Demonstration Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX
pubs.acs.org/jchemeduc
Exploring the Mysterious Substances, X and Y: Challenging Students’ Thinking on Acid−Base Chemistry and Chemical Equilibrium Ingo Eilks,*,† Ozcan Gulacar,‡ and Jose Sandoval‡ †
Department of Biology and Chemistry, University of Bremen, Bremen 26359, Germany Department of Chemistry, University of California−Davis, Davis, California 95616 United States
‡
ABSTRACT: Acid−base chemistry tends to be one of the more challenging concepts for students to master in the high school and undergraduate chemistry curriculum. Mastery of acid−base chemistry requires a concrete understanding of acid−base theories, chemical equilibrium, electronegativity, periodic trends, and the ability to conceptualize intricate processes at a molecular level. Because of the complex nature of the topic, a combination of effective teaching methods should be used. This paper presents an intriguing acid− base demonstration coupled with a detective story that was designed to aid instructors in engaging students while promoting a conceptual understanding of acids and bases. The selected compounds, X and Y, bring additional value and mystery to the demonstration. X and Y are types of faujasite zeolites, three-dimensional crystalline compounds with various useful chemical properties. Although zeolites are used widely in technology and in everyday products such as laundry detergents, their use in the general chemistry curriculum is surprisingly low. This demonstration provides instructors with the opportunity to cover this important group of compounds and elaborate on the behavior of solid state acids and bases at the same time. As a bonus, this demonstration can also be used to revisit Le Châtelier’s principle of chemical equilibrium in a novel context. KEYWORDS: Demonstrations, Acids/Bases, Equilibrium, First-Year Undergraduate/General, High School/Introductory Chemistry
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INTRODUCTION General chemistry courses are some of the major challenges that all STEM majors must face at an early stage of their education.1 These courses introduce a variety of complex concepts that build on each other and require students to apply these concepts to a wide range of multistep problems. Since chemistry is a fundamental course for STEM majors in college and a common high school graduation requirement, chemistry instructors have developed and evaluated different methods of presenting and explaining these rigorous concepts. This paper focuses on the concept of acids and bases.2−4 Acid−base chemistry is one of the many challenging topics covered in advanced high school courses and in general chemistry. As a result, many students fail to develop a conceptual understanding of it.3,5 The mastery of acid−base chemistry is pertinent for chemistry learners because it provides a foundational link among general chemistry, organic chemistry, and other subsequent topics.2,3,5 Due to its essential nature, acid− base chemistry has garnered the efforts of many to develop more effective demonstrations for presenting the material so that students may gain a better understanding. The methods that center on the constructivist theory and focus on providing authentic learning experiences are suggested to be most successful in promoting a meaningful understanding of acids and bases. The success of these methods is mainly attributed to their dynamic approach to learning. Their claim is that learning is mostly a personal endeavor, © XXXX American Chemical Society and Division of Chemical Education, Inc.
influenced as much by reflection and metacognition as it is by what we have experienced. It is influenced by our understanding of the world as well as our own experiences and thoughts as we are presented with something new and challenging.6 In this paper, a focus is placed on the Brønsted−Lowry theory of acid−base chemistry, which defines an acid as a proton donor and a base as a proton acceptor. Due to the difficult nature of the concept and in terms of constructivist learning theory, educators suggest implementing more efficient methods of introducing and explaining this concept. Ideally, this means providing students with hands-on and minds-on experiences. However, due to classroom size, time limitations, and the cost of materials, hands-on activities are often replaced with demonstrations to supplement student understanding of the underlying principles. Demonstrations are utilized in many learning environments and formats. In an effort to increase the benefit of the demonstration and claim students’ attention, a captivating or mysterious story may be coupled with the presentation.7 While the story does not have to be a mystery, it should be geared to the interests of the students, either with respect to their age group, or to their community. Students who are confronted with interesting stories or seemingly mysterious phenomena Received: June 11, 2017 Revised: February 12, 2018
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DOI: 10.1021/acs.jchemed.7b00404 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Demonstration
Figure 1. Zeolite NaX in water with thymol blue (a), and after salt is added and the mixture stirred (b). Zeolite HY in water (c), and after salt is added and stirred (d).
