Stoichiometry: Synthesis of Garden Lime Standard: The student will explore the mathematics of chemical formulas and equations. Have you ever wondered why hot dogs are sold in packages of 10, but hotdog buns are sold in packages of 8? T his is an example of a ratio. Stoichiometry is the scientific word for ratios of moles. It is the study of the quantitative relationships between reactants and products in a chemical reaction. These relationships are often referred to as mole-to-mole ratios. Mole-to-mole ratios help scientists know how much of each reactant is needed to produce a sufficient amount of product. In this lab, you will observe the importance of mole-to-mole ratios when making the main component in lime that is put on a garden! Introduction The manufacturers of hotdogs and hotdog buns would not make very good chemists. Hotdogs are sold in packages of 10 where hotdog buns are sold in packages of only 8. If one package of each is purchased, how many hotdog-on-buns can you make? The answer of course is 8. This means there are 2 hotdogs left over. Now examine the equation below and try to fix their mistake. How many packages of hotdogs and hotdog buns must you purchase in order to not waste any hotdogs? How many hotdog-onbuns does this give you? ___10 Hotdogs + ___ 8 Hotdog buns ___ Hotdog-on-buns If you buy 4 packages of hotdogs and 5 packages of buns you hotdog-on-buns. would get 40 Balancing a chemical equation is done the same way. To balance a chemical equation you place numbers, called coefficients, in front of each type of molecule. Just like you cannot change how many hot dogs there are in a package of hotdogs, you cannot change how many of each type of atom there is in a chemical substance. You can however change the ratio of the chemical substances so that the same number of each type of atom is on both sides of the reaction equation. In the hotdog example there are 40 hotdogs and 40 hotdog buns on both sides of the arrow. But what if instead you buy only 2 packages of hotdogs, and 3 packages of hotdog buns, which would you run out of first? You would have a total of 20 hotdogs and 24 hotdog buns so you would run out of the hotdogs first. This means you could only make 20 of the hotdog-on-buns final product. The number of hotdogs limits the number of hotdog-on-buns you can make. This is similar to what in chemistry is called the limiting reagent. By using stoichiometry it is possible to calculate the limiting reagent of a reaction. Knowing which reactant will be used up first allows chemist to calculate how much product can theoretically be made. This is often compared to the amount that is actually made through calculating what is called the percent yield. The percent yield is found by using the following equation: Percent Yield = __Actual yield__ x 100 Theoretical yield Using the hotdog example, the percent yield would be like only making 19 hotdogs when there were enough hotdogs and buns to make 20 hotdog-on-buns. In this example the percent yield would be calculated as follows: Percent Yield = __19 Actual Hotdog-on-buns___ x 100 20 Theoretical Hotdog-on-buns Percent Yield = 95% Chemist will often try making a desired product many different ways. They will then compare the percent yields to determine which of the methods tried gives the greatest yield. In this laboratory exercise you will explore the concepts of limiting reagents and percent yields through making calcium carbonate, the main component in lime. Lime is used in many different industries, including farming. Farmers spread it on soil to adjust its pH so that their crops will be more productive. Calcium carbonate, CaCO3, can be made by mixing CaCl2 and K2CO3 in water. The trick is to be sure to use the correct amount of each. For this, stoichiometry is used. Look at the following reaction and balance it to determine the mole-to-mole ratio that would be necessary to have the same number of each kind of atom on both sides of the equation. ___ CaCl2(aq) + ____ K2CO3(aq) ____ KCl(aq) + ____ CaCO3(s) When this chemical equation is balanced there is one mole of calcium chloride for every 1 mole of potassium carbonate and together these quantities make 2 moles of potassium chloride and one mole of calcium carbonate. The potassium chloride is soluble in water so it remains in solution. The calcium carbonate will precipitate out as a white solid. CaCl2(aq) + K2CO3(aq) 2KCl(aq) + CaCO3(s) Objectives Demonstrate the use of stoichiometry to synthesize calcium carbonate. Practice using a balance and proper lab techniques. Find the limiting reagent, the theoretical yield, and the percent yield. Date:________ Name:_______________________ Period:_________ Pre-lab Questions 1. What is a limiting reagent? _____________________________________________________________ _____________________________________________________________ 1. A student used 7.15 g of CaCl2 and 9.25 g of K2CO3 to make CaCO3. The actual yield was 6.15 g of CaCO3. Calculate the limiting reagent and the percent yield. Procedure Weigh into a 250 mL beaker the amount of calcium chloride (CaCl2) specified for your group in the chart below. Record the exact mass you weigh out in the data section. 