Lab #9 Stoichiometric Relationships & Theoretical Yield

Transcription

1 Stoichiometric Relationships & Theoretical Yield Name: Nov. 28, 2016 Background and Purpose: Carbonates when reacted with acids will produce carbon dioxide, water and an ionic salt. In this lab, we will perform this reaction of sodium bicarbonate with sulfuric acid and see how close our actual product yield comes to the theoretical yield. Remember that the theoretical yield is the maximum amount of product that could be produced according to stoichiometric relationships. In contrast, due to incomplete reactions, the amount of product that actually forms is known as the actual yield. We will be using a measured amount of sodium bicarbonate as the limiting reagent and thus sulfuric acid will be the reactant in excess, meaning that we have enough to react with all of the sodium bicarbonate plus extra. This will ensure that we have the best chance at maximizing a complete reaction. You will use stoichiometry (unit conversions & dimensional analysis) to calculate your theoretical yield of carbon dioxide gas and use lab techniques to determine the actual yield of carbon dioxide gas after doing the experiment (two times). These two values will allow you to calculate your % yield for each of the 2 trials. Percent Yield is a ratio of your actual yield over your theoretical yield. Start by balancing the chemical equation below. This will be necessary for your theoretical yield calculations. NaHCO3(s) + H2SO4(aq) CO2(g) + H2O (l) + Na2SO4(aq) Equipment: 100mL or 140mL beaker plastic pipette spatula Triple beam balance watch glass Materials: 1.0M H 2SO 4 NaHCO 3 sodium bicarbonate (baking soda) Procedure: 1. Get a clean 100mL or 140mL beaker and watch glass. You will be doing 2 trials of this lab. Complete the first trial all the way through step 9 before repeating for trial Record the mass of the beaker as well as the watch glass. 3. Measure out 0.200g 0.225g of baking soda (NaHCO 3) to the 100mL beaker. Use the scale to determine exactly how much baking soda is added. This is a VERY SMALL amount of baking soda. 4. Record the mass of the beaker with the baking soda. 5. Add the watch glass onto the beaker and record the mass. Find the mass of a plastic pipette. 6. Fill a pipette with 1.0M H 2SO 4. This is sulfuric acid, which is CORROSIVE to body tissue, especially eyes. Please use caution, keep your goggles on, and wash your skin immediately if contact 1

2 is made with this solution. Find the mass of the pipette with sulfuric acid, beaker with baking soda, and watch glass. This is the total mass of the reactants. 7. Do the Reaction: Make sure that most of the baking soda in pushed to one side of the beaker, making it easier to react later on. Put the watch glass on the 100mL beaker as a safety measure to prevent bubbling and splashing of the acid onto the table or your skin. Insert the pipette tip through the hole under the watch glass and begin adding H 2SO 4 drop by drop. You do not need to count the drops since we are considering this to be the excess reagent. Continue adding drops until no more bubbles are produced, signifying the end of the reaction (and that all of the baking soda has completely reacted). 8. After the Reaction: Open the watch glass briefly to allow CO 2 gas to escape. Now reweigh all the equipment and the left over products. Find the mass of the beaker, watch glass, pipette, and product. This will be referred to as the total mass of products. 9. Rinse your beaker contents down the sink, wash the beaker and watch glass. Dry the beaker and watch glass. 10. REPEAT steps 3 through 8 for a second trial. I would like for your initial mass of baking soda to be slightly different than your first trial. That way, you will have a new calculation of theoretical yield for each trial and be testing slight variations. Data Table 1: Limiting Reagent Amounts Trial 1 Trial 2 Mass of beaker Mass of beaker with watch glass Mass of beaker with baking soda Mass of beaker w/ baking soda and watch glass Mass of empty pipette Mass of pipette with 1.0 M H2SO4 11. Subtract the total mass after the reaction from the total mass before the reaction to come up with the amount of CO 2 gas that was produced and lost to the air in this reaction. This is your actual yield or experimental yield. Data Table 2: Mass of Reactants and Mass of Products Trial 1 Total Mass of Reactants & Materials Total Mass of Products & Materials Difference in Mass Due to Loss of CO2 gas produced Trial 2 This column will be your Actual Yield

3 Data Analysis: 12. Now use stoichiometry to calculate the mass of CO 2 gas that should be produced during this reaction, the theoretical yield. Use the mass of baking soda as your given, since this is the limiting reagent. Show your work in the Data Analysis section below. Calculations of Theoretical Yield of CO2 gas in grams Trial 1: Trial 2: 13. Use your theoretical yield (accepted value) and your actual yield (your experimental value) to calculate your percent yield and percent error for each of the two trials. Calculate Percent Yield (% yield) and % error for each of the two trials above. observed value observed value true value % yield 100 % error 100 theoretical value true value Trial 1 Trial 2 Analysis Questions: 1. Discuss your percent yields in this experiment?

4 2. The compound, C 6H 16N 2, is one of the starting materials in the production of nylon. It can be prepared from the following reaction involving adipic acid, C 6H 10O 4: C 6 H 10 O 4 (l) + 2NH 3 (g) + 4H 2 (g) C 6 H 16 N 2 (l) + 4H 2 O(l) What is the percentage yield of C 6H 16N 2 if g of adipic acid (C 6H 10O 4) results in the production of g of C 6H 16N 2? 3. Plants convert carbon dioxide to oxygen during photosynthesis according to the following equation: CO 2 + H 2 O C 6 H 12 O 6 + O 2 Balance this equation, and calculate how much oxygen would be produced if g of carbon dioxide reacts with a percentage yield of 63.4%. 4. Lime, CaO, is frequently added to streams and lakes which have been polluted by acid rain. The calcium oxide reacts with the water to form a base that can neutralize the acid as shown in the following reaction: CaO(s) + H2O(l) Ca(OH)2(s) If mol of lime are needed to neutralize the acid in a lake, and the above reaction has a percentage yield of 54.3%, what is the mass, in kilograms, of base produced?

5 5. Carbon tetrachloride, CCl 4, is a solvent that was once used in large quantities in dry cleaning. Because it is a dense liquid that does not burn, it was also used in fire extinguishers. Unfortunately, its use was discontinued because it was found to be a carcinogen. It was manufactured by the following reaction: CS2 + 3Cl2 CCl4 + S2Cl2 The reaction was economical because the byproduct disulfur dichloride, S 2Cl 2, could be used by industry in the manufacture of rubber products and other materials. If 67.5 g of Cl 2 are used in the reaction and 39.5 g of S 2Cl 2 is produced, what is the percentage yield? Conclusion: State your experimental (observed) yield, the theoretical yield, your percent error, and percent yield for each trial.