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EQUIPMENT AND CHEMICALS The following materials are needed for the demonstration: test tubes (∼25 mm × 150 mm) or small beakers, glass rod, sodium-form zeolite in powder form (e.g., NaX, one can use powdered molecular sieve 13X), hydrogen-form zeolite (e.g., HY, if not available it can be made from sodium-type faujasite zeolites by ion exchange with acids and later calcination), deionized water, thymol blue (w/v 0.2% in ethanol), or any other indicators to indicate acidic, neutral, and basic environments.
will more likely engage in inquiry-based learning and make sense of the phenomena more successfully and readily.7
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BACKGROUND The demonstration described in this paper is about two physically and chemically similar substances: sodium-form and hydrogen-form faujasite zeolites, which show both basic and acidic characteristics. Zeolites are crystalline compounds commonly formed with aluminate (AlO4) and silicate (SiO4) in three-dimensional structures. Their properties include a high degree of hydration, large void volumes when hydrated, low density, and the ability to absorb gases and vapors.8 The two zeolites used in this demonstration are X- and Y-type faujasite zeolites; the name depends on the silica-to-alumina ratio of their framework. X zeolites have such a ratio between 2 and 3, while in Y zeolites this ratio is 3 or higher. Generally, zeolites can act as ion-exchangers, making them interesting for many technical processes or use in everyday objects, such as laundry detergents.9 Hydrogen-form zeolites are also frequently used as heterogeneous acid catalysts in both laboratory and technical applications.10 Many other Na- or H-doped zeolites can be used for demonstration as well; zeolites NaX and HY were used in this demonstration because “X” and “Y” are typically used as placeholder names in mystery stories. As previously mentioned, zeolites are normally formed with either AlO4 or SiO4; the tetrahedron formed by the four oxygen anions surrounding the central atom determines many chemical properties of zeolites. Silicon has an oxidation state of four, and when it is surrounded with the four oxygen atoms, it produces an electrically neutral knot in the structure. However, aluminum has an oxidation state of 3; this produces an anionic spot in the zeolite framework. To establish electrical neutrality, metal ions or protons are bonded to the zeolite matrix in the channels within tetrahedron structures. If the cations are protons, they provide the zeolite with strong acidic characteristics.9 In case of the introduction of certain metal ions into the framework, zeolites can also catalyze oxidation and reduction reactions. However, the chemistry of zeolites is not commonly included in high school or general chemistry textbooks and courses. While there is a great emphasis on the chemistry of commonly known acids and bases and their solutions, solid state acids and bases, such as zeolites, are neglected. Therefore, a demonstration involving these substances could be effective to challenge the learners’ thinking about acid−base chemistry and the corresponding equilibria, forcing them to view the concept from a different perspective.11
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PROCEDURE AND OBSERVATIONS The following protocols describe each demonstration with its associated steps. See Figure 1 for an example of the color changes after the addition of sodium chloride. Part A: Demonstrating Some Properties of Compound X
To demonstrate some visible properties of compound X, follow these instructions. 1. Put 1 g of substance X (a sodium-form zeolite, e.g., NaX; one can use powdered molecular sieve 13X) in a test tube or beaker. 2. Add 20 mL of an aqueous solution of the thymol blue solution (water with a few drops of the indicator solution; one can also use phenolphthalein or other indicators to identify basic solutions). One can see that at the surface of substance X the color starts changing to blue, indicating a basic environment of pH above 8.9. 3. Stir well, and the whole suspension becomes blue. Wait a few seconds for the zeolites to form a sediment. 4. Add 1 g of sodium chloride. After stirring, the whole suspension loses the blue color, indicating a more or less pH neutral environment of pH below 8.9. Part B: Demonstrating Some Properties of Compound Y
To demonstrate some visible properties of compound Y, follow these instructions. 1. Put 1 g of substance Y (a hydrogen-form zeolite, e.g., HY) in a test tube or beaker. 2. Add 20 mL of an aqueous solution of thymol blue. No change can be observed. 3. Add 1 g of sodium chloride. At the surface of the substance, the color starts changing to red. After stirring, the whole suspension turns into red, indicating an acidic environment of pH below 1.7. B
DOI: 10.1021/acs.jchemed.7b00404 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
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Demonstration
HAZARDS The zeolites and sodium chloride are both irritants; they may cause irritation if they make contact with eyes or skin. The dilute acidic and basic solutions can also be corrosive.
