Journal of College Science Teaching :Feature
Throwing Paper Wads in the Chemistry Classroom http://www.nsta.org/publications/news/story.aspx?id=51134&print=true
10/18/2005-Jessica Orvis and Jeffrey A. Orvis
It is well-known that too many students learn to “plug and chug” formulas while gaining very little concomitant understanding of the underlying concepts. Active engagement in the classroom is one of the best tools available for overcoming conceptual difficulties. Science educators agree that students of all ages learn more by participating actively in the interpretation of scientific phenomena (NAS 2003; NSF 1998).
The demonstrations described below are an example of active learning in the chemistry classroom. The equilibrium demonstration involves the entire class and easily acquired paper wads. While using paper wads in the classroom is not necessarily a new idea (Carson 1999), the examples described here represent a useful and fun way to introduce students to two traditionally difficult subjects in general chemistry—equilibrium and reaction mechanisms.
Equilibrium
There are three misconceptions that seem to plague students when they are introduced to this critical topic in chemistry.
· Misconception 1: Many students erroneously assume that the condition of equilibrium means equal concentrations of reactants and products.
· Misconception 2: Many students believe that a reaction at equilibrium has stopped.
· Misconception 3: Students do not easily grasp the notion that one can approach an equilibrium state from either direction.
In our experience, students will memorize the fact that equilibrium refers to rate forward equal to rate backward and thus get the multiple-choice question correct on the exam. However, the misconceptions mentioned above are typically still a part of their thinking.
To best combat these misconceptions, the following demonstration illustrates the difference between equal rates and equal concentrations, shows the active nature of an equilibrium reaction, and demonstrates the approach to equilibrium from either direction. Students begin by making 64 paper wads—paper towels from the restroom work just fine. Any large number will do—we like using at least two for every student. In a class of 32 students, we make 64 paper wads. The instructor then announces that students will be throwing these paper wads at one another, but there are a few ground rules.
· There will be one official counter, one data recorder, and one timekeeper for the whole class.
· Students are allowed to throw only one paper wad at a time. Gathering up a huge pile and tossing them all at once is not allowed.
· When a paper wad lands near a student, the student must pick it up and toss it back.
· When the timekeeper yells “Stop,” all activity ceases.
Students need to be told that the paper wads represent reactants when they are on the left side of the room and products when they are on the right side of the room. The students themselves represent the driving force of the reaction.
A total of three experiments can be run with paper wads that illustrate the nature of equilibrium.
· Experiment 1: Divide the number of students in half and begin with all the paper wads on one side of the classroom.
· Experiment 2: Place 30 students on the left side of the room and two students on the right side of the room. Start with all 64 paper wads on the right side of the room.
· Experiment 3: Keeping 30 students on the left side of the room and two students on the right of the room, begin with all 64 paper wads on the left side of the room.
Experiment 1
With the students divided in half and all of the paper wads starting on the left side of the room, the timekeeper yells “Start!” and the paper wads begin to fly. After five seconds the recorder yells “stop.” The official counter then facilitates the counting of the paper wads on both sides of the room. The results are reported to the data recorder who builds a spreadsheet table that includes the time, the number of paper wads on the left side of the room, and the number of paper wads on the right side of the room. After the data is recorded, the timekeeper yells “Start!” and for five more seconds the throwing continues. The paper wads are counted and recorded again. This continues for a total of at least 20 seconds of throwing. A table resembling that in Figure 1 is generated.
In the formal language of chemistry, the reaction can be represented by this chemical equation:
Pwad(left) ↔Pwad(right)
The equilibrium constant expression is:
Keq= number of paper wads on right/number of paper wads on left
For this reaction,Keq= 1 at the end. Note that, in the last three sessions of paper wad throwing, the numbers change little, yet the activity in the room is as furious at the end as it was at the beginning. This nicely addresses Misconception 2—a reaction at equilibrium has stopped. Clearly, when the students see the activity in the room, the reaction has not stopped, despite the fact that the concentrations (the number of paper wads on each side of the room) is now constant. Using a spreadsheet, a plot of the data generated by the students can be easily produced, showing the approach to equilibrium for the paper wad reaction (Figure 1).
