Laboratory Experiment pubs.acs.org/jchemeduc
Filling a Plastic Bag with Carbon Dioxide: A Student-Designed Guided-Inquiry Lab for Advanced Placement and College Chemistry Courses Laura M. Lanni* Department of Science and Mathematics, Newberry College, Newberry, South Carolina 29108, United States S Supporting Information *
ABSTRACT: A guided-inquiry lab, suitable for first-year general chemistry or high school advanced placement chemistry, is presented that uses only inexpensive, store-bought materials. The reaction of sodium bicarbonate (baking soda) with aqueous acetic acid (vinegar), under the constraint of the challenge to completely fill a sealable plastic bag with the gaseous carbon dioxide product, is presented to students of chemistry. Students must marshal their knowledge of non-STP gas stoichiometry, including the often misunderstood concept of limiting and excess reagents, to successfully complete the task. This contribution is part of a special issue on teaching introductory chemistry in the context of the advanced placement (AP) chemistry course redesign.
KEYWORDS: First-Year Undergraduate, General, High School, Introductory Chemistry, Physical Chemistry, Laboratory Instruction, Hands-On Learning, Manipulatives, Inquiry-Based/Discovery Learning, Acids, Bases, Gases, Stoichiometry
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Herein, a simple, inexpensive guided-inquiry experiment to produce and collect a predetermined volume of carbon dioxide gas is presented, along with suggestions for optimizing success and insight of what to expect from students. The experiment requires materials that can be purchased at the grocery store including vinegar, baking soda, and various sizes of sealable plastic bags. The chemistry concepts of non-STP gas stoichiometry involving a limiting reagent are investigated. This lab has been successfully performed as a whole-group challenge, in teams of 2−4 students, and individually. This flexibility is beneficial; every year, the lesson can be modified based on the abilities of the group.
hen students blindly follow prescribed procedures during a laboratory experiment, they seldom understand the reasoning behind each step. They simply follow along haphazardly, hoping to finish quickly. Often the purpose of the experiment is misunderstood, error is high, and learning is not optimized. An alternative approach involves charging students with the task of developing their own procedure. In doing so, they must consider not only what data to collect but how to best collect it to minimize error. In this approach, students are personally invested in their own success. Yet, this success is not achieved without some help. Guidance from the instructor in a guided discussion, either with the whole class or in groups, is invaluable. The reaction of sodium bicarbonate with acetic acid
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NaHCO3(s) + CH3COOH(aq)
Materials Required
→ CH3COONa(aq) + H 2O(l) + CO2 (g)
Safety goggles, baking soda, vinegar, sealable plastic bags (sandwich, quart, and half-gallon sizes all work well), graduated cylinders (up to 500 mL or 1 L, if available), spatulas, analytical balance with at least ±0.01 g precision, weighing paper, ample paper towels.
has been presented in numerous publications1−7 as a teaching tool because students enjoy reactions that produce gases. Various methods of collecting and measuring the volumes of gaseous products have been reported for use in teaching laboratory experiments.8−10 Use of a sealable plastic bag for collection of the gaseous carbon dioxide product of reaction from common cooking substances makes this reaction one that can be used in the laboratory, as a take-home assignment, and even as a home-school laboratory. © XXXX American Chemical Society and Division of Chemical Education, Inc.
EXPERIMENTAL SECTION
Special Issue: Advanced Placement (AP) Chemistry
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dx.doi.org/10.1021/ed400901x | J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Laboratory Experiment
Time Required
A prelab discussion required approximately 30 min. Performance of the experiment 3 to 5 times required 60−90 min. The time can easily be broken up over several days as it is simple to clean up and restart the experiment. Demonstration and Discussion To Introduce the Experiment
To introduce the lab, 20−30 min of the previous day was used to demonstrate the reaction between baking soda and vinegar in a test tube. A small scoop of baking soda was added to a test tube and then vinegar was added to about one-third of the volume of the test tube. The test tube was shaken or stirred and bubbles were evident. The students were asked the following questions, and the answers were discussed: (1) What are the gas bubbles? (2) What are the chemical identities (names and formulas) for baking soda and vinegar? (3) What is the balanced equation that just occurred in the test tube? (4) What volume of carbon dioxide gas would be produced from 1.00 g of baking soda with excess vinegar at STP (273.15 K and 1 atm)? (5) Is this room at STP? After the discussion, plastic bags were shown to the students and the instructor posed this challenge: Tomorrow in lab, you will perform an experiment, with a procedure that you designed yourself, to completely fill a plastic bag with carbon dioxide gas formed from the reaction of baking soda and vinegar.
Figure 1. Volume of water needed to fill the sealable plastic bag will often be larger than expected.
enough to affect the outcome of the experiment? A student who pushed past his or her mistake tended to learn by experimentation whether the spillage was, in fact, significant. The evidence supplied by multiple unfilled bags of gas eventually led all students to reexamine their procedure steps. Students learned that repeating the process without careful consideration of the source of error cost them time and energy. This is an important lesson during the training stage of any job. The three stages of the reaction are shown in Figure 2: introduction of the vinegar and baking soda, mixing of reagents
Prelab Requirement
To begin the lab, each student submitted a written procedure that included a list of required materials and lab equipment and all of the sequential steps to be performed. Students who submitted procedures that were deficient in material list or other detail were still allowed to carry on with their proposed experiments. They learned from their mistakes. During their attempts in the experiment, they were learning, revising, and in almost all cases, actually caring about the final result; they became invested in their own success. Students were able to revise their procedure after each failure, but the revision had to be written down before each subsequent attempt was allowed.