1. 2. Measure 50 mL of distilled water into a 100 mL graduated cylinder. Pour the water into the 250 mL beaker with the calcium chloride. Group A ~1.00 g CaCl2 Group B ~2.00 g CaCl2 Group C ~3.00 g CaCl2 3. Stir the solution with a stirring rod until all of the calcium chloride is dissolved. 4. Weigh out ~2.50 g of potassium carbonate (K2CO3) in a 50 mL beaker. exact mass in the data section. 5. Measure 25 mL of distilled water into a 100 mL graduated cylinder. Add the water into the 50 mL beaker containing the potassium carbonate. 6. Stir the potassium carbonate in the distilled water with a stirring rod until it is all dissolved. 7. Pour the K2CO3 solution into the 250 mL beaker that has the CaCl2 solution. Rinse the beaker that contained the K2CO3 with a few mL of water and add this to the CaCl2 solution. Stir the mixture. 8. As soon as the reaction begins, record your observations in the data section. Continue stirring until you see no more precipitate forming. 9. Set up the filtering apparatus as shown in Figure 9.1. Secure the Erlenmeyer flask to a ring stand. Rest the funnel in it. HINT: Do NOT begin filtering yet! Record the 10. Zero the balance and weigh a piece of medium flow filter paper and a watch glass. section. Record the masses of both items in the data 11. Fold the filter paper into quarters. Open the filter paper so that a funnel is formed. HINT: Three sides will be together and the other side will open out. 12. Place the paper funnel into the glass funnel and hold it in place with your fingers. Pour a small amount of distilled water onto the filter paper to secure it, so that you do not have to hold it in place. 13. Filter the mixture by pouring it into the filter paper in the funnel. Use the stirring rod and distilled water in a wash bottle to transfer the entire solid into the filter paper. HINT: For best results, be sure to transfer all of the precipitate into the filter paper. Use a rubber policeman if it is available to help with the transfer. 14. Rinse the remaining solid in the filter paper twice with distilled water from a wash bottle to rinse off excess potassium chloride (KCl). After all the liquid has filtered through, rinse the product with approximately 5 mL of ethanol to aid in its drying. Allow the ethanol to completely finish filtering through the paper. 15. Remove the filter paper carefully so as to not lose any product. Gently unfold the filter paper and lay it flat on the pre-weighed watch glass to dry. HINT: Do not throw your product away after you weigh it! 16. Allow the product to air dry until the next laboratory period. 17. In the next laboratory period, weigh the dry product on the filter paper and watch glass. Record the total mass in the data section. Calculate the mass of the product. 18. Complete the data and calculations by completing each step and comparing your results with the other groups. 19. Place the dried product in a designated container for later use in the Rates of Reaction experiment. Wash any dirty glassware Data Record the data for your group. Group ___________ Mass of CaCl2 _______________ Mass of K2CO3 ______________ Mass of filter paper_________________ Mass of watch glass_________________ Mass of product, filter paper, and watch glass____________________ Mass of dry product_________________ Observations for the reaction: Table 1. Final Results for EACH group Group Group A Group B Group C Actual Mass of CaCl2 (g) Actual Mass of K2CO3 (g) Mass of Dry CaCO3 Obtained (g) Calculations 1. Determine the limiting reagent for each group. Show your calculations. (Hint: See the example in the Introduction. ) Group A: Group B: Group C: 2. Calculate the theoretical yield of CaCO3 that could be produced by each group and then fill in Table 2. Table 2: Comparison of Theoretical and Actual Yields for CaCO3 Group Limiting Reagent Theoretical Yield of CaCO3 Actual Yield of CaCO3 Group A Group B Group C 3. Find the percent yield your group obtained for the CaCO3. Post Lab Questions 1. Compare your results with those of the other two groups. How does the amount CaCO3 they obtained compare to your results? of grams of _____________________________________________________________ _____________________________________________________________ 2. Were the results of the other two groups as you expected? Why or why not? _____________________________________________________________ _____________________________________________________________ 3. Predict what would happen if 6.0 grams of CaCl2 were used for the reaction and the amount of K2CO3 remained the same. _____________________________________________________________ _____________________________________________________________ Conclusion: