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5/07/09 Chemistry Unit 8: Stoichiometry Key Learning 1 2 3 4 Chemistry Standard/Outcomes Use of Mole Ratios Calculating Theoretical Yields Percent Yield of a Reaction Determining the Limiting Reactant Mole Ratios Be able to balance reactions. Be able to write mole ratios from the coefficients of balanced reactions. Be able to use the mole ratio to convert moles of a reactant/product to moles of another reactant/product. Calculating Theoretical Yields Be able to convert grams to moles (and the reverse) for a substance. Be able to calculate the number of grams of product that would result from grams of reactant. Percent Yield Be able to calculate percent yield when the actual and calculated yields are given. Be able to calculate percent yield when the calculated yield needs to be determined from the amount of reactant given. Limiting Reactant Know the definition of a limiting reactant. Be able to identify the limiting reactant by calculating the theoretical yield produced by both reactants. Be able to determine the amount of product that would be produced in a reaction. 2 3 Name: _______________ Skill Practice: Interpreting a Balanced Chemical Equation In a chemical equation, the coefficients tell us how many molecules are reacting. Example: 1H2 1 molecule + 1Cl2 → 2 HCl 1 molecule 2 molecules We can represent what is happening with a diagram: Notice that 1 hydrogen molecule and 1 hydrogen molecule make 2 hydrogen chlorides. Complete the blanks: N2 + ____ molecule(s) 3 H2 → ____ molecule(s) 2 NH3 ____ molecule(s) Draw a diagram like the example above that represents what is happening in this reaction: 4 A balanced equation is like a recipe. You can double or triple the recipe if you like. Notice that any change in one coefficient will mean the other coefficients change in the same ratio! 2 H2 + → O2 __4___ molecule(s) 2 H2O __2__ molecule(s) __4__ molecule(s) Complete the blanks: H2 1. + __3__ molecule(s) N2 2. 3. _____ molecule(s) → 3 H2 _20__ molecule(s) 2 NH3 ____ molecule(s) + ____ molecule(s) → O2 ____ molecule(s) H2 2 HCl _____ molecule(s) + 2 H2 → Cl2 2 H2O _1000_ molecule(s) + Cl2 4. __6.02 x 1023__ molecule(s) → ____ molecule(s) 2 HCl _____ molecule(s) _____ molecule(s) Since Avogadro’s number = 6.02×1023 molecules = 1 mole 2 H2 5. __2__ mole + O2 ____ mole → 2 H2O ____ mole 5 The coefficients in a chemical equation give the mole relationships of reactants and products in a reaction. Give the mole relationships for each of the following: H2 6. ____ mole(s) C3H8 7. Cl2 → + + ____ mole(s) 2 HCl ____ mole(s) 5 O2 → ____ mole(s) ____ mole(s) 3 CO2 + ____ mole(s) 4 H2O ____ mole(s) Mole Ratios: 2 H2 + O2 → 2 H2O A mole ratio is the mole relationship between two specific chemicals: Example: The mole ratio between H2/H2O is: 2 mole H2 = 2 mole H2O The mole ratio between H2/O2 is: 2 mole H2 = 1 mole O2 The mole ratio between O2/H2O is: 1 mole O2 = 2 mole H2O 9. Using the reaction in problem 7 above: What is the mole ratio for C3H8/O2? ____________ = ____________ What is the mole ratio for CO2/H2O? ____________ = ____________ What is the mole ratio for H2O/O2? ____________ = ____________ 6 Skill Practice: Using Mole-Mole Relationships Example: C3H8 + 5 O2 → 3 CO2 + 4 H2O 1. How many moles of O2 are needed to completely react with 1 mole of C3H8? Answer: 5 moles 2. How many moles of CO2 form when 5 moles of O2 react? Answer: 3 moles 3. How many moles of H2O form when 1 mole of C3H8 react? Answer: 4 moles 4. How many moles of C3H8 are required to produce 8 moles of H2O? Answer: 2 moles. Practice: 2 Mg + O2 2 MgO 1. How many moles of O2 are needed to completely react with 6 moles of Mg? Answer: ________ 2. How many moles of MgO form when 3 moles of O2 react? Answer: _____ 3. How many moles of MgO form when 8 moles of Mg react? Answer: ______ 4. How many moles of O2 are required to produce 20 moles of MgO? Answer: ________ 7 Mole Ratio Worksheet Name: _____________ *Balance all equations first! 