Lab: STOICHIOMETRY (CALCULATIONS INVOLING REACTIONS) Predicting and Measuring the Mass of NaCl Produced

Transcription

1 Lab: STOICHIOMETRY (CALCULATIONS INVOLING REACTIONS) Predicting and Measuring the Mass of NaCl Produced Making predictions of the outcomes of chemical reactions is an integral part of the scientific process. Not only is it important to be able to predict the identity of chemicals that will be produced but you must also be able to predict the quantities of those chemicals. For the latter we use stoichiometry which involves performing calculations on chemicals reactions. We can find examples of its use in many industrial applications. Stoichiometry allows chemist to determine how much iron is in a barge of iron ore, hoe much sulfur dioxide is in polluted air, or whether a new batch of fertilizer contains all of the nitrogen, phosphorus, and potassium listed on the label. In this experiment, you will study two common reactions between acids and carbonates. In Reaction 1, hydrochloric acid (the acid present in stomach acid) and sodium hydrogen carbonate (sodium bicarbonate or baking soda) will be mixed. The reaction is immediate, impressive, and simple, yet it occurs in two steps. Two products initially result: table salt and carbonic acid, but the carbonic acid is quite unstable and inst5rantly decomposed into water and carbon dioxide gas. It is the fizzing of the carbon dioxide gas that is most noticeable during the progress of this reaction. In the language of chemistry, this reaction can be best described as: Reaction 1: HCI(aq) + NaHCO 3 (s) NaCl(aq) + H 2 O(l) + CO 2 (g) In Reaction 2, hydrochloric acid and sodium carbonate (washing soda) will be mixed. Reaction 2 should behave similarly to Reaction 1, but Reaction 2 is described by the following skeletal (unbalanced) equation: Reaction 2: HCI(aq) + Na 2 CO 3 (s) NaCl(aq) + H 2 O(l) + CO 2 (g) In both reactions, you will be using an excess of hydrochloric acid in order to ensure that the carbonates react completely. In each case, the excess hydrochloric acid solution can be easily boiled away, leaving only solid sodium chloride in the reaction vessel. At the end of the experiment, you will measure the final mass of the product. Boiling away the excess acid solution and obtaining the dry salt requires more time than is normally available in one lab period so this experiment will be typically carried out over two days. It is interesting to note that Reaction 1 describes a way of neutralizing excess stomach acid (heartburn). Of course, this remedy should only be performed with the advice of a physician who would recommend proper substances and dosages. It is also interesting to note that such medically recommended dosages are determined through stoichiometry! OBJECTIVES 1. to observe the reactions between hydrochloric acid and sodium hydrogen carbonate, then hydrochloric acid and sodium carbonate 2. to predict the mass of sodium chloride that produced in each reaction 3. to measure the actual mass of sodium chloride produced in each reaction 4. to determine the percent yield of sodium chloride in each reaction SUPPLIES Equipment Chemical Reagents centigram balance 1.0M hydrochloric acid, HCl drying oven sodium hydrogen carbonate, NaHCO 3 2 evporating dishes sodium carbonate, Na 2 CO 3 beaker (250 ml) dropping pipet (1-3 ml) safety goggles lab apron 6B 1

