Demonstration pubs.acs.org/jchemeduc
Making Water the Exciting Way: A Classroom Demonstration of Catalysis Ryan L. Stowe,† Steven M. Bischof,‡ Michael M. Konnick,‡ Claas H. Hövelmann,‡ Deborah Leach-Scampavia,§ Roy A. Periana,‡ and Brian G. Hashiguchi*,‡ †
Department of Chemistry, ‡The Scripps Energy & Materials Center, §Educational Outreach Office, The Scripps Research Institute, Jupiter, Florida 33458, United States S Supporting Information *
ABSTRACT: An understanding of the factors that govern the rate of chemical reactions has proven elusive for many students who begin a survey course in the chemical sciences. Inquiry-based curricula built upon an understanding of common student misconceptions related to chemical kinetics have proven to be a more effective means by which to develop student understanding than traditional lecture formats. To facilitate teacher-guided discussion regarding the subject of catalysis, we have developed a simple and safe demonstration, whereby the hydrogen and oxygen components evolved upon electrolysis of water are recombined at room temperature with the aid of a platinum/ruthenium catalyst. This demonstration is designed to draw students’ attention toward the dramatic change in reaction rate that can be affected by catalyst involvement and to provoke dialog regarding the mechanistic changes that accompany reaction catalysis. KEYWORDS: First-Year Undergraduate/General, High School/Introductory Chemistry, Second-Year Undergraduate, Upper-Division Undergraduate, Demonstrations, Physical Chemistry, Inquiry-Based/Discovery Learning, Misconceptions/Discrepant Events, Catalysis, Kinetics
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Table 1. Documented Student Misconceptions of Catalysis
he practical utility of a chemical process is very often a question of kinetics. No chemist is interested in a reaction that requires months to achieve appreciable yield of a desired product. Due to the significant industrial emphasis on reaction expediency, an additive is often used in chemical synthesis that serves to accelerate reaction rate without itself being consumed in the reaction. These additive compounds, known as catalysts, are vital to the modern chemical industry and are featured prominently in the synthesis of products ranging from therapeutics to large-scale, commercial chemicals to transportation fuels. It is estimated that over 90% of recently developed chemical processes involve catalysis.1 Additionally, protein catalysts, known as enzymes, play vital roles in accelerating a dizzying array of reactions vital to sustaining life including various aspects of transcription, translation, cell cycle regulation, and protein degradation.2 Due to the significant importance of catalysis in processes both industrial and biological, catalyst-mediated reaction rate acceleration has occupied a central place in advanced high school and introductory undergraduate treatments of chemical kinetics.3 Unfortunately, a detailed understanding of reaction rates and the factors that govern them often proves elusive to budding chemists.3−5 A variety of common conceptual difficulties, which have been documented by several studies,4,6 characterize student misunderstanding of chemical kinetics. Several of the most common misconceptions regarding catalysis are illustrated in Table 1. © 2014 American Chemical Society and Division of Chemical Education, Inc.
Common Misconception A catalyst increases the yield of products A catalyst can affect the rates of forward and reverse reactions differently A catalyst does not change the mechanism of a reaction Activation energy is the (total) amount of energy released in a reaction
Data Source High school students undergraduates6 High school students undergraduates6 High school students undergraduates4 High school students undergraduates4
and and and and
It has been well established in the educational literature that effective instruction on a given topic must begin at a student’s prior level of knowledge (which may include misconceptions).7,8 By focusing on the correction of flaws in a pupil’s concept of chemical reactivity, teachers may engender a robust understanding of mechanistic chemistry in their students. Active-learning approaches, in which teachers act as facilitators for student discovery, have proven especially successful at addressing complex topics. 5 The use of a classroom demonstration as a facilitator for student hypothesis generation is one such active-learning strategy. We have engineered a demonstration that we believe highlights the fundamental rate acceleration achievable via catalysis while simultaneously provoking discussion into catalytic mechanism. Insights gained from this discussion will Published: February 14, 2014 550
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this large outreach event, which involves almost 6 h of continuous engagement with large swathes of the public, this apparatus has proved an extremely robust and effective teaching tool.
