Stoichiometry: Predicting Amounts in Reactions

Transcription

1 Stoichiometry: Predicting Amounts in Reactions Stoichiometry is the process of determining how much product is made or how much reactant is needed during a chemical reaction. As we know, in chemical reactions atoms are conserved. We show this in a balanced chemical equation. The balanced chemical equation tells us two things: 1. Which substances begin with (reactants) and end with (products) during the rearrangement process. 2. The ratio of particles involved. This ratio can be seen either as a ratio of individual particles OR as a ratio of moles. In the lab it is only practical to work with moles of substances rather than individual atoms or molecules, and so we interpret our equations as a ratio of moles, or a mole ratio. Example: 2 Mg + 1O2 2 MgO means for every 2 moles of Mg burned, 1 mole of O2 is required to produce 2 moles of MgO, or a ratio of 2 moles Mg : 1 mole O2 : 2 moles MgO We can use this mole ratio relationship to make predictions about how much we need of something, or how much we can make from what we have. Making Predictions In every reaction, there are three stages we need to consider to make good predictions: 1. What we have before the reaction takes place. 2. : How much of each substance actually changes during the reaction 3. : How much of each substance is present after the reaction is complete. Some good organization can help us in making good predictions. We have an organizational table that can help us track the Before- - for a reaction. Below is an example of a problem involving a chemical reaction. Sample Problem: Hydrogen sulfide gas, which smells like rotten eggs, burns in air to produce sulfur dioxide and water. How many moles of oxygen gas would be needed to completely burn 2.4 moles of hydrogen sulfide? Step 1- Write and Balance the equation (describe the reaction and its mole ratio) 2 H2S + 3 O2 2 SO2 + 2 H2O 271

2 Step 2: Fill in the Before line with the Given information; mark what you must Find on the table (with units) 2 H2S + 3 O2 2 SO2 + 2 H2O 2.4moles xs moles 0 moles 0 moles moles NOTE: Assume reactants you don t have amounts for are present with more than enough available (excess, or xs ) for the reaction to be completed. Step 3: Use ratio of coefficients to determine the made 2 H2S + 3 O2 2 SO2 + 2 H2O 2.4moles xs moles 0 moles 0 moles : 2.4 moles 3.6 moles +2.4 moles +2.4 moles NOTE: Reactants are consumed/decrease (- ), products accumulate/increase (+) Step 4: Complete the table for what remains the reaction is complete 2 H2S + 3 O2 2 SO2 + 2 H2O 2.4 moles xs moles 0 moles 0 moles : 2.4 moles 3.6 moles +2.4 moles +2.4 moles : 0 moles xs moles 2.4 moles 2.4 moles In this case, desired answer is in moles Answer (in moles): 3.6 moles O2 are needed to burn 2.4 moles H2S. If mass is required, convert moles to grams in the usual way 3.6moles O2 * 32 grams = 115 grams O2 1 mole Answer (in grams): 115 grams O2 are needed to burn 2.4 moles H2S. 272

3 Name: Veritas: Unit 8, Worksheet 1 Basic Stoichiometry 1. Lead will react with hydrochloric acid to produce lead (II) chloride and hydrogen gas. How many moles of hydrochloric acid are needed to completely react with 4.0 mole of lead? 2. How many moles of hydrogen gas will be produced if 2.5 moles of calcium hydride react according to the following equation? CaH2 + 2 H2O Ca(OH)2 + 2 H2 3. How many moles of water will be produced if 0.45 mol of oxygen reacts according to the following equation? 2 C6H O2 12 CO2 + 6 H2O? 273

4 4. How many moles of calcium sulfate will be formed if 2.45 moles of calcium hydroxide reacts with an excess of sulfuric acid (the second product is water)? 5. How many moles of barium nitrate will be needed to produce 0.78 moles of barium phosphate, assuming that you have an excess of lead (IV) phosphate? (Knowing the pattern of double replacement reactions, what is the second product?) 6. Sodium burns in air to produce sodium oxide. How many moles of oxygen are needed to completely react with 9.5 moles of sodium? 274

