Understanding Chemical Reaction Kinetics and Equilibrium with

ACTIVITY pubs.acs.org/jchemeduc

Understanding Chemical Reaction Kinetics and Equilibrium with Interlocking Building Blocks Carrie A. Cloonan, Carolyn A. Nichol, and John S. Hutchinson* Department of Chemistry, Rice University, Houston, Texas 77251-1892, United States

bS Supporting Information ABSTRACT: Chemical reaction kinetics and equilibrium are essential core concepts of chemistry but are challenging topics for many students, both at the high school and undergraduate university level. Visualization at the molecular level is valuable to aid understanding of reaction kinetics and equilibrium. This activity provides a discovery-based method to help students visualize a simple reaction at the molecular level using small, plastic brick interlocking building blocks to represent atoms and molecules. By performing and observing model synthesis, decomposition, and competing reactions, the students gain a deeper understanding of bimolecular and unimolecular kinetics, as well as the dynamic state of equilibrium. This activity allows students to dispel some of the common misconceptions about equilibrium and chemical reactions and encourages discussion to facilitate understanding. This flexible activity using building blocks is an appropriate activity for both high school and undergraduate general chemistry students and can be performed in a classroom or laboratory. KEYWORDS: First-Year Undergraduate/General, High School/Introductory Chemistry, Physical Chemistry, Hands-On Learning/Manipulatives, Inquiry-Based/Discovery Learning, Equilibrium, Kinetics, Student-Centered Learning

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major goal of any chemistry laboratory is to help students connect the molecular concepts introduced in class with macroscopic observations seen through experiments.1
3 However, research has shown that most students do not visualize the microscopic level when performing laboratory experiments or thinking about their observations unless specifically challenged to do so.4 An activity is presented that uses small, plastic brick interlocking building blocks to visualize and to demonstrate chemical reaction kinetics and dynamic equilibrium by expanding on an idea initially proposed by Stacy.5 Other building-block activities have been developed for chemistry classes,6
8 but there are no published reports of building blocks being used to model chemical reaction kinetics and equilibrium. Stacy suggested that by using building blocks to simulate elementary reactions, the concepts of concentration and collision rates can also be examined.5 By using small, macroscopic objects to represent atoms and molecules, this activity encourages students to visualize on the molecular level by putting the particles in their hands.9 This activity goes beyond the static visualization that occurs when model kits are used to represent a single molecular structure. Through hands-on macroscopic experimentation, students can discover the concentration dependence of the reaction rate, observe a reaction system in dynamic equilibrium through competing reaction rates, and dispel some of the common alternate conceptions regarding equilibrium. Equilibrium is a core concept throughout chemistry, and the underlying concept of dynamic equilibrium permeates the discussion of all chemical reactions, as well as phase equilibrium. However, students often have misconceptions about Copyright r 2011 American Chemical Society and Division of Chemical Education, Inc.

chemical equilibrium because of vocabulary issues (e.g., the meaning of shift, balance, reversibility in chemistry versus the everyday usage), confusion about the difference between rate constants and equilibrium constants, misinterpretation of Le Chatelier’s principle, and lack of knowledge about the dynamic nature of equilibrium.10
14 A variety of approaches have been developed to elucidate the nanoscopic world of chemical equilibrium including macroscopic analogies, models, writing assignments, and computer simulations.15
19 In general, students find concrete examples more helpful than the abstract symbols that are traditionally used to illustrate chemical equilibrium. Students find the concept of dynamic equilibrium particularly challenging and only a few lab protocols exist to help students examine this concept.20 The activity presented here helps students dispel some of these misconceptions by actually seeing and experiencing a dynamic model for how reactions depend on concentrations of reactants and come to equilibrium. Students can actually see that equilibrium is not a static state on the particulate level, but one in which the macroscopic properties are not changing even while the microscopic particles are in continuous motion. In a separate article in this Journal, we present a laboratory activity that also encourages students to understand dynamic equilibrium via constancy of the vapor pressure equilibrium of a liquid
vapor system as volume changes.21

Published: July 14, 2011 1400

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’ OVERVIEW The purpose of this activity is to demonstrate the dependence of the reaction rate on concentration and to illustrate how the rates of the forward and reverse reactions become equal when those reactions compete and come to equilibrium. The basic system is an elementary reaction involving synthesis and decomposition in equilibrium A þ B / AB

ð1Þ

where A is a green (or any color) building-block atom, B is a yellow (or any other color) building-block atom, and AB is a molecule of connected green and yellow atoms. First, the bimolecular reaction of the synthesis of molecules from atoms is performed and the reaction rate monitored. Second, the unimolecular reaction of the decomposition of molecules into atoms is performed and again the reaction rate is recorded. The kinetics of both bimolecular and unimolecular reactions become apparent as students perform the reactions themselves and reconcile the differences between the two. Finally, both reactions are performed simultaneously to simulate competing forward and reverse reactions. This provides a vivid illustration of how equilibrium occurs when the forward and reverse rates are equal and gives the students context to build their understanding of equilibrium as macroscopically static but microscopically dynamic.