help students recognize the mechanistic change implicit in reaction catalysis, as well as appreciate the difference between heterogeneous and homogeneous catalysis. This demonstration involves the combination of gaseous hydrogen and oxygen (generated in small and safe amounts via the electrolysis of water) catalyzed by a heterogeneous platinum/ruthenium (Pt/ Ru) catalyst. Although the reaction of molecular hydrogen and oxygen to form water is a thermodynamically favored process, the significant activation energy required cannot be achieved at ambient pressure without elevated reaction temperatures. The contrast between the catalyzed reaction and the unreactive character of the uncatalyzed process may be used to illustrate the rate acceleration implicit in catalysis and serves as an excellent segue into discussions of catalytic mechanisms. A generic reaction coordinate diagram depicting both a catalyzed and uncatalyzed reaction process, as shown in Figure 1, may be helpful in this discourse.9
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HAZARDS The presenter should wear gloves, safety glasses, and a lab coat. Be mindful of the location of flame sources and catalyst-coated sticks when the electrolysis apparatus is filled with H2 and O2 as premature exposure of the gaseous mixture to either could result in an accidental explosion. Only plug in the power supply when prepared to carry out the experiment. Turn on the power supply last when assembling the electrolysis apparatus. When the power supply is engaged and clipped to the electrodes, do not touch the bicarbonate solution within the beaker.
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DEMONSTRATION The electrolysis apparatus developed for our demonstration consists of a gas collector (assembled from 4 in. plastic tubing, a powder funnel, and a 2-way valve) weighed down by a 1 lb ring, an electrode assembly (composed of two coiled stainless-steel electrodes braced by a support constructed from polyethylene), a 2 L beaker filled with sodium bicarbonate solution and a dc power supply (Figure 2). Detailed instructions regarding the
Figure 1. A generic reaction coordinate diagram depicting catalyzed and uncatalyzed reactions. Note: the multiple transition states depicted in the catalyzed reaction signify a change in mechanism.
Electrolysis followed by gas sequestration via soap bubbles serves as a simple and safe manner by which to generate the correct stoichiometry of gaseous oxygen and hydrogen for this demonstration. All of the materials for this demonstration, including supplies for the construction of the electrolysis apparatus, can be purchased for approximately $350. Due to this modest expense and the relatively benign nature of the electrolysis process, this demonstration should be suitable for students in undergraduate general chemistry courses or their high school equivalent. The ability of this demonstration to generate useful dialogue regarding various aspects of catalysis has been repeatedly shown during publically accessible outreach events. Each year the Florida campus of The Scripps Research Institute (TSRI) participates in a day of community outreach known as the “Scripps Florida CELLebrate Science Day”. Nearly all aspects of TSRI’s biomedical research enterprise are showcased in some manner as a part of this extensive event. The chemistry booth for the “CELLebrate Science” event always features catalysis as a major theme and provokes discussion regarding catalytic action and mechanism using the demonstration detailed herein. During the five years we have participated in
Figure 2. The fully assembled electrolysis apparatus.