5 Name: Veritas: Unit 8, Worksheet 2 Basic Stoichiometry Part II For each of the problems below: a. Write the balanced chemical equation b. Identify what is given (with units) and what you want to find (with units) c. Use coefficients from balanced equation to determine mole ratio. d. Show set up (organize it!). 1. Hydrogen sulfide gas, which smells like rotten eggs, burns in air to produce sulfur dioxide and water. How many moles of oxygen gas would be needed to completely burn 8 moles of hydrogen sulfide? H 2 S (g) + O 2 (g) SO 2 (g) + H 2 O (g) 2. Propane, C3H8, burns in air to form carbon dioxide and water. If 12 moles of carbon dioxide are formed, how many moles of propane were burned? 3. Ammonia, NH3, for fertilizer is made by causing hydrogen and nitrogen to react at high temperature and pressure. How many moles of ammonia can be made from 0.15 moles of nitrogen gas? 275

6 4. The poison gas phosgene, COCl2, reacts with water in the lungs to form hydrochloric acid and carbon dioxide. How many moles of hydrochloric acid would be formed by moles of phosgene? 5. Iron metal and oxygen combine to form the magnetic oxide of iron, Fe3O4. How many moles of iron can be converted to magnetite by 8.80 moles of pure oxygen? (make your BCA table) How many moles of iron oxide would be produced? 6. The recipe for Coca- Cola Classic is a closely guarded secret. Researchers outside the company believe the flavoring mixture, known as 7X, contains oils of orange, lemon, nutmeg, cinnamon, and coriander. The original mixture also contained caffeine, vanilla, caramel, lime juice, sugar or artificial sweetener, and citric acid. Over the years, the recipe has changed. For example, the original recipe contained citric acid but this was combined with phosphoric acid to cut production costs. Corn syrup replaced sugar for the same reason. To produce 1000 cans of Coca- Cola Classic, 40g (0.21 moles) of caffeine are reacted with phosphoric acid and other ingredients. How many moles of phosphoric acid are required? How many moles of carbon dioxide are required? (caffeine) (phosphoric acid) (potassium benzoate) C 8H 10N 4O H 3PO CO 2 + other ingredients à C 6H 5CO 2K + other products 276

7 Unit 8, Lab 1 In this experiment, a solution of copper (II) chloride will react with aluminum wire. Careful measurements and calculations will allow you to determine mole relationships. Title: Purpose/Question: Procedure: Day 1 1. Obtain a 30-cm length of aluminum wire, a vial of copper (II) chloride, and a dixie cup. Label the cup with the period and group number so you can find it again. 2. Measure out 2.0 g of copper (II) chloride. Record the actual amount used (ex or 2.08). Be sure to use the same balance for all measurements. 3. Record the mass of the aluminum wire. 4. Record the mass of the labeled cup. 5. Clamp a clean test tube to a ring stand. 6. Add 20.0 ml of distilled water to the test tube. 7. Very carefully add the copper (II) chloride from the weigh boat to the test tube. Stir gently with a glass-stirring rod to dissolve the crystals completely. 8. Coil the aluminum wire by wrapping it around a pencil. Stretch the wire until it is about 2 cm longer than the test tube. Place the coiled wire into the test tube with the copper (II) chloride solution so that the straightened end is out of the solution. 9. Note the reaction that occurs; record your observations. 10. Allow the reaction to continue for 20 minutes or as long as possible. 11. Shake the copper from the aluminum wire and remove the wire from the test tube. Using the distilled water bottle, rinse the wire into the weighed, labeled cup. Dip the wire in the acetone located on the stock table, then set it aside to dry. 12. Pour the contents of the test tube into the labeled cup. Use the distilled water wash bottle to rinse the test tube into the beaker. 13. Carefully decant the solution from the copper into another beaker. Rinse the copper with about 10 ml of distilled water, stirring gently with a clean stirring rod. Allow the copper to settle, then decant into the other beaker. Repeat this 3 more times. To minimize error, try not to allow any of the copper to escape during decantation. Discard the liquid from the other beaker into the waste container on the stock table. 14. the final rinse, place the labeled cup on the stock table. 15. Record the mass of the aluminum wire to the nearest 0.01g, then put the wire in the designated location on the central stock table. 16. Clean the used beaker and your lab station. 277

8 Procedure Day When the copper is dry, find the mass of the cup and copper to the nearest 0.01g. Return the cup that contains the copper to your instructor. Data Construct a data table from the measurements recorded in lab. Calculations Show all work, units, and appropriate sig figs. 1. Determine the mass (in grams) of Al that reacted during the experiment. 2. Determine the moles of Al reacted. 3. Determine the mass of Cu produced during the experiment. 4. Determine the moles of Cu produced during the experiment. 5. Determine the value of the ratio mol Al: mol Cu. Make sure that this ratio is in its simplest form by dividing each quantity by the smaller of the two. Report the exact ratio. 278