After further interviews, several accounts confirmed that the victim was a chemical researcher. Could that be a lead? The police were stumped, so they decided to follow through with the test tubes. Would you like to learn how my colleague helped solve the case? Here are the samples of the substances found. I will show you part of the chemical analyses my colleague did to shed light on the case.
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DISCUSSION The sodium-type zeolite reacts with water to form an alkaline solution as
After the demonstration, students are to be split into small groups to discuss what they have seen. In this setting, the instructor should encourage students to think of chemical concepts that they have learned and determine which concept of chemistry is being demonstrated. In addition, they should aim to explain the demonstration. After students share their initial predictions, the instructor may decide to provide the students with background information about zeolites and their behavior as ion-exchangers (the information given in the background section of this paper) with the purpose of aiding the formulation of potential reaction equations. At the instructor’s discretion, he or she may finish the demonstration and conclude with the following remarks regarding the mystery storyline: As the substances failed the drug tests, my colleague decided to go back to the basics and keep it simple. The chemical analysis showed that the substances were made up of silicon, aluminum, and a heavy concentration of oxygen. He decided to test for acidic and basic properties and found what you all just witnessed. He revealed the substances to be zeolites, solids that can have both acidic and basic properties. Zeolites are ion-exchangers, making them suitable for everyday objects, such as water filters and laundry detergents. It turns out, the murdered chemical researcher was on the brink of a breakthrough, one that, if used correctly, would make a revolution in the laundry detergent market to remove calcium and magnesium ions from water and to allow the replace of environmentally unfriendly phosphates from detergents. Zeolites can also filter out metals such as copper, cadmium, and zinc from water. They could also take out the chlorine, which would improve the taste and odor of water. In the 1980s, these new filters would revolutionize the water market. With this knowledge, police focused their search on the water companies, and by following this loose end, were able to capture the victim’s attackers.
NaX(s) + H 2O(l) ⇌ XH(aq) + OH−(aq) + Na +(aq)
A basic medium is indicated by the acid−base indicator. Adding NaCl raises the concentration of Na+ ions and directs the equilibrium toward the side of NaX and H2O, following Le Châtelier’s principle. The solutions are partially neutralized. The hydrogen-form zeolite hardly shows any effect in deionized water because there are nearly no ions available to exchange with hydrogen atoms bonded to the zeolite matrix. When NaCl is added, hydrogen ions are released from the matrix, forming H3O+ ions in the solution, as shown by the acid−base indicator: HY(s) + NaCl(s) + H 2O(l)
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⇌ NaY(aq) + H3O+(aq) + Cl−(aq)
INTRODUCING THE DEMONSTRATION WITH A STORY ON THE MYSTERIOUS SUBSTANCES X AND Y This demonstration focuses on the properties of solid state acids to create a visual aid that describes the relationship between acids and bases in a chemical reaction. The goal is for students to be able to analyze an observed physical phenomenon and relate it to previous knowledge of acid−base chemistry and Le Châtelier’s principle. In addition, students should be able to understand the meaning of Brønsted−Lowry acids and bases in terms of zeolites. More specifically, students should categorize the zeolites as Brønsted−Lowry acids or bases. In order to better engage students in the demonstration, it is recommended that instructors develop a corresponding storyline, one that sparks students’ curiosity and aids in the learning process.7 The following is an example of a storyline that an instructor may present along with the demonstration. The professor may begin: In the early 1980s, a colleague of mine received a call from the FBI about an ongoing investigation of a 38-year-old Caucasian male. They required an expert opinion on the case from a chemist’s point of view. Some background: The victim was found dead near one of the popular walking trails in Central Park by an elderly couple on their morning walk. The couple frantically called the police, hoping that justice could be found for this young man. The couple recognized the male. He was a regular visitor to the park. He always greeted everyone as he jogged by in his fluorescent athletic clothes and was not overlooked by anyone in the park. The crime scene investigators quickly evacuated the premises to avoid any tampering of evidence or contamination of the crime scene. Investigators were quick to note the blunt force trauma the victim received to the head and chest; however, no weapons were found at or near the crime scene. In the victim’s pocket, the police found two test tubes stashed in an unmarked envelope. Both contained white powders: one labeled X and the other Y. The police considered the possibility that they were drugs; however, the substances did not react to the typical drug tests performed at the scene.
