In the Classroom edited by
Secondary School Chemistry
Erica K. Jacobsen University of Wisconsin–Madison Madison, WI 53706
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Applications of Reaction Rate Kevin Cunningham Department of Chemistry, University of Wisconsin–Madison, Madison, WI 53706;
[email protected] Many articles concerning chemical kinetics appropriate for high school or introductory college chemistry have appeared in past issues of the Journal. These can generally be grouped into one of three categories: demonstrations and investigations of factors (such as the temperature, concentration, surface area of the reactants, or the presence of a catalyst or inhibitor) that affect the rates of chemical reactions (1– 18), theoretical and empirical determinations of reaction rates and rate constants (19–28), and models and analogies that illustrate how and why different factors influence reaction rates (29–35). Knowing that the rates of different chemical reactions can vary significantly and that the rate of a particular reaction depends upon the nature of its reactants and the conditions under which it is occurring are also featured prominently in the National Science Education Standards (36). Although much has been written about how instructors can help students understand reaction rate, little has been said regarding how an appreciation for the centrality and direct applicability of this key concept might be cultivated. Textbooks generally contain examples in which increasing or decreasing the rate of a specific reaction has important implications, and teachers of chemistry undoubtedly provide students with more cases through lectures, demonstrations, and laboratory experiences. There is nevertheless something substantial to be gained from asking students to find their own meaning and relevance in such content. This article presents an assignment in which students are asked to research and write about a chemical reaction whose increased or decreased rate is of practical importance. The assignment is designed to develop and assess a number of valuable skills and understandings, including: • Can the student identify a change that is clearly chemical, as opposed to physical, in nature?
samples of student work may be found in the Supplemental Material.W The remainder of this article will examine the skills and understandings the assignment is intended to develop and some of their associated pedagogical issues. Can the student identify a change that is clearly chemical, as to physical, in nature? It has been the experience of this author that few high school students (and, presumably, fewer first-year college students) have trouble with this requirement. Nevertheless, a student may point out that the wax of a burning candle melts more quickly at a higher temperature. Although true, this and similar misidentifications of chemical change would suggest the presence of alternative conceptions regarding the distinction commonly made between chemical and physical transformations. Can the student identify a chemical reaction whose increased or decreased rate is of some interest or practical importance? Students could conceivably select nearly any chemical reaction for this assignment and legitimately state that increasing the surrounding temperature would increase its rate.1 This exercise, however, asks students to consider the subject in a deeper and more practical sense. Requiring students to find some meaningful relevance in the concept of reaction rate and the particular reaction they have selected is an important part of this activity. There are many types of chemical reactions of special significance that students may select from, including:
• Can the student correctly identify the reactants and products of the chemical change they have selected?
• Industrial reactions, particularly those that produce vital materials. An excellent example is the extraction of iron from its oxide through reduction with carbon (coke). The resulting metal is the primary ingredient of steel, a significant industrial and commercial alloy. This reaction is temperature dependent and will not occur below approximately 700 K.
• Can the student clearly and correctly explain the mechanism by which the factor identified (temperature, concentration, surface area, or the presence of a catalyst or inhibitor) increases or decreases the rate of the selected reaction?
• Biochemical reactions. There are few such reactions that are not mediated by enzymes—biological catalysts that increase the rate of otherwise unfavorable reactions often critical to living systems. The digestion of proteins with the help of pepsin, trypsin, and chymotrypsin is a good example.
• Can the student effectively apply the standard conventions of written English?
• Environmental reactions. The erosion of limestone by acid rain is an example of a deleterious reaction whose rate is accelerated by an increase in the concentration of the acids present.
• Can the student identify a chemical reaction whose increased or decreased rate is of some interest or practical importance?