1) Given this equation: ___N2 + ___H2 ---> ___NH3, write the following molar ratios: a) N2 / H2 _____________ = ______________ b) N2 / NH3 _____________ = ______________ c) H2 / NH3 _____________ = ______________ 2) Given the following equation: ___H2 + ___S8 ---> ___H2S, write the following molar ratios: a) H2 / H2S _____________ = ______________ b) H2 / S8 _____________ = ______________ c) H2S / S8 _____________ = ______________ 3) Answer the following questions for this equation: ___ H2 + ___O2 ---> ___ H2O a) What is the H2 / H2O molar ratio? _____________ = ______________ b) Suppose you had 20 moles of H2 on hand, how many moles of H2O could you make? ______________ c) What is the O2 / H2O molar ratio? _____________ = ______________ d) Suppose you had 20 moles of O2, how many moles of H2O could you make? ____________ 4) Use this equation: ___N2 + ___H2 ---> ___NH3 for the following problems. a) If you used 1 mole of N2, how many moles of NH3 could be produced? __________ b) If 10 moles of NH3 were produced, how many moles of N2 would be required? _________ c) If 3.00 moles of H2 were used, how many moles of NH3 would be made? ___________ d) If 0.600 moles of NH3 were produced, how many moles of H2 are required? _________ 8 Balanced Equations and Mole Ratios Name: ____________ 1. Balance each equation. 2. Write a mole ratio between the two chemicals with a star (*). Follow the example in the first problem. Mole ratio 1. * * __1___NH4NO2 --> __2___H2O 2. _____H2 3. _____MgCO3 --> _____MgO + _____CO2 4. _____P4 + _____Cl2 --> _____PCl5 5. _____CrO3 6. _____IF5 7. _____NH3 + _____O2 --> _____NO + _____H2O 8. * * _____HBrO3 --> _____Br2O5 9. * _____NO2 + _____H2O * + ___1__N2 * + _____N2 --> _____NH3 * * * * * * --> _____Cr2O3 + _____O2 * + _____H2O * --> _____HF + _____HIO3 * 10. 1 mole NH4NO2 = 2 mole H2O * + _____H2O * --> _____HNO3 * * _____NH4NO3 --> _____N2O + _____H2O + _____NO 9 Using Balanced Chemical Equations Predicting Moles of Reactants/Products Answer the following questions in the space provided. Use the chemical reaction and show your dimensional analysis! TiCl4 + 2H2O 4HCl + TiO2 1. How many moles of HCl would you produce if you started with 3.11 moles of TiCl4? 2. How many moles of TiO2 would you produce if you started with 3.11 moles of TiCl4? 3. If you needed 0.003 mol of TiO2 how many moles of TiCl4 would you need to start with? 4. If you needed 0.003 mole of TiO2, how many moles of H2O would you need to start with? 5. If you needed 4.91 moles of HCl, how many moles of TiCl 4 should you start with? 10 Converting Grams to Moles Name: ______________ Use dimensional analysis and show your work! 1. How many grams of NH3 are in 12 moles of NH3? 2. How many grams of K2CO3 are in 2.4 moles? Use this equation to solve the problems below. (Balance it first!) ____CaH2 + ____H2O ____Ca(OH)2 + ____H2 3. If you started with 2.50 g of CaH2, how many grams of H2 would be produced? What mole ratio will you use? ___________=___________ What molar masses will you use? 1mole = ________ g CaH2 1mole = ________ g H2 Solve the problem: (Fill in the missing numbers) 2.50 g CaH2 1 mole CaH2 ____mol H2 _____ g CaH2 ____ mol CaH2 ___g H2 = 1 mole H2 4. If you started with 0.112 g of H2O, how many grams of H2 would be produced? What mole ratio will you use? ___________=___________ What molar mass will you use? 1mole = ________ g H2O 1mole = ________ g H2 Solve the problem: (Watch out!!! The mole ratio is different for this one!) 5. If you wished to produce 4.7 g of Ca(OH)2, how many grams of H2O would be required? What mole