9 Analysis 4. Because of our deep and abiding belief that atoms react in simple whole number ratios, what do you suppose is the actual ratio of mol Al: mol Cu? (Note: Your team should use class results to help you decide the correct ratio.) 5. Write a balanced equation for the single replacement reaction between solid aluminum and aqueous copper (II) nitrate. 6. Complete a BCA table for this chemical reaction. (NOTE: Be sure to enter the exact number of moles of Al and the exact number of moles of copper (II) sulfate that were present before the reaction began.) 7. How many moles of copper does the BCA Chart predict should be produced in this chemical reaction? Convert this quantity to grams 8. How many grams of copper were actually recovered at the end of the lab procedure? 9. Using the actual yield of Cu and the theoretical yield of Cu, determine the percent yield. 279

10 Conclusions If your ratio mol Al: mol Cu is greater than the accepted value, then either the moles of Al is too high, or the moles of Cu is too low. If your ratio is lower than the accepted value, then either the moles of Al is too low, or the moles of Cu is too high. Use the % yield to help you decide whether the problem is due to the value you have obtained for the moles of copper. Discuss as many specific experimental errors (minimum 3) that could account for the ratio being too high or too low. (Your lab errors should make sense with your data.) 280

11 Name: Veritas: Unit 8, Worksheet 3 Stoichiometry with Gram-Mole Conversions Write the balanced chemical equation for each problem. Use the information that is given to complete a BCA Chart. Some conversions will be necessary. Show the math for the conversions off to the side of the BCA Chart. Circle or box in the final answer. 1. Solid aluminum metal reacts with chlorine gas to produce solid aluminum chloride crystals. Assuming 100% yield, how many grams of aluminum chloride crystals will be produced if there are 61.5 grams of solid aluminum available? grams of solid potassium chlorate decomposes when heated to form solid potassium chloride and oxygen gas. Assuming 100% yield, how many grams of solid potassium chloride will be produced? 3. Solid zinc and aqueous copper (II) bromide react in a single replacement reaction producing zinc bromide and solid copper. How many grams of solid zinc nuggets must be put into this reaction in order to recover 25.0 grams of solid copper metal. (Assume 100% yield.) 281

12 4. Solid iron (III) hydroxide decomposes when heated into solid iron (III) oxide and water vapor. If 5.25 grams of iron (III) hydroxide decomposes completely in this reaction, how many grams of iron (III) oxide will be formed? (Assume 100% yield.) 5. Solid magnesium combines with fluorine gas producing solid magnesium fluoride. How much magnesium metal is required to produce exactly 50.0 grams of the magnesium fluoride product? 6. Exactly grams of solid calcium oxide is combined with exactly grams of solid diphosphorus pentoxide. This reaction produces solid calcium phosphate. Assuming 100% yield, how many grams of solid calcium phosphate will be produced? 282

13 Name: Veritas: Unit 8, Worksheet 4 Stoichiometry with Percent Yield Write the balanced chemical equation for each problem. Use the information that is given to complete a BCA Chart. Some conversions will be necessary. Show the math for the conversions off to the side of the BCA Chart. Circle or box in the final answer. 1. Using the Hoffman apparatus for electrolysis, a chemist decomposes 36 g of water into its gaseous elements. How many grams of hydrogen gas should she get (theoretical yield)? 2. Recall that liquid sodium reacts with chlorine gas to produce sodium chloride. You want to produce 581 g of sodium chloride. How many grams of sodium are needed? 3. You eat g of glucose (90 M&Ms). If glucose, C6H12O6, reacts with oxygen gas to produce carbon dioxide and water, how many grams of oxygen will you have to breathe in to burn the glucose? 4. Suppose 4.61 g of zinc was allowed to react with hydrochloric acid to produce zinc chloride and hydrogen gas. How much zinc chloride should you get? Suppose that you actually recovered 8.56 g of zinc chloride. What is your percent yield? 283

14 5. Determine the mass of carbon dioxide that should be produced in the reaction between 3.74 g of carbon and excess O2. What is the % yield if g of CO2 is recovered? 6. In the reaction between excess K(s) and 4.28 g of O2(g), potassium oxide is formed. What mass would you expect to form (theoretical yield)? If g of K 2 O is actually produced, what is the percent yield? 7. Determine the mass of carbon dioxide one could expect to form (and the percent yield) for the reaction between excess CH 4 and 11.6 g of O2 if 5.38 g of carbon dioxide gas is produced along with some water vapor. 8. Determine the mass of water vapor you would expect to form (and the percent yield) in the reaction between 15.8 g of NH3 and excess oxygen to produce water and nitric oxide (NO). The mass of water actually formed is 21.8 g. 284

