 + H2O

  4. Percent Yield Reasons for % yield < 100 % • The reactants are not pure. • Reactants are involved in “side” reactions • Products are lost in transfer, isolation, or purification steps. Can the % yield be more than 100%??? • Theoretically…..NO! (violates the Law of Conservation of Mass)

  5. Objective • Define the terms limiting reactant and excess reactant. • Identify the limiting reactant in a balanced equation.

  6. Remember the following problem: You are building model cars. Each car has four tires, two doors, and one body. If you have 16 tires, 10 doors, and 7 car bodies, how many model cars can you build? What part limits the number of model cars that can be built?

  7. A. The Concept of Limiting Reactants • Stoichiometric mixture • N2(g) + 3H2(g) 2NH3(g)

  8. A. The Concept of Limiting Reactants • Limiting reactant mixture • N2(g) + 3H2(g) 2NH3(g)

  9. A. The Concept of Limiting Reactants Limiting Reactant – reactant that limits the amount of product that can form; it is the reactant that gets used up in a reaction Excess Reactant – reactant that is left over at the end of a reaction

  10. Finding the Limiting Reactant N2H4(l) + 2 H2O2(l) N2 (g) + 4 H2O(l) Which is the limiting reactant when 0.750 mol of N2H4 is mixed with 0.500 mol of H2O2? • A given amount of a reactant is used to determine the required amount of the other reactant. That amount is then compared to the actual amount.

  11. A. The Concept of Limiting Reactants • Limiting reactant mixture • N2(g) + 3H2(g) 2NH3(g) • Limiting reactant is the reactant that runs out first. • H2

  12. B. Calculations Involving a Limiting Reactant

  13. B. Calculations Involving a Limiting Reactant

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