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EXPERIENCES This demonstration was used in several undergraduate teacher education courses for chemistry teachers in Germany, as well as in teachers’ continuous professional development seminars. The implementation technique was inspired by the 5E-model for scientific inquiry learning (engage, explore, explain, elaborate, evaluate)12 to promote inquisitive learning by using the captivating effect of mysteries.7,13 The demonstration is recommended to engage students with the exploration and the explanation of the phenomenon. The introduction to zeolites and their use in laundry detergents was meant to extend students’ conceptual understanding of the issue and allow them to practice skills and behaviors. In the final stage, students are encouraged to evaluate the use of zeolites in different industrial applications, and the teacher has a chance to assess students’ understanding of the topic. In almost all cases, the phenomenon that a solid and insoluble substance behaves as an acid or base was very surprising even to experienced teachers. This uncommon experience first C
DOI: 10.1021/acs.jchemed.7b00404 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
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led to predictions and then to intense discussions among the audience. The students or teachers rarely reached the conclusion that this was an experiment with zeolites. Without any knowledge about the basic nature of the zeolites, the learners first started to explore the more familiar parts of the demonstration, such as the changes in pH values as indicated by the acid− base indicator. In their sketches of potential chemical equations, the participants started to analyze the demonstration step by step. For most of them, it became clear quite soon that these were questions dealing with acid−base chemistry. We collected the sketches the participants did during the group work. Because most participants did not have any idea about zeolites as being the substances X and Y, they started creating reaction equations based on the appearance and disappearance of H+ and OH− ions. When motivated to use X− and Y− as placeholders for the acid anions, most groups of participants were able to write down meaningful reaction equations. Using cooperative work in small groups, the learners in most cases were able to set up the explanation by the equilibrium equations for both demonstrations. Both students and teachers were surprised when told that there are substances in chemistry that have the names “X” and “Y”. Finally, as an extension of the 5E-model for scientific inquiry learning,12 information about industrial use of zeolites as ionexchangers in laundry detergents or acidic catalysts in chemical reactions was provided.
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CONCLUSIONS AND IMPLICATIONS
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AUTHOR INFORMATION
Demonstration
REFERENCES
(1) Cook, E.; Kennedy, E.; McGuire, S. Y. Effect of Teaching Metacognitive Learning Strategies on Performance in General Chemistry Courses. J. Chem. Educ. 2013, 90 (8), 961−967. (2) Silverstein, T. P. Weak vs Strong Acids and Bases: The Football Analogy. J. Chem. Educ. 2000, 77, 849−850. (3) Romine, W. L.; Todd, A. N.; Clark, T. B. How Do Undergraduate Students Conceptualize Acid−Base Chemistry? Measurement of a Concept Progression. Sci. Educ. 2016, 100 (6), 1150−1183. (4) Nakhleh, M. B.; Krajcik, J. S. Influence of Levels of Information as Presented by Different Technologies on Students’ Understanding of Acid, Base, and pH Concepts. J. Res. Sci. Teach. 1994, 31 (10), 1077− 1096. (5) McClary, L. M.; Bretz, S. L. Development and Assessment of a Diagnostic Tool To Identify Organic Chemistry Students’ Alternative Conceptions Related to Acid Strength. Int. J. Sci. Educ. 2012, 34 (15), 2317−2341. (6) Bodner, G. M. Constructivism: A Theory of Knowledge. J. Chem. Educ. 1986, 63 (10), 873−878. (7) Peleg, R.; Katchevich, D.; Yayon, M.; Mamlok-Naaman, R.; Dittmar, J.; Eilks, I. The Magic Sand Mystery. Sci. Sch. 2015, 32, 37− 40. (8) Win, T. D. Zeolites: Earliest Solid State Acids. AU J. Techn. 