’ ACTIVITY DETAILS Detailed guides for students and teachers are included in the Supporting Information. Materials include boxes containing 50 small, plastic brick interlocking building blocks of each of two colors for a total of 100 bricks similar in size. Each building block represents an individual atom. Elements are distinct by color. When multiple building blocks are combined, they represent a molecule made of those particular atoms. Students work in teams, each with different tasks. The tasks are as follows: • Assembler: Reach two hands into the box and pull out two particles without looking. If the particles are individual atoms (unattached building blocks) of different colors, they are assembled into a molecule and returned to the box. If the particles are the same color or if already a molecule, both are returned to the box with no changes. Repeat for the duration of the reaction time. • Disassembler: Reach one hand into the box and pull out one particle without looking. If the particle is a molecule, it is disassembled and the individual atoms are returned to the box. If the particle is an individual atom, it is returned to the box. Repeat for the duration of the reaction time. • Agitator: Shake the box of building blocks to mix them during the reaction time. This is necessary as the most recently assembled or disassembled pieces will otherwise always be at the top. • Timer: Indicate start and stop times to the team. Procedure 1: Kinetics of Bimolecular Synthesis

Students perform a synthesis reaction by starting with all individual atoms and only having assemblers work to make molecules with no disassemblers. The reaction runs for 1 min, then the numbers of atoms and molecules are counted. The reaction is performed again starting from the previous number of molecules for two additional 1-min intervals.

Procedure 2: Kinetics of Unimolecular Decomposition

Students perform a decomposition reaction the same manner as outlined in procedure 1 but starting with all assembled molecules and no individual atoms. For a decomposition reaction, only disassemblers work and there are no assemblers. The reaction is performed for three 1-min intervals, similar to procedure 1. Procedure 3: Competing Reactions and Equilibrium

Students perform both synthesis and decomposition reactions simultaneously to represent competing forward and reverse reactions. The starting number of molecules can be varied or students can predict what they expect will be equilibrium and start at that point. The reactions are best performed in 2-min intervals when both reactions occur together. Equilibrium is typically attained after three 2-min intervals.

’ HAZARDS Physical hazards include pinching or cuts from the building blocks. Such hazards can be avoided by connecting the bricks with an offset or perpendicular orientation to allow for easy separation. ’ RESULTS After completion of the reactions, students compute “mole fractions” (in this case, simple fractions of particles of each type) to represent a concentration of each Xi ¼

Ni NT

ð2Þ

where Xi is the mole fraction of species i, Ni is the number of i species, and NT is the total number of species. The mole fractions are calculated after each reaction interval. Students are asked to compare and contrast the synthesis and decomposition reactions, their reaction rates, and the time to get to completion. For the forward and reverse reactions together, students are asked if they achieved equilibrium and to calculate an equilibrium constant, K, using the mole fractions: K ¼

XAB XA XB

ð3Þ

Students can then compare their observations to theoretical concentrations and equilibrium constant under the simple assumption that the forward and reverse reaction rate constants are equal, in which case K = 1. Included in the Supporting Information are the calculations of the equilibrium constant and equilibrium concentrations, which highlight the fascinating aspect that if K = 1, there would be 15 molecules and 35 of each atom at equilibrium. Thus, even if K = 1 is assumed, the result is an unequal number of molecules and particles. Moreover, the data will differ if the reaction rate constants are not equal. Actual student data varied, but many groups found an equilibrium number of molecules to be between 10 and 12. This is because the forward and reverse rate constants are not equal for this system, because, as students find out quickly, it takes less time to pull two connected building blocks apart than to put two pieces together. As noted below, however, this observation challenges students’ preconceptions about dynamic equilibrium and what it means to have equal forward and reverse reaction rates. 1401

