Activity Stoichiometric Calculations with Excess Reactants

Transcription

1 Activity Stoichiometric Calculations with Excess Reactants Directions: This Guided Learning Activity (GLA) focuses on completing stoichiometric calculations to predict the amount of excess reactant that will remain after the reaction. Part A reviews how to identify the limiting reactant and determine the theoretical yield, Part B introduces determining the mass of the excess reactant, and Part C discusses finding the concentration of an excess reactant. The worksheet is accompanied by an in-depth key. See for additional materials. Part A Limiting Reactants and Theoretical Yields Please refer to GLA 151 9: Introduction to Stoichiometry for a detailed discussion of how to convert mass or volume of a substance into moles of that substance. In chemistry, often we aim to predict the outcome of a chemical reaction both qualitatively (what will my products be?) and quantitatively (how much product will I get?) To predict the amount of product that will be formed, we must relate the mass or volume of each reactant to its molar amount. In chemical reactions that consume two or more reactants, the limiting reactant is the reactant that will yield the least amount of product. The theoretical yield is the amount of product that can form if 100% of the limiting reactant is converted into product. To determine the limiting reactant and the theoretical yield, calculate the amount of product that will be formed by each of the reactants. Consider the following reaction between vinegar and baking soda: HHHH 2 HH 3 OO 2 (aaaa) + NNNNNNNNOO 3 (aaaa) NNNNCC 2 HH 3 OO 2 (aaaa) + HH 2 OO(ll) + CCOO 2 (gg) Vinegar Baking soda Sodium acetate Water Carbon dioxide If 25.0 ml of a M solution of vinegar is mixed with 1.23 g of baking soda, which is the limiting reactant? What is the theoretical yield of carbon dioxide? Calculate the theoretical yield of CO 2 for HC 2H 3O 2: ( mmmm HHCC 22 HH 33 OO 22 ) mmmmmm HHCC 22HH 33 OO mmmmmm CCOO 22 = mmmmmm CCOO mmmm HHCC 22 HH 33 OO mmmmmm HHCC 22 HH 33 OO Activity Page 1 of 5

2 Calculate the theoretical yield of CO 2 for NaHCO 3: ( gg NNNNNNNNOO 33 ) 11 mmmmmm NNNNNNNNOO mmmmmm CCOO 22 = mmmmmm CCOO gg NNNNNNNNOO mmmmmm NNNNNNNNOO In this case, the vinegar can produce moles of CO 2, which is less than the moles of CO 2 that the baking soda could produce. The limiting reactant is vinegar and the theoretical yield is moles of CO 2. Keep in mind that each product has a theoretical yield. If more than one product forms, you only need to find the theoretical yield of one product, since all products will have the same limiting reactant. Whether or not all of the limiting reactant is converted into product depends on a two things: 1. In many chemical reactions, equilibrium exists between the reactants and products, so the reverse reaction occurs simultaneously with the forward reaction. This equilibrium limits the amount of reactant that will be converted to product. 2. Sometimes there are competing reactions that consume some of the reactant. These competing reactions prevent the reactant from being converted into the product of interest, and thereby limit the yield. For many reactions, we can assume the reaction goes to completion and the actual yield is equal to the theoretical yield. Example #1 The zinc core of a penny can be dissolved in hydrochloric acid. If a penny that contains g of zinc is placed in 20.0 ml of a 6.00 M hydrochloric acid solution, will the zinc in the penny dissolve completely? ZZZZ(ss) + 2HHHHHH (aaaa) ZZZZZZll 2 (aaaa) + HH 2 (gg) Activity Page 2 of 5

3 Part B Finding the Amount of the Excess Reactant In the reaction between vinegar and baking soda, we found that the limiting reactant was vinegar. When the reaction is complete, the vinegar has been completely consumed. However, some of the baking soda is still present. In this case, the baking soda is the excess reactant. There may be only one excess reactant, or there may be several, depending on the chemical reaction. In all cases, a portion of the excess reactant remains at the end of the reaction. To find the amount of excess reactant that remains after the reaction, the product yield is used to calculate the amount of excess reactant that was consumed during the chemical reaction. This quantity is then subtracted from the initial amount of excess reactant. In the above reaction between vinegar and baking soda, how much baking soda remains after the reaction is complete? Since vinegar is the limiting reactant, calculate the amount of NaHCO 3 that was consumed in the chemical reaction to form mol CO 2: ( mmmmmm CCOO 22 ) 11 mmmmmm NNNNNNNNOO gg NNNNNNNNOO 33 = gg NNNNNNNNOO 11 mmmmmm CCOO mmmmmm NNNNNNNNOO 33 uuuuuuuu 33 Subtract this amount from the initial amount of baking soda: gg NNNNNNNNOO gg NNNNNNNNOO 33 = gg NNNNNNNNOO 33 rrrrrrrrrrrrrr After the reaction is complete, 0.54 g of the baking soda remains. The vinegar has been completely depleted, so the remaining baking soda does not form product. Example #2 How much excess reactant is present in Example #1? Activity Page 3 of 5