ratio will you use? ___________=___________ Record your molar masses here: Solve the problem: 11 Use this equation to solve the problems below. (Balance it first!) ____NaN3 ____Na + ____N2 6. If you started with 3.8 g of NaN3, how many grams of Na would be produced? What mole ratio will you use? ___________=___________ Record your molar masses here: Solve the problem: 7. If you started with 0.86 g of NaN3, how many grams of N2 would be produced? What mole ratio will you use? ___________=___________ Record your molar masses here: Solve the problem: 8. If you wished to produce 5.7 g of N2, how many grams of NaN3 would be required? What mole ratio will you use? ___________=___________ Record your molar masses here: Solve the problem: 12 Introduction to Percent Yield: You have learned from laboratory experience that there is sometimes a difference between an experimental result and a mathematical result. Using stoichiometry, we can mathematically determine the amount of a product that should be formed during an experiment, yet we sometimes find that we don't end up with exactly the right amount of product. Percentage yield tells us how close to “perfect” we were. Specifically, it allows us to calculate what percent of the expected product we are able to account for by the end of our experiment. The formula that we use is: actual amount of product percentage yield = ------------------------------------------- x 100 expected amount of product Example 1: This is just like finding your grade after a test. You scored 28 questions correct out of 30. We compare the “actual” number of questions correct (28) to the number you “should have answered correctly” (30): Answer: Questions correct (28) percentage yield = ------------------------------------- x 100 = Questions on the test (30) 93 % Example 2: A student conducts a reaction that produces 2.755 grams of copper. Mathematically he determines that 3.150 grams of copper should have been produced. Calculate the student's percentage yield. Answer: actual amount of product: 2.755 g expected amount of product: 3.150 g actual amount of product percentage yield = ------------------------------------------- x 100 expected amount of product 2.755g percentage yield = --------------- x 100 3.150g percentage yield = 87.5 % Percent Yield Practice Name:_________________ 13 Be sure to show how you plugged the numbers into the formula as was shown in the answer to example #2. No credit will be given for answers only. 1. In the reaction shown below, a student produces 22 grams of water. Mathematically he determines that 29 grams of water should have been produced. Calculate the student's percentage yield. 2H2 + O2 2H2O 2. The actual amount of MgO synthesized in the reaction shown below was 0.77 g. If the theoretical amount of MgO was calculated to be 1.30 g, what is the percent yield of the reaction? 2Mg + O2 2MgO 3. A chemist starts with 11.2 g of KClO3 and calculates that it should be possible to make 7.6 g of KCl with that much KClO3. After having conducted the reaction, she finds that she recovered only 6.8 g of KCl. What is her percent yield? 2KClO3 2KCl + 3O2 4. You have 6.2 g of NH3 in the cabinet. You do a short calculation and figure out that this much NH3 will yield 0.92 g of H2 in the reaction shown below. You conduct the reaction and find that you actually get 0.90 g of H2. What is your percent yield? 2NH3 N2 + 3H2 14 15 16 17 Limiting Reagents (mole comparisons) Name: _______________ In each case, identify the limiting reagent. (Show your work!) 1. C2H4 + 3O2 2CO2 + 2H2O a) 2.00 moles of C2H4 are reacted with excess O2 b) 3.00 moles of C2H4 are reacted with 5.00 moles of O2 c) 1.45 moles of C2H4 are reacted with 5.00 moles of O2 2. Zn + 2 HCl ZnCl2 + H2 a) 27 moles of Zn are reacted with excess HCl. b) 12 moles of Zn are reacted with 12 moles of HCl. c) 1.49 moles of Zn are reacted with 2.81moles of HCl. 3. 