Math-tastic! Lesson 9.5 Limiting Reagent & Percent Yield 2/21/2015. Identify the limiting reagent in a reaction. Limiting Reactants OBJECTIVES:

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1 . Math-tastic! Unit 9: Math of Chemistry Part II - Stoichiometry Lesson # 9.5: Limiting Reagent & Percent Yield 121 Lesson 9.5 Limiting Reagent & Percent Yield OBJECTIVES: Identify the limiting reagent in a reaction. Calculate theoretical yield, percent yield, and the amount of excess reagent that remains unreacted given appropriate information. Limiting Reactants If the amounts of two reactants are given, the reactant used up first determines the amount of product formed

2 Analogy Suppose you are preparing cheese sandwiches. Each sandwich requires 2 pieces of bread and 1 slice of cheese. If you have 4 slices of cheese and 10 pieces of bread, how many cheese sandwiches can you make? 124 Cheese Sandwich Products Sandwich = Sandwich = 125 Learning Check #1 How many sandwiches can you make? slices of bread + slices of cheese = sandwiches What is left over? What is the limiting reactant? 126 2

3 Solution # 1 How many sandwiches can you make? 10 slices of bread + 4 slices of cheese = 4 sandwiches What is left over? _2 slices of bread What is the limiting reactant? cheese 127 Hints for LR Problems 1. For each reactant amount given, calculate the moles (or grams) of a product it could produce. 2.The reactant that produces the smaller amount of product is the limiting reactant. 3. The number of moles of product produced by the limiting reactant is ALL the product possible. There is no more limiting reactant left. 128 Limiting Reagents - Combustion 3

4 How do you find out which is limited? The chemical that makes the least amount of product is the limiting reagent. You can recognize limiting reagent problems because they will give you 2 amounts of chemical Do two stoichiometry problems, one for each reagent you are given. If 10.6 g of copper reacts with 3.83 g sulfur, how many grams Cu of the is the product (copper (I) sulfide) will be formed? Limiting 2Cu + S Cu Reagent, 2 S since it 1 mol Cu 1 mol Cu 10.6 g Cu 2 S g Cu 2 S produced less 63.55g Cu 2 mol product. Cu 1 mol Cu 2 S = 13.3 g Cu 2 S 3.83 g S 1 mol S 32.06g S 1 mol Cu 2 S 1 mol S g Cu 2 S 1 mol Cu 2 S = 19.0 g Cu 2 S Another example: If 10.3 g of aluminum are reacted with 51.7 g of CuSO 4 how much copper (grams) will be produced? 2Al + 3CuSO 4 3Cu + Al 2 (SO 4 ) 3 the CuSO 4 is limited, so Cu = 20.6 g How much excess reagent will remain? Excess = 4.47 grams 4

5 The Concept of: A little different type of yield than you had in Driver s Education class. Percent Yield You prepared cookie dough to make 5 dozen cookies. The phone rings while a sheet of 12 cookies is baking. You talk too long and the cookies burn. You throw them out (or give them to your dog.) The rest of the cookies are okay. How many cookies could you have made (theoretical yield)? How many cookies did you actually make to eat? (Actual yield) 134 Vocabulary There are three different types of yield: Actual yield is the amount of product actually recovered from an experiment Theoretical (possible) yield is the maximum amount of product that could be produced from the reactant. Percent Yield is the actual yield compared to the maximum (theoretical yield) possible

6 Percent Yield Calculation What is the percent yield of cookies? Percent Yield = Actual Yield (g) recovered X 100 Possible Yield (g) % cookie yield = 48 cookies x 100 = 80% yield 60 cookies 136 Learning Check Without proper ventilation and limited oxygen, the reaction of carbon and oxygen produces carbon monoxide. 2C(g) + O 2 (g) 2CO(g) What is the percent yield if 40.0 g CO are produced when 30.0 g O 2 are used? 1) 25.0% 2) 75.0% 3) 76.2% 137 Solution theoretical yield of CO 30.0 g O 2 x 1 mol O 2 x 2 mol CO x g CO g O 2 1 mol O 2 1 mol CO = 52.5 g CO (theoretical) percent yield 40.0 g CO (actual) x 100 = 76.2 % yield 52.5 g CO (theoretical) 3) 76.2 % yield 138 6

7 Copyright 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings 139 Learning Check When N 2 and 5.00 g H 2 are mixed, the reaction produces 16.0 g NH 3. What is the percent yield for the reaction? N 2 (g) + 3H 2 (g) 2NH 3 (g) 1) 31.3 % 2) 56.5 % 3) 80.0 % 140 N 2 (g) + 3H 2 (g) Solution 2NH 3 (g) 5.00 g H 2 x 1 mol H 2 x 2 mol NH 3 x g NH g H 2 3 mol H 2 1 mol NH 3 = 28.2 g NH 3 (theoretical) Percent yield = 16.0 g NH 3 x 100 = 56.7 % 28.2 g NH 3 2) 56.5 % 141 7

8 Practice: 6.78 g of copper is produced when 3.92 g of Al are reacted with excess copper (II) sulfate. 2Al + 3 CuSO 4 Al 2 (SO 4 ) 3 + 3Cu What is the actual yield? = 6.78 g Cu What is the theoretical yield? What is the percent yield? = 49.1 % = 13.8 g Cu Details on Yield Percent yield tells us how efficient a reaction is. Percent yield can not be bigger than 100 %. Theoretical yield will always be larger than actual yield! Why? Due to impure reactants; competing side reactions; loss of product in filtering or transferring between containers; measuring End of Unit 9 8