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Limiting Reagents & Percent Yield

Limiting Reagents & Percent Yield. Chapter 9 Notes Part III. What are limiting reagents?. Up until now, we have assumed that all reactants are used up in a reaction. In actuality, usually one chemical runs out first, stopping the reaction—that chemical is called the limiting reagent.

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Limiting Reagents & Percent Yield

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  1. Limiting Reagents& Percent Yield Chapter 9 Notes Part III

  2. What are limiting reagents? Up until now, we have assumed that all reactants are used up in a reaction. In actuality, usually one chemical runs out first, stopping the reaction—that chemical is called the limiting reagent.

  3. For Example: How many sandwiches can you make from a full jar of peanut butter, a full jar of jelly and 4 slices of bread?

  4. For Example: How many sandwiches can you make from a full jar of peanut butter, a full jar of jelly and 4 slices of bread?

  5. For Example: How many sandwiches can you make from a full jar of peanut butter, a full jar of jelly and 4 slices of bread? +

  6. For Example: How many sandwiches can you make from a full jar of peanut butter, a full jar of jelly and 4 slices of bread? +

  7. For Example: How many sandwiches can you make from a full jar of peanut butter, a full jar of jelly and 4 slices of bread? + +

  8. For Example: How many sandwiches can you make from a full jar of peanut butter, a full jar of jelly and 4 slices of bread? + +

  9. For Example: How many sandwiches can you make from a full jar of peanut butter, a full jar of jelly and 4 slices of bread? + + =

  10. For Example: How many sandwiches can you make from a full jar of peanut butter, a full jar of jelly and 4 slices of bread? + + =

  11. The limiting reagent limits or determines the amount of product that can be formed in a reaction. On the flip side, the excess reagent is the one not completely used up.

  12. How do you find what the limiting reagent is? To find the limiting reagent, use dimensional analysis to determine how much of the product will be formed by both reactants. The smaller of the two results will indicate which reactant is the limiting reagent.

  13. Here’s an example: In the reaction: Mg + HCl a MgCl2 + H2 you combine 35.5 grams of magnesium with 28.1 grams of hydrochloric acid. Which one is the limiting reagent, and how much magnesium chloride is produced?

  14. Mg + 2 HCl a MgCl2 + H2

  15. Mg + 2 HCl a MgCl2 + H2 35.5 g Mg x 1 mole Mg x 1 mole MgCl2 x 95.3 g MgCl2 = 139 g MgCl2 24.3 g Mg 1 mole Mg 1 mole MgCl2

  16. Mg + 2 HCl a MgCl2 + H2 35.5 g Mg x 1 mole Mg x 1 mole MgCl2 x 95.3 g MgCl2 = 139 g MgCl2 24.3 g Mg 1 mole Mg 1 mole MgCl2 28.1 g HCl x 1 mole HCl x 1 mole MgCl2 x 95.3 g MgCl2 = 36.6 g MgCl2 36.5 g HCl 2 moles HCl 1 mole MgCl2

  17. Mg + 2 HCl a MgCl2 + H2 35.5 g Mg x 1 mole Mg x 1 mole MgCl2 x 95.3 g MgCl2 = 139 g MgCl2 24.3 g Mg 1 mole Mg 1 mole MgCl2 28.1 g HCl x 1 mole HCl x 1 mole MgCl2 x 95.3 g MgCl2 = 36.6 g MgCl2 36.5 g HCl 2 moles HCl 1 mole MgCl2 HCl is the limiting reagent and 36.6 g of MgCl2 are produced.

  18. Percent Yield In the real world, you never form 100% of the products that you think you should based on theoretical calculations. By comparing how much of a product forms to how much should form based on stoichiometry, you can find the percent yield.

  19. Percent Yield %Yield= actual yield x 100 theoretical yield

  20. Practice Problem In the reaction of nitrogen gas and hydrogen gas producing ammonia (NH3), 250.2g H2 produces 1095.1g NH3. What is the % yield of this reaction?

  21. N2 + 3 H2→ 2 NH3 First find out how much NH3 should be produced:

  22. N2 + 3 H2→ 2 NH3 First find out how much NH3 should be produced: 250.2 g H2 x 1 mole H2 x 2 moles NH3 x 17 g NH3 = 2836 g NH3 1 g H2 3 moles H2 1 mole NH3

  23. N2 + 3 H2→ 2 NH3 First find out how much NH3 should be produced: 250.2 g H2 x 1 mole H2 x 2 moles NH3 x 17 g NH3 = 2836 g NH3 1 g H2 3 moles H2 1 mole NH3 Then calculate the percent yield:

