Research: Science and Education
Introducing Dynamic Equilibrium as an Explanatory Model Jan H. Van Driel* ICLON Graduate School of Education, Leiden University, The Netherlands Wobbe de Vos Centre for Science and Mathematics Education, Utrecht University, The Netherlands Nico Verloop ICLON Graduate School of Education, Leiden University, The Netherlands
In the usual structure of chemistry courses, chemical reactions are initially taught as processes that proceed in one direction only until one of the original reactants is exhausted (“go to completion”) (1). The introduction of chemical equilibrium at a later stage, however, implies the reversibility of chemical reactions and the possibility that chemical reactions do not proceed to completion. Moreover, the dynamic nature of chemical equilibria requires students to assume that two opposite chemical reactions are taking place, in spite of the fact that this cannot be deduced from observations. Consequently, the introduction of chemical equilibrium requires students to revise the conception of chemical reactions they initially developed. Although many articles have documented learning difficulties, specific student misconceptions, and strategies for teaching chemical equilibrium (2–11), relatively little attention has been paid to the revision of students’ conceptions of chemical reactions as a consequence of the introduction of the chemical equilibrium concept. There is, nonetheless, evidence of students’ failure to conceive the dynamic nature of chemical equilibria (8, 9). Instead, chemical equilibria are perceived as static, that is, “nothing happens” in a system in a state of chemical equilibrium. This problem may be anticipated by illustrating the dynamic nature of chemical equilibrium by means of analogies (e.g., physical equilibria or everyday situations) (12, 13 ), simulations (14 –17 ), or computer animations (18). This article presents an educational design in which students perform hands-on experiments that explicitly demonstrate the reversibility of chemical reactions and the possibility that chemical reactions do not proceed to completion. Next, the dynamic chemical equilibrium concept is offered as a model that explains the results of these experiments. Specific types of reasoning that students use while actually working with this educational design, will be described. Finally, some implications for teaching will be discussed. Educational Design Adopting a constructivist view of learning, our educational design applies specific conceptual change strategies (19, 20). The course design (i) includes assignments to challenge students’ existing conceptions and (ii) stimulates active student engagement through small-group discussions and hands-on experiments. These experiments were chosen not only to highlight aspects of chemical equilibrium that are relevant
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from a cognitive perspective, but also to appeal to students from an affective point of view (e.g., fascinating colors and unexpected events), thus stimulating them to search for explanations (21, 22). The students perform and discuss the experiments in small groups (3 or 4 students), guided by questions in the course material, to facilitate the process of explaining the observations. The entire equilibrium program requires 6 to 7 consecutive chemistry lessons of 50 minutes. Below, the features essential for the introduction of the equilibrium concept are described in more detail.
Reversibility For the introduction of the reversibility of chemical reactions, the system of cobalt(II) tetrachloro and cobalt(II) hexahydrate complexes in isopropanol was chosen. This system can be either pink, owing to an abounding presence of the hydrate complex, or deep blue, indicating the presence of the chloro complex (23): Co(H2O) 62+ + 4 Cl{ pink
CoCl42{ + 6 H 2O blue
The color of the system is subject to the influence of both temperature (i.e., raising the temperature from room temperature to about 85 °C results in a change from pink to blue) and additions of water or chloride. In our educational design, the students are presented with a test tube containing 4 mL of a pink solution. This solution was prepared in advance by dissolving 1.0 g of CoCl 2? 6 H2O in 25 mL of isopropanol under magnetic stirring and adding 5 mL of water to turn the blue solution pink. As our students are unfamiliar with complex ions, we chose not to bother them with the chemical formulas mentioned above. Instead, the students are informed that the test tube contains a solution of a “pink substance” in isopropanol. Next, they are asked to heat and cool this solution by repeated alternating treatments in a warm and a cold water bath and to discuss their observations in terms of chemical reactions. These experiments and discussions require approximately 40 minutes. In another experiment, students are handed a test tube containing 4 mL of a solution of a “blue substance” in isopropanol (prepared by dissolving 1.0 g of CoCl2?6H2O in 100 mL of isopropanol), to which they add drops of water until the color has turned completely pink. Next, they add about 1.5 mL of a (saturated) solution of anhydrous calcium chloride in isopropanol until the pink solution has turned blue. In a parallel experiment, the sequence of the additions is reversed. That is, students start by adding chloride to a pink solution, followed by the addition of water. Once again,
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they are asked to describe the observed phenomena in terms of chemical reactions, using only the names of the substances involved (i.e., blue substance, water, pink substance, chloride). It takes the students about 60 minutes to carry out these experimental and discussion assignments. Through this series of experiments and questions, we hope students come to understand that they have observed two chemical reactions which are opposite to each other in terms of reactants and products (i.e., blue substance + water → pink substance + chloride, and vice versa). For this phenomenon, the term “reversible reaction” is introduced in the course material.