A complete set of materials necessary for administering the assignment, including a student sheet describing the activity and its expectations, a guide for assessment, descriptions of 30 potential topics, a model report, and five additional 430
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The reaction selected, of course, need not be important to society as a whole. It may be one in which students have a
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personal interest, such as the rusting of their car and how rust proofing can slow this process, or lactose intolerance and why a friend or relative has difficulty digesting milk products. Students need only describe the context in which their reaction is significant, and any reaction is important in some context. For students who have trouble identifying a relevant reaction and a context in which it is meaningful, brief descriptions of and references for 30 additional potential topics are available in the Supplemental Material.W Some specific suggestions include warming to hasten the rising of baked goods containing baking soda; using warm water to accelerate the action of household bleach; fanning or blowing on hot coals to increase their rate of combustion; mixing oxygen with the gaseous fuel of high temperature torches used for cutting and welding; coal dust, grain-elevator, and fuel-air explosions; and the role of catalysts in generating hydrogen from coal, preparing aspirin, and the destruction of ozone in the upper atmosphere. Regardless of the specific reaction chosen, students (and their instructors) should remain focused on the fact that the reaction in question will or will not occur at an appreciable rate (or perhaps not at all) under conditions of high or low temperature, concentration, surface area, or the presence or lack of a catalyst or inhibitor. This will in turn, presumably, have meaningful consequences (either positive or negative) for the associated application. Can students correctly identify the reactants and products of the chemical change they have selected? Furthermore, can they represent that reaction with a reasonable chemical equation? At first glance, these may seem to be trivial tasks for even high school students, and for those who select a transformation common to the literature (such as the oxidation of iron) this may be true. However, if the student chooses a complex chemical change, or one whose complete equation is not readily accessible, these questions will become important and may pose a serious challenge. The spoilage of food, for instance, clearly represents a relevant and significant chemical change (or, more accurately, a complex host of transformations) whose rate we commonly attempt to slow through refrigeration and the use of preservatives. Can the student who has selected this reaction correctly identify its principal reactants and products? Can he or she demonstrate this understanding using an acceptable chemical equation (even if simplified by relying on words rather than traditional formulas)? The following student equations suggest the presence of alternative conceptions regarding chemical change in the thinking of their authors (as well as the nature and role of energy in such processes): energy + bananas → ripe milk + temperature → spoiled milk cheese + time + temperature → mold Without further investigation, it is impossible to determine whether such representations are indicative of a poor understanding of chemical reactions, how to effectively represent such changes with chemical equations or equation-like symbolism, a combination of these two factors, or something else www.JCE.DivCHED.org
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entirely. What is important here is that these problems might never have been exposed had this assessment not been employed. Additionally, such revelations present an excellent learning opportunity for students (and perhaps their instructor) by raising several relevant and interesting questions: What is the purpose of a chemical equation? What are the criteria for a “proper” chemical equation? Must a chemical equation always be balanced in order to be useful? How does one accurately yet concisely describe a complex chemical transformation in which not all of the participants (or their chemical formulas) may be known?2 For any given classroom, these questions may be entirely new, a review of concepts already considered, or a combination thereof. Regardless, they are certainly worthy of discussion when responses like those above are observed. An even better approach would involve a proactive strategy. Realizing that students might harbor similar misconceptions, an instructor may share these or similar examples with students while introducing them to the assignment. Students can then be asked to comment on and offer suggestions for improving the accuracy and effectiveness of these notations in conveying the nature of the changes they were intended to represent. A teacher who has conducted this activity with students in the past has the ability to anticipate common problems and illustrate them by referring (anonymously) to specific examples of past mistakes. Such preparatory work is intrinsically motivating for most students and does much to facilitate their learning and improve their work. In this respect, it is both appropriate and desirable to “teach to the test”. Can the student clearly and correctly explain the mechanism by which the factor identified increases or decreases the rate of the selected reaction? It is here that this assessment focuses on an understanding of the crucial chemical concepts of interactions at the molecular level. In spite of having already received instruction in this regard, as well as having their notes and other reference material to consider as they address this requirement, students often have difficulty accurately and effectively applying this information to the reaction they have selected. A common mistake is not explaining specifically why the chosen factor influences reaction rate. For example, a student may correctly state that increasing temperature results in greater motion of reactant molecules. This would suggest an understanding of how an increase in temperature influences reactant molecules. However, such an “explanation” fails to demonstrate a complete and accurate understanding of why temperature influences reaction rate. In order to fulfill this requirement, students must establish a direct causal relationship between the factor in question and the resulting rate change. Students would successfully indicate their understanding of why an increase in temperature increases reaction rate by stating that the increased motion of the reactant molecules produces more energetic collisions between those particles. More energetic collisions in turn result in a greater likelihood of surmounting the energy barrier to reaction (the activation energy) and increase the number of collisions between reactant molecules, affording more opportunities to react in a given period of time.