  24. N2 + 3 H2→ 2 NH3 First find out how much NH3 should be produced: 250.2 g H2 x 1 mole H2 x 2 moles NH3 x 17 g NH3 = 2836 g NH3 1 g H2 3 moles H2 1 mole NH3 Then calculate the percent yield: % Yield = Actual x 100 % Yield = 1095.1 g x 100 Theor. 2836 g % Yield = 38.6 %

  25. N2 + 3 H2→ 2 NH3 First find out how much NH3 should be produced: 250.2 g H2 x 1 mole H2 x 2 moles NH3 x 17 g NH3 = 2836 g NH3 1 g H2 3 moles H2 1 mole NH3 Then calculate the percent yield: % Yield = Actual x 100 % Yield = 1095.1 g x 100 Theor. 2836 g % Yield = 38.6 %

  26. Reaction #1 K2CrO4 + Pb(NO3)2a KNO3 + PbCrO4 If 0.947g K2CrO4 are combined with 0.331g Pb(NO3)2 what will be the limiting reagent, and how much lead chromate is produced?

  27. K2CrO4 + Pb(NO3)2a 2 KNO3 + PbCrO4 0.947g K2CrO4 x 1 mole K2CrO4 x 1 mole PbCrO4 x 323.2 g PbCrO4 = 1.97 g PbCrO4 155.1 g K2CrO4 1 mole K2CrO4 1 mole PbCrO4 0.331g Pb(NO3)2 x 1 mole Pb(NO3)2 x 1 mole PbCrO4 x 323.2 g PbCrO4 = 0.323 g PbCrO4 331.2 g Pb(NO3)2 1 mole Pb(NO3)2 1 mole PbCrO4 Pb(NO3)2 is the limiting reagent and 0.323 g PbCrO4 is produced.

  28. Reaction #2 C3H8O + O2a CO2 + H2O If 3.78 L of O2 are combined with 7.9 g of C3H8O, what is the limiting reagent, and how much water is produced?

  29. 2 C3H8O + 9 O2a 6 CO2 + 8 H2O 3.78 L of O2 x 1 mole O2 x 8 moles H2O x 18 g H2O = 2.7 g H2O 22.4 L O2 9 moles O2 1 mole H2O 7.9 g of C3H8O x 1 mole C3H8O x 8 moles H2O x 18 g H2O = 9.48 g H2O 60 g C3H8O 2 moles C3H8O 1 mole H2O O2 is the limiting reagent and 2.7 g of H2O are produced.

  30. Reaction #3 K2CrO4 + AgNO3a KNO3 + Ag2CrO4 If 1.17g K2CrO4 are combined with 1.23g AgNO3, what is the limiting reagent and how much Ag2CrO4 is produced?

  31. K2CrO4 + 2 AgNO3a 2 KNO3 + Ag2CrO4 1.17g K2CrO4 x 1 mole K2CrO4 x 1 mole Ag2CrO4 x 331.8 g Ag2CrO4 = 1.99 g Ag2CrO4 194.2 g K2CrO4 1 mole K2CrO4 1 mole Ag2CrO4 1.23g AgNO3 x 1 mole AgNO3 x 1 mole Ag2CrO4 x 331.8 g Ag2CrO4 = 1.20 g Ag2CrO4 169.9 g AgNO3 2 moles AgNO3 1 mole Ag2CrO4 AgNO3 is the limiting reagent and 1.20 g Ag2CrO4 are produced.

  32. Reaction #4 Zn + HCl a ZnCl2 + H2 What is the limiting reagent and how many liters of hydrogen are produced if 15.5 grams of zinc are combined with 26.3 grams of hydrochloric acid?

  33. Zn + 2 HCl a ZnCl2 + H2 15.5 g Zn x 1 mole Zn x 1 mole H2 x 22.4 L H2 = 5.31 L H2 65.4 g Zn 1 mole Zn 1 mole H2 26.3 g HCl x 1 mole HCl x 1 mole H2 x 22.4 L H2 = 8.07 L H2 36.5 g HCl 2 moles HCl 1 mole H2 Zn is the limiting reagent and 5.31 Liters of H2 are produced.

  34. Reaction #5 Na + H2O a NaOH + H2 If 2.1g of sodium is added to 3.8g of water, what is the limiting reagent, and how many grams of sodium hydroxide are produced?

  35. 2 Na + 2 H2O a 2 NaOH + H2 2.1 g Na x 1 mole Na x 2 moles NaOH x 40 g NaOH = 3.7 g NaOH 23 g Na 2 moles Na 1 mole NaOH 3.8 g H2O x 1 mole H2O x 2 moles NaOH x 40 g NaOH = 8.4 g NaOH 18 g H2O 2 moles H2O 1 mole NaOH Na is the limiting reagent and 3.7 g NaOH are produced.

  36. In Review: What is a limiting reagent? What is the excess reagent? How does a limiting reagent control the amount of product?

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