15 Name: Veritas: Unit 8, Worksheet 5 Stoichiometry with Limiting and Excess Reactants 1. Write the balanced equation for the reaction between hydrogen and oxygen. Balanced Suppose that 4 molecules of hydrogen gas and 4 molecules of oxygen gas react to form water. Make a drawing that represents the reaction container before and after the reaction. Before How many molecules of water can be produced? Which reactant is in excess? Why? How many molecules of excess reactant are there? Construct a Before- - Table for this reactant mixture: Bal. : : According to the table you just made, How many molecules of water can be produced? Which reactant is in excess? Why? How many molecules of excess reactant are there? Based on your two methods of analysis above, what determines how much product can be made from a particular reactant mix? 285

16 2. Write the equation for the formation of ammonia from nitrogen gas and hydrogen gas. Balanced Given 6 molecules of nitrogen and 12 molecules of hydrogen, make a drawing that represents the reaction container before and after the reaction. Before How many molecules of ammonia can be produced? Which reactant is in excess? Why? How many molecules of excess reactant are there? Construct a Before- - Table for this reactant mixture: Bal. : : According to the table you just made, How many molecules of ammonia can be produced? Which reactant is in excess? Why? How many molecules of excess reactant are there? Describe what you must look for in a particular reactant mixture to decide which reactant will be in excess (have some left over after the reaction): 286

17 3. When 0.50 mole of aluminum reacts with 0.72 mole of iodine to form aluminum iodide, how many moles of the excess reactant will remain? How many moles of aluminum iodide will be formed? 4. When sodium hydroxide reacts with sulfuric acid (H 2 SO 4 ), water and sodium sulfate are the products. Calculate the mass of sodium sulfate produced when 15.5 g of sodium hydroxide are reacted with 46.7 g of sulfuric acid. 5. A 14.6 g sample of oxygen gas is placed in a sealed container with 2.5 g of hydrogen gas. The mixture is sparked, producing water vapor. Calculate the mass of water formed. Calculate the number of moles of the excess reactant remaining. 287

18 6. Neuroscientists believe that the only chemical in chocolate that may have a feel- good effect on the human brain is phenylethylamine (PEA). Although the PEA in chocolate occurs naturally, PEA can be made in the laboratory by the following reaction: How much PEA can be made from 75.0g of ammonium formate and 125g of acetophenone? What mass of the excess reactant remains? phenylethylamine CH 5NO 2 + C 8H 8O à C 8H 11N + CO 2 + H 2O ammonium formate acetophenone phenylethylamine (PEA) g of ammonia is reacted with 4.00 g of oxygen to produce nitrogen monoxide and water. How many grams of water are produced? How many grams of excess reactant is remaining? 8. Suppose 456 g of carbon monoxide and 65.0 g of hydrogen gas are mixed and allowed to react. What mass of ethanol (CH 3OH) can be produced? How many grams of the excess reactant remain? 288

19 Name: Veritas: Unit 8 Extra Practice Problems 1. Calculate the number of moles of potassium chlorate, KClO 3 (s), that must decompose to produce potassium chloride, KCl (s), and 1.8 moles of oxygen gas. 2. In a single displacement reaction, magnesium metal reacts with hydrochloric acid to produce magnesium chloride and hydrogen gas. How many moles of hydrochloric acid are needed to completely react with 2.43 g of magnesium? 3. Ethane, C 2 H 6 reacts with oxygen gas to produce carbon dioxide gas and water vapor. What mass of oxygen gas is required to react with 2.20 moles of ethane? 289

20 4. Determine the mass of sodium nitrate produced when 0.73 g of nickel (II) nitrate reacts with sodium hydroxide according to the following equation: 5. In the copper silver nitrate lab copper metal and silver nitrate solution reacted to produce silver metal and copper(ii) nitrate in solution. A student placed a copper wire with a mass of 2.93 g in the reaction test tube. The silver nitrate solution contained 1.41 g of silver nitrate. He obtained 0.87 g of silver metal. Calculate the percent yield of silver. 6. When hydrochloric acid (HCl) is added to sodium hydrogen carbonate, the products are water, aqueous sodium chloride and carbon dioxide gas. What is the per cent yield if 4.68 g of CO 2 are collected when 10.0 g of sodium hydrogen carbonate reacts with excess HCl? 290