2007, 11 (1), 36−41. (9) Cleaning up with Chemistry: Investigating the Action of Zeolite in Laundry Detergent. J. Chem. Educ. 1999, 76 (10), 1416A.10.1021/ ed076p1416A (10) Bibby, D. M.; Johnston, P.; Orchard, S. W.; Copperthwaite, R. G.; Hutchings, G. J. Conversion of Methanol to Hydrocarbons Using a Zeolite Catalyst: An Undergraduate Chemistry Laboratory Experiment in Heterogeneous Catalysis. J. Chem. Educ. 1986, 63 (7), 634. (11) Eilks, I.; Gulacar, O. A Colorful Demonstration To Visualize and Inquire into Essential Elements of Chemical Equilibrium. J. Chem. Educ. 2016, 93 (11), 1904−1907. (12) Bybee, R.; Taylor, J. A.; Gardner, A.; Van Scotter, P.; Carlson, J.; Westbrook, A.; Landes, N. The BSCS 5E Instructional Model: Origins, Effectiveness, and Applications; BSCS: Colorado Springs, CO, 2006. https://www.bscs.org/sites/default/files/_legacy/BSCS_5E_ Instructional_Model-Executive_Summary_0.pdf (accessed Jan 2018). (13) Lin, J.-L.; Cheng, M.-F.; Chang, Y.-C.; Li, H.-W.; Chang, J.-Y.; Lin, D.-M. Learning Activities That Combine Science Magic Activities with the 5E Instructional Model To Influence Secondary-School Students’ Attitudes to Science. Eurasia J. Math. Sci. Techn. Educ. 2014, 10 (5), 415−426. (14) Hackathorn, J.; Solomon, E. D.; Blankmeyer, K. L.; Tennial, R. E.; Garczynski, A. M. Learning by Doing: An Empirical Study of Active Teaching Techniques. J. Effect. Teach. 2011, 11 (2), 40−54. (15) Peleg, R.; Yayon, M.; Katchevich, D.; Mamlok-Naaman, R.; Fortus, D.; Eilks, I.; Hofstein, A. Teachers’ Views on Implementing Storytelling as a Way To Motivate Inquiry Learning in High-School Chemistry Teaching. Chem. Educ. Res. Pract. 2017, 18 (2), 304−309. (16) Carpineti, M.; Childs, P.; Dittmar, J.; Eilks, I.; Fortus, D.; Giliberti, M.; Hofstein, A.; Jordan, J.; Katchevich, D.; MamlokNaaman, R.; Peleg, R.; Sherborne, T.; Yayon, M. How Using Mysteries Supports Science Learning; The TEMI Project: London, 2015.
Student success and understanding in chemistry is not achieved by solely memorizing facts; instead, understanding requires that students develop a conceptual understanding of the material, all while maintaining a constant determination and interest to master and apply their knowledge.14 While teachers and professors may find it difficult to instill determination and interest in chemistry, a conceptual understanding of the material might be more easily established with the help of demonstrations, especially ones that portray such abstract concepts visually and make them as concrete as possible.11 In this demonstration, students can observe an acid−base reaction as a means to solidify a better foundational understanding of acid−base chemistry. In addition, instructors can use this demonstration as a tool to present Le Châtelier’s principle by illustrating how the addition of a product shifts the direction of the reaction. The versatility of this demonstration is also seen with the storyline, which has proved useful in motivating students. Instructors do, however, need time and experience to become acquainted with the storytelling approach.15 This paper includes an example of such a story, which instructors can modify to better appeal to their students.16 With the overall goal of instruction being increasing knowledge and instilling the desire to continue learning, this demonstration may prove helpful in making students’ experiences in general chemistry more valuable.
Corresponding Author
*E-mail:
[email protected]. ORCID
Ingo Eilks: 0000-0003-0453-4491 Ozcan Gulacar: 0000-0001-7709-0524 Notes
The authors declare no competing financial interest. D
DOI: 10.1021/acs.jchemed.7b00404 J. Chem. Educ. XXXX, XXX, XXX−XXX