Experiment 2
With 30 students on the left side of the room and two students on the right side of the room and all 64 paper wads on the right side of the room, the same process is repeated. Here the final equilibrium constant,Keq, is seven. By itself this experiment shows that, at equilibrium, the concentrations of reactant and product do not have to be equal, addressing Misconception 1. A plot of the data can be instructive as well (Figure 2).
Experiment 3
With 30 students on the left side of the room and two students on the right side of the room and all 64 paper wads on the left side of the room, the same process is repeated. Here the final equilibrium constant,Keq, is five, comparable to Experiment 2. Together, Experiments 2 and 3 address the third misconception, demonstrating that the same equilibrium can be approached from either direction. A plot of the data can be easily generated as well (Figure 3).
The fact that the counts are not rock steady makes a fine illustration of random scatter in scientific data and the need to run an experiments many times to achieve solid, reproducible results.
Reaction mechanism and rate-limiting step
A pile of about 20 paper wads can be used to teach about reaction mechanisms and the significance of the rate-limiting step. The overall reaction will be one in which paper wads (reactants) starting out on the front student table are tossed in the trash (products). The equation is represented as follows:
Pwad(table) →Pwad(trash)
One purpose of the demonstration is to illustrate elementary steps that make up a reaction mechanism and how these steps relate to the overall reaction. A second purpose is to illustrate how the rate-limiting step controls the overall rate of reaction.
The instructor can indicate to the students that there are plenty of ways to carry out this reaction. For example, Alice, Bob, Chris, and Donna are all sitting at a table near the front of the classroom. Alice picks up a paper wad and then passes it to Bob, who passes the paper to Chris, who passes the paper to Donna. Donna must get out of her seat and walk six feet to the trashcan and drop the paper in the trashcan before returning to her seat and sitting completely down. Only one paper wad can be handled at a time, and Donna must complete the entire process of standing, walking, and returning each time. The reaction mechanism is given by the following:
Pwad(table) ↔Pwad(Alice)
Pwad(Alice) ↔Pwad(Bob)
Pwad(Bob) ↔Pwad(Chris)
Pwad(Chris) ↔Pwad(Donna)
The classroom procedure is entertaining. Alice picks up a paper wad and quickly passes it to Bob. Alice immediately picks up another wad, and so forth. The paper wads are very rapidly passed down the line. Of course Donna must get out of her seat to throw away the first paper wad. By the time she returns to her seat, several paper wads are waiting to be thrown away. She must sit in her chair, pick up the second paper wad, and walk it to the trashcan. The situation becomes somewhat comical when Donna has to fight the urge to pick up all the paper wads and throw them away at once. The other students on the line gleefully pass paper wads to her as quickly as they can. When the demonstration is over, students are very quick to identify the rate-limiting step of the mechanism, the step that required the most energy in the mechanism. Donna’s reward for putting up with paper wads everywhere is that she is identified as being the most important step in the mechanism—she is the one that controls how fast the wads appear in the trash, which is the overall reaction in this scenario.
Conclusion
It should come as no surprise that students enjoy tossing paper wads around the classroom. It’s quite a liberating experience to throw them. After all, students have been told since kindergartennotto throw things in the classroom, and certainly for good reason. These very active demonstrations give everyone a chance to play in the classroom. Most importantly, these activities give students another opportunity to learn difficult concepts and to challenge strongly held misconceptions. Our favorite student comment during the demonstration was, “I wish we could do this every day!”
Jessica Orvis()is an assistant professor of chemistry andJeffrey A. Orvis()is an associate professor of in organic chemistry, both at Georgia Southern University in Statesboro, Georgia.
References
Carson, S.R. 1999. An interactive pupil demonstration of the approach to dynamic equilibrium.Teaching Physics34 (1).
National Academy of Sciences (NAS). 2003.Evaluating and improving undergraduate teaching in science, technology, engineering, and mathematics. Washington, DC: The National Academies of Science Press.
National Science Foundation (NSF). 1998.Shaping the future, VolumeII: Perspectives on undergraduate education in science, mathematics, engineering, and technology. Arlington, VA: NSF.