21 7. Phosphorus and bromine react vigorously together to form phosphorus tribromide. If 5.0 g of phosphorus and 35 g of bromine react, how many grams of PBr3 could be produced? 8. Zinc sulfide and oxygen gas react to form zinc oxide and sulfur dioxide. Determine the amount of ZnO that should be produced in a reaction between 46.5 g of ZnS and 13.3 g of oxygen. What is the mass of the xs reactant? 9. Sodium hydroxide reacts with sulfuric acid, H2SO4, to produce water and sodium sulfate. How many grams of sodium sulfate will be formed if you start with 200. g of sodium hydroxide and you have an excess of sulfuric acid? 291

22 10. The combustion of a sample of butane, C4H10 (lighter fluid), produced 2.46 grams of water. Carbon dioxide was also produced. (a) How many moles of water formed? (b) How many moles of butane burned? (b) How many grams of butane burned? (d) How much oxygen was used up in moles? (e) How much oxygen was used up in grams? 11. How many grams of lithium nitrate will be needed to make 250 grams of lithium sulfate, assuming that you have an adequate amount of lead (IV) sulfate to do the reaction? : 292

23 12. Calculate how many grams of iron can be made from 16.5 grams of iron (III) reacting with an excess of hydrogen gas. (Water is the second product.) : 13. Calculate how many grams of iodine are needed to prepare 28.6 grams of ICl by the following reaction: 2 I2 + KIO3 + 6HCl à 5 ICl + KCl + 3H2O 14. How many grams of KMnO4 are needed to carry out the following reaction with 11.4 grams of KNO2? 5 KNO2 + 2 KMnO4 + 3 H2SO > 5 KNO3 + 2 MnSO4 + K2SO4 + 3 H2O 293

24 15. How many grams of oxygen gas are needed to react with 56.8 grams of ammonia (NH3) to produce nitrogen monoxide and water? 16. Calculate the number of grams of iodine (I2) that can be made in the following reaction with 16.4 grams of NaIO3. NaIO3 + 6 HI > 3 I2 + NaI + 3 H2O 17. A 50.6 g sample of Mg(OH)2 is reacted with 45.0 g of HCl producing magnesium chloride and water. What is the theoretical yield of magnesium chloride? 294

25 18. Methane, CH4, burns in oxygen to give carbon dioxide and water. In one experiment, a mixture of mol of methane was burned in 1.25 mol of oxygen in a sealed steel vessel. Find the limiting reactant, if any, and calculate the theoretical yield, (in moles) of water. 19. Chloroform, CHCl3, reacts with chlorine to form carbon tetrachloride and hydrogen chloride. In an experiment 25 grams of chloroform and 25 grams of chlorine were mixed. Which is the limiting reactant? What is the theoretical yield of CCl4 in grams? 20. Aluminum chloride can be made by the reaction of aluminum with chlorine. What is the limiting reactant if 20.0 grams of Al and 30.0 grams of Cl2 are used, and how much AlCl3 can theoretically form? 295

26 21. An organic chemist reacted 10. g CH4 with excess chlorine and obtained 10. g of CH3Cl. What was his percent yield? 22. If 20.0 g of aluminum and 30.0 g of chlorine are used, what is the theoretical yield of aluminum chloride? 23. If 80.0 grams of I2O5 react with 28.0 g of carbon monoxide, what is the percent yield if moles of iodine were actually produced? 296

27 24. If you start with 45 grams of ethylene (C2H4), how many grams of carbon dioxide will be produced? C2H4 + 3 O2 à 2 CO2 + 2 H2O 25. If you start with 5.5 grams of sodium fluoride, how many grams of magnesium fluoride will be produced? Mg + 2 NaF à MgF2 + 2 Na 26. If you start with 20 grams of hydrochloric acid, how many grams of sulfuric acid will be produced? 2 HCl + Na2SO4 à 2 NaCl + H2SO4 27. If you start with 10.0 grams of lithium hydroxide, how many grams of lithium bromide will be produced? LiOH + HBr à LiBr + H2O 297

28 28. Write the balanced equation for the double replacement reaction of lead (II) nitrate with sodium iodide. If I start with 25.0 grams of lead (II) nitrate and 15.0 grams of sodium iodide, how many grams of sodium nitrate can be formed? What is the limiting reagent? How many grams of the non-limiting reagent will be left over? 29. If I start with 5 grams of C3H8, what is my theoretical yield of water? C3H8 + 5 O2 à 3 CO2 + 4 H2O I got a percent yield of 75% How many grams of water did I make? 298