2 PROCEDURE Part I: Reactions 1 and 2 (Day 1) 1. Put on your lab apron and safety goggles. 2. Obtain two clean, dry evaporating dishes, identify them with your name, and then number them 1 and 2. Use the centigram balance to measure the empty dishes masses. Record these values in your copy of Table 1 of Experimental Results in your notebook. Reaction 1: HCI and NaHCO 3 3. Place about 2.50 g of powdered sodium hydrogen carbonate in dish #1. Using too much powder will spoil your results. Find the combined mass of the dish and its contents and record this value. 4. Place the watch glass on top of the evaporating dish, curved side down. 5. Slowly add 6 M HCl from a dropper bottle to the NaHCO 3 in the dish, a few drops at a time, through the spout of the dish. Using a dropping pipet, slowly add a pipet full of the acid to the dish containing the sodium hydrogen carbonate. Carefully tilt the dish back & forth a couple of times to make sure that the acid has reacted with all of the NaHCO 3. Continue adding acid until the bubbling stops (you should add almost all of the HCl). Remove the watch glass and place it curved side up on the lab table. 6. Place the watch glass on top of the evaporating dish, curved side down. 7. Place the evaporating dish in the drying oven overnight. Reaction 2: HCI and Na 2 CO 3 8. Repeat Step 3 through 7, but this time use dish #2 with sodium carbonate. 9. Before leaving the laboratory, wash your hands thoroughly with soap and water. Part II: Measuring the Masses of NaCl Produced (Day 2) 1. Obtain your 2 evaporating dishes from the drying oven and a centigram balance. 2. What remains in both dishes is now simply the dry NaCl (s) (table salt). Measure and record, in your copy of Table 2, the masses of dishes containing the dry solid. 3. Wash out the dishes and put all equipment away. 4. Before leaving the laboratory, wash your hands thoroughly with soap and water. REAGENT DISPOSAL Rinse all solutions down the sink with copious amounts of water. Any solid waste should go into the designated waste container. POST LAB CONSIDERATIONS In each of the two reactions in this experiment, three products are formed: carbon dioxide, water, and sodium chloride. The only product that remains in the dish after heating is solid sodium chloride (table salt). Before attempting any stoichiometric calculations, you will need to determine the balanced equation for the reactions so that you can correctly predict the amounts of products that should have formed. Chemists are often interested in how well they did in manufacturing a certain chemical. One way of measuring this is to calculate the percent yield of that particular chemical by using this formula: Percent yield = (actual mass produced) x 100% (theoretical mass produced) 2

3 EXPERIMENTAL RESULTS Part I: Reactions 1 and 2 (Day 1) Table 1 Part I, Reaction 1: HCl and NaHCO 3 (Day 1) Part I, Reaction 2: HCl and Na 2 CO 3 (Day 1) Mass of dish #1 Mass of dish #2 Mass of dish #1 + NaHCO 3 Mass of dish #2 + Na 2 CO 3 Part II: Measuring the Masses of NaCl Produced (Day 2) Table 2 Part II, Measuring the Masses of NaCl Produced (Day 2) Reaction 1 Mass of dish #1 + dry solid NaCl (after the reaction) Reaction 2 Mass of dish #2 + dry solid NaCl (after the reaction) 3

4 Salt Lab Analysis Show all work!! Reaction 1 1.Copy the equation for the reaction of HCl and sodium hydrogen carbonate from the introduction to this lab in the space below. Double check: Is it balanced? 2. The dry sodium chloride was left in your evaporating dish, but what happened to the other two products? 3. a. What was the mass of NaHCO 3 that you reacted?(check your data) b. Using the correct molar mass from the periodic table, convert the grams of NaHCO 3 into moles. Show work!! 4.Now reexamine the equation for this reaction. a.what is the molar ratio of NaHCO 3 to NaCl? b.using this ratio, the answer to 3b above, and your common sense, how many moles of NaCl should theoretically have been produced? c.convert the moles of NaCl in answer b above into grams. 5.How many grams of NaCl were actually produced? 6.Use this formula to calculate the percent yield for the grams of salt produced in your lab: % Yield = actual grams of product x 100 theoretical grams of product Reaction 2 7.Copy the equation for the reaction of HCl and sodium carbonate from the introduction to this lab in the space below. Double check: Is it balanced? 8. The dry sodium chloride was left in your evaporating dish, but what happened to the other two products? 4

5 9. a. What was the mass of Na 2 CO 3 that you reacted?(check your data) b. Using the correct molar mass from the periodic table, convert the grams of Na 2 CO 3 into moles. Show work!! 10.Now reexamine the equation for this reaction. a.what is the molar ratio of Na 2 CO 3 to NaCl? b.using this ratio, the answer to 9b above, and your common sense, how many moles of NaCl should theoretically have been produced? c.convert the moles of NaCl in answer b above into grams. 11.How many grams of NaCl were actually produced? 12.Use this formula to calculate the percent yield for the grams of salt produced in your lab: % Yield = actual grams of product x 100 theoretical grams of product Follow up questions: 1.Suggest two reasons why your yield is not exactly 100%. That is, what are two possible sources of error in this lab? 2. If 6.0 grams of CaCO 3 reacts with HCl, what theoretical mass of carbon dioxide should be formed? Write the balanced equation first! 5