Figure 2. Expected sequence: (A) isolated baking soda and vinegar prior to reacting, (B) reaction beginning as noted by bubbling, and (C) full bag of carbon dioxide.
and bubbling, and full bag of carbon dioxide and excess reagent upon completion of reaction. The bag must be completely dried after pouring out the water or the baking soda will stick to it. The baking soda must be isolated to one corner of the bag and the vinegar poured to the other corner prior to sealing the bag and allowing the reaction to begin (Figure 2A). If the baking soda and vinegar were allowed to react prior to sealing the bag, a partially filled, wimpy bag resulted, due to escaped carbon dioxide product. It was also necessary to use the same bag for the reaction that was filled with water, as not all sealable plastic bags are exactly the same size. These are points of advice that instructors may dole out as needed. Some groups of students discovered these nuances on their own, whereas others needed more guidance. Some student produced an overfilled bag that burst or broke the seal, the opposite of wimpy bag. This scenario was most often encountered due to erroneous stoichiometry calculations. Often students in this situation would overfill two bags, that is, they failed twice, before they recognized their error or even considered that their calculations were at fault. The typical first response to a popped bag was an attempt to modify the procedure. Students were required to record data for every attempt to fill the bag with carbon dioxide.
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HAZARDS There are no significant hazards in this experiment. RESULTS AND DISCUSSION Students tried many methods for finding the volume of the bag. Though the best way was to fill the plastic bag with water and measure the volume of the water, as shown in Figure 1, some students measured the length and width of the bag with a ruler and calculated its area, thinking that this was the same as the volume. A geometry lesson ensued when they failed to convert cm2 to liters. If the dimension error did not stop them, most often, when they tried to perform the reaction, they produced a “wimpy bag”, one incompletely filled with carbon dioxide gas, and were required to start over. It is important to completely fill the bag with water, no air bubbles, to accurately find the volume of the bag. Students who spilled some water were allowed to decide whether to note the spillage in their procedure writeup or to start again. This prompted thinking about error analysis: was this spill significant B
dx.doi.org/10.1021/ed400901x | J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Laboratory Experiment
stoichiometry calculations can be repeated at the measured temperature and pressure of the room, and the procedure followed at least one more time. In all, each student, or group of students, performed the experiment 4−5 times and became more proficient each time, just like a scientist in the laboratory. An example of a good student procedure is show in Box 1.
Observant students noticed that the bag became cold during the reaction, which prompted discussion about comparisons of exothermic and endothermic processes. This also led to the important realization that the gaseous products were not at STP. Some students realized that the bag should be allowed time to warm to room temperature. If desired, the
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SUMMARY Inquiry laboratories require extra effort from both the instructor and the students. They take more time and energy than cookbook procedures. But they also, most often, provide the greatest intellectual reward. When the students are invested in solving a problem, when performing a task is enjoyable and challenging, and when they are internally motivated to succeed, their growth, learning, and mastery of concepts are maximized. As educators, these are the moments when we most enjoy our role in student learning. These are the moments for which we first chose to serve as educators, for moments like these when our students are learning, and even enjoying the challenge of learning, right before our eyes.
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freshman chemistry, without whom her life would be less colorful, her clothes drier, and the insights presented here nonexistent.
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ASSOCIATED CONTENT
* Supporting Information S
A teacher-preparation list, including materials and a sample procedure. This material is available via the Internet at http:// pubs.acs.org.
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REFERENCES
(1) Whitmer, J. C. Kitchen Chemistry. J. Chem. Educ. 1975, 52 (10), 665. (2) Carlson, G. L. A New Approach to the Baking Soda-Vinegar Reaction. J. Chem. Educ. 1990, 67 (7), 597. (3) Ed. Staff.. How Big is the Balloon? Stoichiometry Using Baking Soda and Vinegar. J. Chem. Educ. 1997, 74 (11), 1328A−1328B. (4) Duffy, D. Q.; Shaw, S. A.; Bare, W. D.; Goldsby, K. A. More Chemistry in a Bottle: A Conservation of Mass Activity. J. Chem. Educ. 1995, 72 (8), 734−736. (5) Rohrig, B. Fizzy Drinks: Stoichiometry You Can Taste. J. Chem. Educ. 2000, 77 (12), 1608A−1608B. (6) Antony, E.; Mitchell, L.; Nettenstrom, L. When A + B ≠ B + A. J. Chem. Educ. 2000, 77 (9), 1180−1181. (7) Artdej, R.; Thongpanchang, T. A Dramatic Classroom Demonstration of Limiting Reagent Using the Vinegar and Sodium Hydrogen Carbonate Reaction. J. Chem. Educ. 2008, 85 (10), 1382− 1384. (8) Thorsen, K. Chem 13 News: Ways to Teach in the Classroom and Beyond. J. Chem. Educ. 2000, 77 (7), 824. (9) Criswell, B.; Bennett, C.; Bevsek, H. M. Two “Gas-in-a-Bag” Reactions to Show the Predictive Power of the Relative Acid-Base Strength Chart. J. Chem. Educ. 2006, 83 (8), 1167−1169. (10) Mattson, B.; Hoette, T. Incomplete Combustion of Hydrogen: Trapping a Reaction Intermediate. J. Chem. Educ. 2007, 84 (10), 1668−1670.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The author declares no competing financial interest.
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ACKNOWLEDGMENTS The author wishes to thank her hundreds of students, from two decades of teaching Advanced Placement and college-level C
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