Stoichiometry Analysis: Measuring CO2 Production in Antacid Reaction, Study notes of Stoichiometry

Download Stoichiometry Analysis: Measuring CO2 Production in Antacid Reaction and more Study notes Stoichiometry in PDF only on Docsity! Bellevue College CHEM& 121 Page 1 of 8 Experiment: Stoichiometric Analysis of an Antacid1 Introduction In this lab, you will use the concept of stoichiometry to solve two sequential problems. First, you will try to determine the products of a certain reaction (below), choosing between three possibilities. Then, you’ll use your results of this first part to determine the amount of sodium bicarbonate in a common household substance. CH3COOH (aq) + NaHCO3(s) → CO2(g) + ??? You’ve probably seen this reaction in elementary school -- add a few drops of red food coloring, and you have the classic “volcano reaction.” Or, you can perform it easily in your kitchen by mixing vinegar (dilute acetic acid) and baking soda (sodium bicarbonate). The most noticeable sign of the reaction is vigorous bubbling, a result of very rapid carbon dioxide generation. Gaseous carbon dioxide is one of the products, as you can see with your own eyes. (You can prove the gas to be carbon dioxide by collecting it in a flask, and inserting a burning match into the flask. The flame will be immediately extinguished.) Aside from carbon dioxide, what else is produced by the reaction? A chemist approaching this problem would most likely form some hypotheses about the other products, and then design experiments to evaluate which hypothesis is best supported by experimental evidence. For this experiment, we’ll supply three possible reactions, shown below. Notice that they are all balanced equations. A. CH3COOH (aq) + NaHCO3(s) → 2 CO2(g) + CH2O(aq) + Na+(aq) + 3H+(aq) B. CH3COOH (aq) + NaHCO3(s) → CO2(g) + H2O(l) + CH3COO-Na+ (aq) C. CH3COOH (aq) + 2 NaHCO3(s) → CO2(g) + Na2CO3(aq) + H2O(l) + 2 CH2O(aq) Your job is to determine which of these three possibilities is correct, using some simple laboratory measurements. Focus on the differences between the three proposals: Reactions A and C produce formaldehyde (CH2O), but Reaction B doesn’t. The products of Reaction A are acidic (H+ is produced); those of B and C are basic (CH3COO-Na+ and Na2CO3 are produced). These things could be tested, but an even simpler method would be take advantage of the different amounts of carbon dioxide produced, relative to sodium bicarbonate: In Reaction A, 1 mole NaHCO3 produces 2 moles CO2 In Reaction B, 1 mole NaHCO3 produces 1 mole CO2 In Reaction C, 2 mole NaHCO3 produces 1 mole CO2 You will measure the ratio of moles NaHCO3 used to moles of CO2 produced, and if it is approximately 1:2, you may conclude that Reaction A is correct; if the ratio is around 1:1, you can bet that Reaction B is correct, and if it’s about 2:1, you should choose Reaction C. 1 Used with permission from Seattle Central Community College, 2011 Bellevue College CHEM& 121 Page 2 of 8 Keep in mind that your results may not give you exact whole number mole-to-mole ratios because of basic experimental errors. Your results may be off by as much as 20% for this experiment, but you will still be able to choose between the three reactions (A, B, or C) with a fair amount of confidence if you work carefully and collect good data. Determining the moles of NaHCO3 is easy: Use the measured mass you scoop out of the container to use. (The other reactant, acetic acid will be used in excess, so its exact amount will have no relationship to the amount of carbon dioxide generated.) Determining the moles of CO2 is less straightforward; it’s not so simple to collect and measure the mass of a gaseous substance, as you can imagine. In each of the three reactions above, carbon dioxide is the only gas, and all other reactants and product are liquids, solids, or aqueous. As the reaction occurs, carbon dioxide will bubble out of the reaction solution and escape into the laboratory. Therefore, the mass of your reaction mixture after the reaction will be lighter due to the loss of carbon dioxide, and a simple subtraction tells you how much carbon dioxide was produced: ⎟ ⎠ ⎞⎜ ⎝ ⎛⎟ ⎠ ⎞⎜ ⎝ ⎛= reactionafter mixturereaction of mass reaction before mixturereaction of massCO of Mass -2 One small complication is that some of the CO2 produced will remain dissolved in the reaction mixture because carbon dioxide is somewhat soluble in water. This