In the Classroom edited by
Applications and Analogies
Arthur M. Last University College of the Fraser Valley Abbotsford, BC, Canada
An Interactive Classroom Activity Demonstrating Reaction Mechanisms and Rate-Determining Steps Laura D. Jennings and Steven W. Keller* Department of Chemistry, University of Missouri–Columbia, Columbia MO 65211; *
[email protected] Multistep reactions and the rate laws describing them are difficult concepts to grasp for many general chemistry students. Several automobile analogies have been described in this Journal for various topics in kinetics (1, 2), and a wonderfully simple and effective dishwashing analogy for ratedetermining steps and reaction mechanisms was described by Last many years ago (3). We also sought to employ a process familiar to students, but wanted to incorporate some active class participation and interaction in the activity, which the above analogies lack. Our understanding of the main conceptual difficulty students have with reaction mechanisms is the disconnect between the written (or witnessed) chemical reaction and the reaction coordinate diagram. The confusion is not surprising considering a gaseous reaction like NO2(g) + CO(g)
NO(g) + CO2(g)
that appears to be a simple oxygen-atom transfer, but at temperatures below about 500 K has the following rate law: Rate = k[NO2]2 As professional kineticists well know, even detecting intermediates can be a daunting task, let alone understanding their role in multistep reactions. The two-step “reaction” of unwrapping and eating chocolate candies described here brings not only the “reaction intermediate”, but also the reactants and products into macroscopic view. Further, the qualitative “activation barriers” of both steps can be adjusted independently and the effects on the resulting overall rates predicted and then tested. Class Preparation For the activity to be most effective, several introductory kinetics lectures should precede this activity so that the concepts of reaction rates, rate laws and orders, reaction coordinates and activation energies are familiar to the students. We performed this activity in a large-lecture, second-semester general chemistry class (ca. 250 students) and about a dozen students were able to participate in the process. Alternatively, discussion sections or laboratory preparation sections could be used to conduct the demonstration. Advanced high school classes should also be able to benefit. The class time investment is quite flexible. The “reactions” can be performed very quickly, and several variations were completed in 20– 25 minutes including some time to give students a chance to sketch diagrams and make suggestions or predictions.
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Initial Setup Six to eight students are selected from the class and divided into two groups: the unwrappers and the eaters. (It may be helpful to assign roles without naming the categories, or there may be very few of the former). The groups are positioned on either side of an overhead projector and given the following instructions: • The unwrapper (we suggest starting with only one member from each group) is to unwrap Hershey’s Kisses one-at-a-time and place them on the overhead projector. • The eater is to eat one chocolate at a time, only after it has been placed on the overhead.
The Reaction When instructed, the unwrapper starts unwrapping and depending on the students (and the time of day) the initial “reaction” can go on for quite some time, but after three or four candies are consumed, a fairly constant rate is established.1 At this point, it becomes clear that this is a multistep process: • The students see the unwrapping of the chocolates. • The unwrapped candies (intermediates) are visible on the overhead. • They are consumed.
Furthermore, it becomes clear (if it was not already) that the unwrapping is the “hard part” (slow) and the eating is the easy part (fast). It is convenient at this point to relate the demonstration back to the language of chemistry by writing the overall reaction
wrapped candy
eaten candy
and then break this down into its two “elementary steps”:
wrapped candy (reactant)
unwrapped candy (intermediate)
eaten candy (product)
With the appropriate introduction as outlined above, the class can then be asked to draw a reaction coordinate dia-
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gram that hopefully will look like Figure 1. Reactants, products, and intermediates can be identified, and activation energies (at least qualitatively) described. Alternatively, this can be a very good situation for the predict-observe-explain activity (4, 5), where each student draws and labels her or his own diagram and then is asked to explain it to a classmate or group of classmates. With suitable leadership from the instructor, peer discussions can resolve any disagreements or misunderstandings at this point so that all of the students maintain correct focus on the chemical principles being demonstrated. Suitably armed with a reaction coordinate diagram for the process, and the ability to explain the diagram, the students can establish a connection between the process and the representation of the process; the effects of changing certain experimental conditions can then be predicted, observed, and further explained. Concentration Variations Initially, we sought to demonstrate that in multistep processes some changes in the experimental conditions alter the rate, while others do not. One easy way to accomplish this is by increasing the number of unwrappers or eaters. It was our experience that more of the latter did not increase the rate of consumption, but doubling the number of unwrappers approximately doubled the rate2; connections between reaction rate and concentration can then be explored with some additional discussion with the instructor.3 Again, connecting the results back to the reaction coordinate diagram is illustrative. In this case, changing the number of participants does not change the difficulty of either of the individual steps, so the reaction coordinate diagram is unchanged; the rate changes are a result of concentration changes.4 Other Variations If there is enough time, the relative difficulty of the steps can be controlled and changed (often in very entertaining ways!). The student interactivity is highlighted when the class is asked to suggest modifications in “reaction” parameters (independent of the number of eaters and unwrappers) accompanied by predictions as to the effects the variations would have on the rate. The changes are implemented and new rates are observed and compared with predictions. Suggestions from the class to make the first-step more difficult included: unwrappers using one hand (or have one hand each from two different persons “working together”), using chopsticks, or putting the wrapped chocolates in a bag at some distance from the overhead. More entertaining were the suggestions as to how to slow down the second step. Eaters not using their hands, or using chopsticks or forks, or being blindfolded are possibilities. By combining several variations, (e.g., lots of unwrappers and the eaters using a fork) a large buildup of the intermediate (unwrapped but uneaten chocolates) will be observed by the students. At some point the class should draw reaction coordinate diagrams indicating increases in the activation barriers for these variations and observe the accompanying changes in rate. Class Discussion With several scenarios presented, a discussion can be developed that connects the activity back to the kinetics termi550
Journal of Chemical Education
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“Energy”
In the Classroom
wrapped candy
unwrapped candy
eaten candy
Reaction Profile Figure 1. Typical reaction coordinate diagram for the initial conditions described.
nology that had been previously discussed abstractly, as well as creating a tangible relationship between reactions, reaction mechanisms, and rate laws, which can be especially emphasized if one of the variations makes the second step rate-determining. Finally, this activity can lead into a discussion of catalysts. By leaving out the unwrapping, eating a Hershey’s Kiss can be a one-step process; of course the activation energy for that step (especially for those of us with fillings) would be gigantic! Notes 1. A note about hygiene. We suggest bringing in some wetnaps or other wipes for the Unwrappers to clean their hands prior to participating. In addition, a clean transparency (also wiped and dried) should be placed on the overhead prior to the beginning of the demonstration to avoid melting chocolate directly onto the glass. Alternatively, a clear plastic plate can be placed on the overhead. 2. We suggest alternating the eaters to avoid “saturation”, especially for early morning classes! 3. We chose to compare all of the different reaction rates qualitatively, and all of the variations produced identifiable changes. Quantitative rates (i.e., candies兾second) could also be measured with additional student volunteers having digital stopwatches. 4. There is not a direct link between the number of eaters and the concentration of “product”, but the idea of one parameter affecting the rate and the other having no effect is clear.
Literature Cited 1. 2. 3. 4.
Potts, R. A. J. Chem. Educ. 1985, 62, 579. Ball, D. W. J. Chem. Educ. 1987, 64, 486. Last, A. M. J. Chem. Educ. 1985, 62, 1015. White, R.; Gunstone, R. Probing Understanding; Falmer: London, 1992; p 5. 5. Rickey, D.; Stacy A. M. J. Chem. Educ. 2000, 77, 915.
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