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UNIT VII Excess & Percentage Yield Unit 7: Lesson 2

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L IMITING REAGENTS & E XCESS AMOUNTS SO far, we’ve only considered reactions assuming that ALL of the reactants will be USED UP entirely so zero is left at the end of the reaction. Wellllll, this is not always true! WHAT to do?!!?

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VII.4 STOICHIOMETRY OF EXCESS QUANTITIES Often in a reaction, not all of the reactants are used up – frequently one or more reactants are in excess LIMITING REAGENT : is the reactant you run out of first; the smaller number! EXCESS REAGENT: is the one you have left over; the bigger number!

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VII.4 STOICHIOMETRY OF EXCESS QUANTITIES Ex: you are given one dozen loaves of bread, a gallon of mustard and three pieces of salami – how many salami sandwiches can you make??? Limiting reagent = Excess reagents = The limiting reagent determines how much product you can make

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VII.4 STOICHIOMETRY OF EXCESS QUANTITIES Well, how do we find out the limiting reagent in a chemical reaction!? In order to FIND OUT which reactant is limiting…you do TWO stoichiometry problems: The one that makes the least product is the limiting reagent!!

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VII.4 STOICHIOMETRY OF EXCESS QUANTITIES Ex: #1 Copper reacts with sulfur to form copper (I) sulfide. If 10.6g of copper reacts with 3.83g of S how much product will be formed?

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VII.4 STOICHIOMETRY OF EXCESS QUANTITIES Ex: #2 Mg (s) + 2HCl (g) → MgCl 2(s) + H 2(g) If 10.1 mol of magnesium and 4.87 mol of HCl gas are reacted, how many moles of gas will be produced? How much excess reagent remains?

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VII.4 STOICHIOMETRY OF EXCESS QUANTITIES TRY: If 10.3 g of aluminum are reacted with 51.7g of CuSO 4 how much copper will be produced? How much excess reagent will remain?

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VII.4 STOICHIOMETRY OF EXCESS QUANTITIES Ex: #3 When 79.1 g of zinc are reacted with 1.05L of 2.00M HCl, to produce ZnCl 2, and H 2 gas, which reactant will be in excess and by how much?

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VII.5 PERCENTAGE YIELD Percentage Yield: is used to describe the amount of product actually obtained as a percentage of the expected amount. 2 reasons: 1. reactants may not all react 2. some products are lost Yield : is the amount made in a chemical reaction

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VII.5 PERCENTAGE YIELD: THREE TYPES of YIELD 1. Actual Yield – what you get in the lab when the chemicals are mixed 2. Theoretical Yield – what the balanced equation says you should make 3. Percent Yield - Actual Yield (obtained) x 100% Theoretical Yield (expected) Percent purity- pure x 100% impure

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VII.5 PERCENTAGE YIELD: There are two possible situations: 1. Actual amount of PRODUCTS FOUND is LOWER than the value expected. To solve: calculate “amount of product expected” using stoichiometry then multiply (x) “amount of product expected” by the decimal equivalent of the % yield or % purity Products multiply.

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VII.5 PERCENTAGE YIELD: 2. Actual amount of REACTANTS NEEDED is GREATER than the value expected To solve: calculate the “amount of reactant needed” using stoichiometry then divide (÷) “amount of reactant needed” by the decmial equivalent of % yield or % purity Reactants divide

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VII.5 PERCENTAGE YIELD: Ex: #1 When 45.8g of potassium carbonate, K 2 CO 3 are reacted completely with an excess of HCl, 46.3 g of KCl are produced. Water and CO 2 also are formed. Calculate the theoretical yield and the percent yield of KCl.

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VII.5 PERCENTAGE YIELD AND PERCENTAGE PURITY: Ex: #2 2Al + 3CuSO 4 → Al 2 (SO 4 ) 3 + 3Cu When 3.92 g of Al are reacted with excess copper(II) sulfate, 6.78 g of copper is produced. a) What is the actual yield?

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VII.5 PERCENTAGE YIELD AND PERCENTAGE PURITY: Ex: #2 (continued) 2Al + 3CuSO 4 → Al 2 (SO 4 ) 3 + 3Cu When 3.92 g of Al are reacted with excess copper(II) sulfate, 6.78 g of copper is produced. b) What is the theoretical yield?

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VII.5 PERCENTAGE YIELD AND PERCENTAGE PURITY: Ex: #2 (continued) 2Al + 3CuSO 4 → Al 2 (SO 4 ) 3 + 3Cu When 3.92 g of Al are reacted with excess copper(II) sulfate, 6.78 g of copper is produced. c) What is the percentage yield?

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VII.5 PERCENTAGE YIELD AND PERCENTAGE PURITY: Ex: #2 (continued) 2Al + 3CuSO 4 → Al 2 (SO 4 ) 3 + 3Cu When 3.92 g of Al are reacted with excess copper(II) sulfate, 6.78 g of copper is produced. d) If you started with 9.73g of Al how much copper would you expect?

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VII.5 PERCENTAGE YIELD AND PERCENTAGE PURITY: Ex: #3 What mass of Fe 2 O 3 is produced when 2.30 g of FeCO 3 are reacted with an excess of O 2 according to the reaction: 4FeCO 3 + O 2 → 2Fe 2 O 3 + 4CO 2, if the reaction has a 67.0 % yield?

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VII.5 PERCENTAGE YIELD AND PERCENTAGE PURITY: Ex: #4 What mass of CO 2 is required to make 8.00g of K 2 CO 3(s) according to the reaction: 4KO 2(s) + 2CO 2(g) → 2K 2 CO 3(s) + 3O 2(g) if the reaction has a 85.0% yield?

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HOMEWORK Excess amounts: p.133 # 26-32 % questions: p. 137 # 33-37 UNIT 7 STOICHIOMETRY : FINITOOOOOOO!

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