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Instructors can emphasize the importance of demonstrating complete and accurate understandings by telling students that it is specifically what they have written and not what they may have been thinking when they were writing that will be assessed. Too often, students provide half-answers and expect the reader to supply missing material, establish necessary relationships, or draw their own conclusions. Consistently accepting such responses for full-credit legitimizes these practices and conveys a detrimental lesson to students. One way to avoid such deficiencies is to direct students to complete a provided statement in their written responses. For example, in the case of temperature, a student might be specifically required to complete the assertions “Temperature increases or decreases the rate of this chemical reaction because it has this effect on the reactant molecules” (italicized terms would be replaced by the student’s own words), and “This change in the reactant molecules increases or decreases the rate of this reaction because…”. Compelling students to complete such “because” statements forces them to establish a direct causal relationship between the factor in question and the resulting change in the rate of the reaction. A strategy for more self-directed students is to refer to specific examples of fair, good, and superior responses periodically as they complete their own work (a model report and several student samples are available in the Supplemental MaterialW for this purpose). Teachers can also save examples of common errors to serve as points of discussion with future students, who can be surprisingly adept at recognizing and correcting problems in the work of others that would have been disregarded in their own efforts. In sharing a range of such examples with students, it is important to explicitly identify, explain, and justify the specific characteristics that distinguish each. Has the student effectively applied the standard conventions of written English in completing the assignment? Any instructor who asks students to write and assesses that work is obliged to help them improve their efforts in doing so. As pointed out earlier, an effective way to do this is by identifying (anonymously) common deficiencies in examples of past work and eliciting student suggestions for improvement. This helps students to strengthen their communication skills, develops their ability to critically evaluate their own work, and produces final reports of higher overall quality that are more readily understood and thus easier to assess. Again, in this respect, it is both appropriate and desirable to “teach to the test”. Acknowledgments The author wishes to thank his wife, Julie Cunningham, a chemistry instructor at Lake Mills High School and 2005 Milken Family Foundation National Educator Award winner, for testing this assignment with her students and providing her insight into its effective application. The author is also grateful to Peter Hewson of the University of Wisconsin–Madison, Erica Jacobsen of the Journal of Chemical Education, Craig Akey, and the manuscript’s reviewers for their helpful suggestions in preparing this article. 432
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Supplemental Material
A student assignment sheet, scoring guide for peer, self, and teacher assessment of the assignment, descriptions of 30 research topics for this assignment, and a model report and five samples of student work are available at JCE Online. Notes 1. This is, of course, an over generalization of the relationship between reaction rate and temperature. The rate of some reactions, such as those mediated by enzymes, will decrease above a certain temperature as the protein begins to denature through thermal agitation. 2. “Real” changes, as opposed to the neat and tidy reactions represented by the equations found in most chemistry textbooks, are usually very messy and complicated affairs. We misrepresent nature when we suggest otherwise through the exclusive use of concise notations in which every atom is precisely accounted for (although the conservation of matter and the rearrangement of atoms in chemical reactions are unquestionably crucial concepts). The decomposition of foodstuffs illustrates this point nicely. The complexity and high degree of variability found in the chemical composition of food and the reactions experienced by its many constituents in both spoilage and digestion pose a challenge for those who seek to express these changes with a single, simple, chemical equation. The best way to effectively deal with this is to simplify the process. This can be accomplished in a number of ways, the most general of which might be
(1) These changes are the result of microbial, catabolic activity in which bacteria are digesting and breaking down food molecules. This is done to extract the energy stored within their rich chemical bonds and provide simpler building blocks from which to build the molecular structures necessary for life, growth, and reproduction. In using this representation, it is important to note that the oxygen is not “disappearing” but being incorporated into the product molecules. Another possible equation, containing more detail, is
(2) The comments made regarding the first equation apply also to the second equation. In this case, one should point out that the sources of the carbon and hydrogen on the product side are primarily the sugars and fats or oils on the reactant side, while amino acids are the monomers of protein polymers and can be recombined to produce the protein structures needed by the bacteria generating the spoilage. A final suggestion is (3) Here we have a very specific example of the oxidation of a simple carbohydrate (sugar) common to many foods that can serve to il-
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