2 Mg + O2 2 MgO a) 0.8 moles of Mg are reacted with excess O2. b) 0.72 moles of Mg are reacted with 0.22 moles of O2. c) 2.61 moles of Mg are reacted with 1.26moles of O2. 18 Limiting Reagents Name: _______________ In each case, identify the limiting reagent and the amount of product that would be produced. (Show your work!) Strategy: Convert g of each reactant to g of product. (Do two separate train track problems.) The one that makes less product was the limiting reagent. 1. For this equation: C2H4 + 3O2 2CO2 + 2H2O What is the limiting reagent if 2.00 g of C2H4 are reacted with 5.00 g of O2? How much CO2 would be made in this reaction? What is the limiting reagent? 2. For this equation: Zn + 2 HCl How much CO2 would be produced? ZnCl2 + H2 What is the limiting reagent if 27 g of Zn are reacted with 0.79 moles of HCl? How much H2 would be produced in this reaction? What is the limiting reagent? How much H2 would be produced? 19 3. _____ Cu(s) + _____ AgNO3(aq) _____ Cu(NO3)2(aq) + _____ Ag(s) If 2.5 g of copper and 5.5 moles of silver nitrate are available to react, what is the limiting reactant? 4. _____ CaO(s) + _____ H2O(l) _____ Ca(OH)2(aq) How many grams of calcium hydroxide will be formed in this reaction when 4.44 g of calcium oxide and 7.77 g of water are available to react? Also identify the limiting reagent. 20 Introduction to Stoichiometry Lab Name: ____________________ Goal: To make 0.5 g of CuO. (And to make an educated guess as to the amount of starting material that will give us 0.5 g of CuO.) Safety: Tie back long hair and loose clothing near the Bunsen burner. Copper carbonate is toxic. Do not ingest it and wear goggles at all times. Be careful with the flame. Turn it off if you are leaving your station for any amount of time. Waste: After they cool down, the products can go in the garbage. Carbonates are a special class of compounds that do this reaction when heated: CuCO3 CuO + CO2 What kind of reaction is this? ________________ (choose from synthesis, decomposition, double and single displacement.) What you need to do: We want to make exactly 0.5 g of the CuO product. To do this we need to decide how much CuCO3 to start with. Weigh between 0.5 and 1.5 g of CuCO3 into your evaporating dish. Mass of evaporating dish empty ________ g Mass of evaporating dish filled with CuCO3 ________ g Mass of CuCO3 that you are starting with ________ g Set up a Bunsen burner as shown below: Put the evaporating dish on the wire guaze and start heating. Stir the powder with your glass stirring rod until all of the green has turned to black. Let the dish cool until you can safely touch it. Weigh the evaporating dish again. ________ g 21 We need to be sure that all of the stuff has changed to product. Heat it again for another minute, then cool, and weigh the dish again. ________ g Did the mass change from the last time you weighed it? If so repeat heating until it doesn’t change by more than 0.1 g. Record the final mass of the evaporating dish here: _________ g Mass of empty evaporating dish __________g (You already weighed this above.) Mass of black CuO after you subtract out the dish: _________ g Did you get exactly 0.5 g? (0.45-0.55g is okay.) If you didn’t, predict how much CuCO3 you should start with to get exactly 0.5 g of CuO in the end. Try it again with your new predicted amount. Record the data that you collected for your second attempt here: Mass of CuCO3 at start Mass of the CuO at the end. Remember units! Conclusion: What mass of CuCO3 do you need to start with to get 0.5 g of CuO at the end? _____g (SHOW THIS TO YOUR INSTRUCTOR FOR CONFIRMATION.) Why does the mass change going from reactants to products?