construction of this apparatus including an itemized list of required supplies may be found in the Supporting Information. Adaptations of this apparatus design may be capable of facilitating this demonstration so long as they feature some manner of gas collector with a 2-way valve capable of releasing the gases resultant from electrolysis in a controlled manner. Approximately 30 min prior to the demonstration, prepare a number of catalyst-coated wooden dowels by spraying the dowel end with a light adhesive layer and rolling the stick in Pt/ Ru black to give a coating 1/2 to 3/4 in. long. The Pt/Ru black catalyst must be a fine, dry powder. Commercially available catalyst may be stored in a cool, dry place for extended periods. 551
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The demonstration may proceed once the electrolysis apparatus has been vented and filled with H2 and O2. With bubble solution, prepare a bubble film using the included “bubble wand” and place the film over the top of the 2-way valve. Following this, slowly release pressure to create a small bubble. Close the 2-way valve and slowly move the wand and accompanying bubble an arm’s length away from the electrolysis apparatus. Select a volunteer from the class and have them rupture the bubble with an unaltered wooden stick. There is not sufficient activation energy to combine the two gaseous reactants at room temperature and thus this first rupture will be decidedly unexciting. Following this, fill another bubble, move the bubble away from the electrolysis apparatus and have your volunteer rupture it with a lit, long-reach match. The class will observe that a loud noise and flash of light accompany this rupture as the flame provides the needed activation energy for the formation of water. Finally, fill a third bubble, remove the bubble from the electrolysis apparatus and encourage your volunteer to rupture it with the previously prepared catalyst-coated dowels. The noise and flash accompanying this will be analogous to that observed upon bubble rupture with a lit dowel.
Figure 3. A plausible mechanism for the synthesis of water catalyzed by Pt/Ru black. Steps include (1) H2 reaction with the catalyst surface to generate Pt/Ru−H, (2) molecular oxygen reaction with the Pt/Ru− H, and (3) the release of water from the catalyst surface.
demonstration, together with appropriate discussion, will help students discover first-hand how the dramatic rate acceleration observed in the catalyzed combination of H2 and O2 is possible. Additionally, the electrolysis apparatus constructed as a part of this demonstration may find further utility in discussions of electrochemistry that often follow treatments of chemical kinetics in general chemistry courses.
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DISCUSSION The demonstration detailed above is intended primarily as an aide to provoke classroom discussion regarding the specifics of catalytic processes. Students will readily observe the contrast between the catalyzed combination of H2 and O2 gas relative to the unreactive character of the same gaseous mixture in the absence of the catalyst but may not understand how the catalyst acts to accelerate reaction progress. The mechanistic change, which enables the lowered activation energy barrier between reactants and products observed in catalytic processes, is not often recognized.4 Additionally, the instructor may wish to highlight that both the catalyzed and uncatalyzed reaction pathways have the same difference in free energy between the reactants and products (as ΔG is a state function). One could further mention the spontaneous character of both processes and use this occasion to highlight the difference between the scientific and colloquial definitions of “spontaneous”. Finally, the instructor could provoke discussion into the differing modes of reaction facilitation provided by the flaming stick and the dowel coated in Pt/Ru black. One might explicitly ask whether the heat provided by the burning stick constitutes a catalyst to initiate this discourse. The authors strongly encourage all classes who perform this demonstration to dedicate some time to a discussion of plausible catalytic mechanisms for the observed reaction. One plausible mechanism is illustrated in Figure 3. In this scenario, molecular hydrogen reacts with the catalyst surface to generate highly reactive Pt/Ru metal hydrides. The activated hydride then reacts with molecular oxygen to form molecules of water. Similar mechanisms are known in the literature including the generation of powerful hydride reducing agents from Pt-group metals.10−13 Alternatively, molecular oxygen may react with the catalyst surface to produce activated metal-oxides capable of reacting with hydrogen (not shown).14,15 Given the significant importance of catalytic processes in both industrial and biological contexts, it is unsurprising that they are discussed in nearly all college-level general chemistry textbooks.3 Unfortunately, reading and lecture-based instruction may not be sufficient for most students to gain a deep knowledge of catalyst action. It is our hope that this
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ASSOCIATED CONTENT
S Supporting Information *
A list of materials together with detailed instructions for electrolysis apparatus construction and safety hazards associated with this demonstration. This material is available via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge financial support of our research by the Chevron Corporation for R.A.P. and C.H.H., The Center for Catalytic Hydrocarbon Functionalization, a DOE Energy Frontier Research Center (DOE DE-SC000-1298) for S.M.B., B.G.H., and M.M.K., and The Scripps Research Institute and the William R. Kenan, Jr. Charitable Trust for D.L.-S. and R.L.S. We would also like to thank William R. Roush for his assistance editing this manuscript.
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REFERENCES
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