4 Part C Finding the Concentration of the Excess Reactant In many chemical reactions, the excess reactant is in the aqueous phase. For aqueous substances, we generally give the amount in terms of concentration (molarity) instead of mass (grams). To find the concentration of an excess reactant, the molar amount of the excess reactant is divided by the total volume of the resulting solution. Typically, this volume is a sum of the reactant volumes. At the end of the vinegar and baking soda reaction, you measure the volume of the remaining solution to be 26.3 ml. What is the concentration of the NaHCO 3 that remains in the solution? We know: [NNNNNNNNOO 33 ] = mmmmmmmmmm NNNNNNNNOO 33 LLLLLLLLLLLL ssssssssssssssss Find moles of baking soda remaining: ( gg NNNNNNNNOO 33 ) 11 mmmmmm NNNNNNNNOO gg NNNNNNNNOO 33 = mmmmmm NNNNNNNNOO 33 Divide by liters of the final solution to determine concentration: [NNNNNNNNOO 33 ] = mmmmmmmmmm NNNNNNNNOO 33 LLLLLLLLLLLL ssssssssssssssss = mmmmmm NNNNNNNNOO LL ssssssssssssssss = MM NNNNNNNNOO 33 In the final solution, the baking soda is present at 0.24 moles per Liter, or 0.24 M NaHCO 3. Example # ml of a M potassium iodide solution is mixed with 2.00 ml of a M mercury (I) nitrate solution. Assuming the reaction goes to completion, what is the excess reactant, and what is its concentration in the final solution? 2KKII(aaaa) + HHgg 2 (NNOO 3 ) 2 (aaaa) 2KKKKOO 3 (aaaa) + HHgg 2 II 2 (ss) Activity Page 4 of 5

5 Part D Extra 1. A solution containing 5.00 g of lead (II) acetate is mixed with a solution containing 3.00 g of ammonium iodide. PPPP(CC 2 HH 3 OO 2 ) 2 (aaaa) + 2NNHH 4 II (aaaa) PPPPII 2 (ss) + 2NNHH 4 CC 2 HH 3 OO 2 (aaaa) a. What is the limiting reactant for the above reaction? b. Assuming the reaction occurred in a 1.0 L solution, what is the final concentration of the excess reactant? 2. Zinc chloride (0.55 M, 50.0 ml) reacts with ammonium phosphate (0.75 M, 55.0 ml) to completion. 3ZZZZZZll 2 (aaaa) + 2(NNHH 4 ) 3 PPOO 4 (aaaa) ZZnn 3 (PPOO 4 ) 2 (ss) + 6NNHH 4 CCCC (aaaa) a. What mass of zinc phosphate is produced? b. Assuming the volumes are additive, what is the final concentration of ammonium chloride? c. What is the excess reactant, and what is its final concentration? 3. Consider the reaction between silicon dioxide and carbon: SSSSOO 2 (ss) + 3 CC(ss) SSSSSS(ss) + 2CCCC(gg) a g of silicon dioxide is mixed with 4.21 g of carbon. What is the limiting reactant? Calculate the theoretical yield for SiC. b. If the reaction goes to completion, how many grams of SiO 2, C, SiC and CO are present after the reaction is complete? Challenge Problems For the following problems, first write the chemical reaction. 4. Antacids are used to neutralize the stomach acid and decrease acid reflux. If Dave s stomach contains 150 ml of M hydrochloric acid, will two antacid tablets (each containing 500. mg of calcium carbonate) neutralize all of the acid? How much excess reactant will remain? 5. A solution of potassium hydroxide (0.540 M, 50.0 ml) is mixed with iron (III) chloride ( M, ml). How much of each product is formed if the reaction goes to completion? What is the excess reactant, and what is its concentration at the end of the reaction? Activity Page 5 of 5