means that the mass you calculate by subtraction in the above equation is somewhat too low—i.e., you have not accounted for the carbon dioxide that goes into the water. You will account for this with a correction factor in your calculations. Once you have chosen the correct reaction between acetic acid and sodium bicarbonate, you can use it to measure the amount of sodium bicarbonate in Alka-Seltzer tablets using a similar methodology. In this case, the mass of sodium bicarbonate will be an unknown. You can measure the amount of CO2 produced as you did before, and use the mole-to-mole ratio of the chosen reaction to calculate the number of moles and the mass of sodium bicarbonate which reacted. Finally, you will determine your experimental error by comparing your experimentally determined mass of sodium bicarbonate present with what the manufacturer reported on the package of Alka-Seltzer. Objectives In this experiment, you will ü Determine the stoichiometry of a reaction experimentally. ü Weigh by difference a reaction mixture before and after the reaction in order to find the mass of a gas produced. ü Practice molar mass and mole ratio calculations. ü Calculate a percent error and determine how an inaccuracy in a specific measurement affects the outcome. Hazards M Even though the reagents in this lab are fairly safe, please wear safety goggles and dispose of waste in the labeled waste container. Bellevue College CHEM& 121 Page 5 of 8 Report Name _____________________Section______ Stoichiometric Analysis of an Antacid Lab Partner ____________________________ Data, Calculations, and Results Part A: Reaction Stoichiometry Trial 1 Trial 2 Average Mass of NaHCO3 Mass of beaker + acetic acid + NaHCO3 + watch glass before reaction Mass of beaker + reaction mixture + watch glass after reaction Mass of CO2 gas released Moles of CO2 gas released Moles of CO2, corrected for amount dissolved* (see below) Moles of NaHCO3 used NOTE: *Calculate by adding 0.0040 moles to the “Moles of CO2 gas released”. This correction accounts for the amount of CO2 that dissolves in 50 mL of aqueous solution. Calculate (and show your work for at least trial #1) for the following: 1. The number of moles of CO2 for Trial #1, 2, and 3. 2. The number of moles of CO2, corrected, for Trial #1, 2, and 3. 3. The number of moles of NaHCO3 for Trial #1, 2, and 3. Bellevue College CHEM& 121 Page 6 of 8 4. Compare the calculated number of moles of CO2 and NaHCO3 in the table above. Which of the three possible reactions, A, B or C, is best supported by these results? Write out the balanced equation you chose, and explain your reasoning. (Note that results may not be exactly whole numbers.) Part B: Analysis of an Antacid Tablet 1 Tablet 2 Mass of beaker + acetic acid + tablet + watch glass before reaction Mass of beaker + reaction mixture + watch glass after reaction Mass of CO2 gas released Moles of CO2 gas released Moles of CO2, corrected for amount dissolved* (see note below) Moles of NaHCO3** (see note below) Mass of NaHCO3 Average Mass of NaHCO3 for two tablets *Calculate by adding 0.0040 moles to the “Moles of CO2 gas released”. **Use the balanced chemical equation from Part A for the mole-to-mole ratio to convert moles of CO2 (corrected) to moles of NaHCO3. Calculate the following (and show your work for at least trial #1): 1. The number of moles of CO2 gas released by the tablet. Bellevue College CHEM& 121 Page 7 of 8 2. The corrected moles of CO2 released by the tablet. 3. Show the calculation for the moles of NaHCO3, using the mole ratio in the balanced chemical equation determined from Part A. 4. Show the calculation for the mass of NaHCO3 for the tablet. 5. Show the calculation for the average mass of NaHCO3 for the two tablets. 6. The mass of sodium bicarbonate in each tablet of Alka-Seltzer is reported as 1916 mg on the package (which we will call the “actual value”). Using your experimental value for the average mass of NaHCO3, calculate the percent error. Show your work. 100% valueactual | valuealexperiment - valueactual | error % ×= 7. In Part B, suppose the tablet was mostly dissolved when some of your solution splashed out of the beaker as CO2 continued to evolve. How would this affect the perceived mass of CO2? Would your final calculated mass of sodium bicarbonate in the tablet be artificially high or artificially low as a result of this splashing? Choose one, and explain why.