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’ DISCUSSION The equilibrium activity was performed by general chemistry laboratory students. After completing the activity, some students did not understand that their system had reached equilibrium even if the number of molecules remained constant after each reaction period. Through the students’ written responses and discussion, we discovered the students were unable to let go of their alternate conceptions regarding equilibrium. The main misconception the students struggled to reconcile is their belief that at equilibrium the concentration of reactants equals the concentration of products. Students were also encountering confusion regarding the difference of the chemistry definition of equilibrium with the everyday meaning of equal. Thus, the students expected equilibrium to only be achieved if they found 25 molecules and 25 of each atom. Although not all students understood equilibrium immediately after the activity and some expressed disappointment even when the numbers of particles of each type became static after multiple trials, guided discussions helped clarify misconceptions. Students came to understand the dynamic aspect of equilibrium at the particulate level while the macroscopic state appears static. Conversation forced students to reconcile that equilibrium was represented by the building blocks when the number of molecules remained constant while both the forward and reverse reactions continued. Students reconciled that equilibrium means equal forward and reverse reaction rates. After discussion in lecture, the majority of students showed understanding on an exam question on equilibrium that referenced the building blocks activity (available in Supporting Information). Feedback from interviews with students also revealed the building blocks activity to be successful for understanding equilibrium (these students only did the equilibrium activity, not the kinetics as that topic came later in the sequence of the semester). Students were able to understand reactions and equilibrium on a molecular level because the molecules were big enough to be in their hands and they were able to visualize the process. When asked to respond about the effectiveness of the building blocks lab, one student gave the following response: It really helped me understand the equilibrium thing and that it depends on the rate of forward and reverse reactions and the number of reactants...The whole equilibrium thing made sense. ’ ACTIVITY WITH SECONDARY SCHOOL TEACHERS Both the kinetics and equilibrium activities were performed with 35 secondary science teachers as part of a professional development course. The teachers enjoyed the activity and saw its value for illustrating principles of reaction rates, dependence on concentration, and equilibrium. In those curricula where mole fractions are not discussed, an alternative can be to use percentage of particles or refer somewhat loosely to the mole fractions as concentration. A secondary science teacher from this cohort took this activity back to the classroom and the students found the experience valuable and understood concentration dependence by looking at percentages. That same teacher also noted that the students seemed to gain the most understanding when performing the role as agitator, as they were able to watch how the reactions occurred. Thus, the ideal setup would be to allow

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students to rotate through each of the roles. Other teachers also planned to use this activity in their classrooms using building blocks or cheaper materials such as paper clips.

’ FURTHER STUDENT ACTIVITIES The building blocks activities could be modified in various ways to look more at reaction rates and equilibrium by starting with differing numbers of molecules or using differing sets of atoms. Enzyme kinetics could be simulated by including another building block piece as an enzyme. Also, other reactions could be performed rather than merely the combination of two distinctly colored bricks. The possibilities are vast and offer an informative way to visualize at the molecular level. ’ ASSOCIATED CONTENT

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Supporting Information Instructions for students and teachers, sample exam question, and calculation of an equilibrium constant. This material is available via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported in part by the Center for Biological and Environmental Engineering (CBEN) NSF EEC-0118007 and EEC-0647452 and National Science Foundation Graduate Research Fellowship under Grant No. 0940902. ’ REFERENCES (1) Schlesinger, H. I. J. Chem. Educ. 1935, 12, 524–528. (2) Reid, N.; Shah, I. Chem. Educ. Res. Pract. 2007, 8, 172–185. (3) Hofstein, A.; Lunetta, V. N. Sci. Educ. 2004, 88, 28–54. (4) Lazarowitz, R.; Tamir, P. Research on Using Laboratory Instruction in Science. In Handbook of Research on Science Teaching and Learning; Gabel, D. L., Ed.; MacMillan: New York, 1994. (5) Stacy, A. M. Unit 6: Showtime. In Living by Chemistry; Key Curriculum Press: Emeryville, CA, 2010. (6) Witzel, J. E. J. Chem. Educ. 2002, 79, 352A–352B. (7) Sharma, C. V. K. J. Chem. Educ. 2001, 78, 617–622. (8) Mind and Hand Alliance and MIT Edgerton Center. LEGOO Chemistry. http://web.mit.edu/edgerton/outreach/ACT_LC.html (accessed July 9, 2010). (9) Rhodes, G.; Daly, J. M. J. Chem. Educ. 1977, 54, 12–13. (10) Ozmen, H. Chem. Educ. Res. Pract. 2008, 9, 225–233. (11) Quílez, J. Chem. Educ. Res. Pract. 2004, 5, 281–300. (12) Banerjee, A. C. Int. J. Sci. Educ. 1991, 13, 487–494. (13) Wheeler, A. E.; Kass, H. Sci. Educ. 1978, 62, 223–232. (14) Bergquist, W.; Heikkinen, H. J. Chem. Educ. 1990, 67, 1000–1003. (15) Wilson, A. H. J. Chem. Educ. 1998, 75, 1176–1177. (16) Rudd, J. A.; Greenbowe, T. J.; Hand, B. M. J. Chem. Educ. 2007, 84, 2007–2011. (17) Raviolo, A.; Garritz, A. Chem. Educ. Res. Pract. 2009, 10, 5–13. (18) Niaz, M. Sci. Educ. 1998, 7, 107–127. (19) Stieff, M.; Wilensky, U. ChemLogo; Center for Connected Learning and Computer Based Modeling, Northwestern University: 1402

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