Incomplete Conversion To experience the incompleteness of a chemical conversion, students perform additional experiments with the system of cobalt(II) complex ions described above. After having heated and cooled this system repeatedly, the students place a test tube containing 4 mL of a pink solution at room temperature and a test tube containing an equal amount of a deep-blue solution at 85 °C in a water bath at an intermediate temperature (55 °C). As a result, both solutions turn the same purple color. The students are then asked to discuss the composition of these purple solutions. It is hoped that they understand that both the “blue” and the “pink” substances are present herein, in constant concentrations. These assignments require about 20 minutes. In another experiment, students prepare an aqueous solution of iron(III) thiocyanate complex by first adding one drop of a solution of ammonium iron(III) sulfate (0.5 M) to a test tube half filled with water, and then adding one drop of potassium thiocyanate solution (1.0 M) (24): Fe3+(aq) + SCN{(aq)
Fe(SCN)2+(aq) red
Again, the students are not informed about the structure of these ions. Instead, the species in this system are referred to as “iron(III) ions”, “thiocyanate ions”, and “rhodanide”, respectively. Next, the “rhodanide solution” is divided among three test tubes. Another drop of the ammonium iron(III) sulfate solution is added to the first and another drop of the potassium thiocyanate solution is added to the second. By comparing the colors of the resulting solutions with each other and with the third tube as a reference, the students are expected to observe that the red color is intensified through both additions. They are then asked to discuss this phenomenon in relation to their understanding of reactions going to completion. These assignments take about 30 minutes.
Dynamic Equilibrium After having performed and discussed the experiments and assignments described above, the students are expected to understand that Chemical reactions may be reversible; that is, a change in experimental conditions may result in the conversion of A and B into C and D, and vice versa. Chemical reactions do not always proceed to completion; that is, A and B do not always react until one of these reactants is exhausted.
At this point, the term “chemical equilibrium” is introduced in the course material to describe situations in which all substances concerned are present, whereas no changes at the macroscopic level are detectable. The dynamic equilib560
rium concept is not yet introduced. Instead, the students are asked to answer the following questions with respect to the “pink-and-blue” system. 1. Do you think that both the forward and the backward reaction actually take place if the system is at equilibrium? 2. Suppose that both reactions do indeed take place.What can you then say about their respective rates?
After these questions, the term “dynamic equilibrium” is finally introduced in the course material. The students are then asked to discuss this concept in corpuscular terms and specifically to resolve the anomaly that it appears as if, in a state of equilibrium, some particles of a species have reacted while other particles of the same species have not. This question serves mainly to reveal whether, at this point, the students are able to implement statistical notions in their corpuscular conceptions. Students’ Reasoning in Chemistry Classes
Method Research data were collected in regular chemistry classes in a dozen secondary schools in the Netherlands. The data included (i) the audiotaped discussions of about 20 groups of students (aged 15 to 16 years) working with our course and (ii) the written answers of some 40 to 50 groups of students to all the questions and assignments included in this course. The purpose of this research was to identify the types of reasoning students apply when reversible reactions and chemical equilibrium are introduced to them in the manner described above. Specifically, we aimed to explore which types of reasoning, at both the macroscopic and at the corpuscular levels, would promote or hinder the students’ conceptual change processes in this context. The research methodology incorporated the constant comparative method (25), which involved the comparison of students’ reasoning in discussions with their written answers, and researcher triangulation (26 ), which aimed at agreement among individual researchers (viz., the first and second authors) about the interpretation of students’ reasoning in the data. Results When studying the reversibility of the “pink-and-blue” system, students would usually be fascinated by the color changes. Their observation that the color of the system could be changed repeatedly helped students interpret their observations in terms of reversible chemical reactions. For instance, students would adopt this interpretation by reasoning “You can always get it to turn blue again.” Many students, however, experienced difficulties in accepting that the equations of the two chemical reactions involved are indeed the opposite of each other. Specifically, students objected to the idea that the addition of, for example, chloride to the pink solution could result in the production of both the blue substance and water. Typically, students would reason that “Water cannot be produced in this reaction, since it would immediately react with the blue substance, resulting again in a pink solution.” The results of the experiments demonstrating incomplete conversion usually surprised the students. In many groups, students engaged in long and sometimes heated discussions. Eventually, almost every student accepted as an empirical fact
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Research: Science and Education
that, in these cases, the chemical conversion had not proceeded to completion. At the same time, students were eager to find an adequate explanation for the unexpected experimental results. Often, they would spontaneously ask questions in terms of the availability of reactants (“Why doesn’t this reaction proceed until the reactants are exhausted?”), whereas some framed their problem in corpuscular terms (“Why do some molecules react, while others don’t?”). From their answers to the questions preceding the introduction of the dynamic equilibrium concept (see above), it appeared that the vast majority of the students believed that both the forward and the backward reactions actually take place in a state of equilibrium. Thus, they were ready to accept the dynamic conception. In most instances, the students’ line of reasoning proved to revolve around the idea that all conditions for the reactions to take place are satisfied. A typical written answer was “Yes, when all substances are present together, they will always react.” Once the students had adopted the view that both the forward and the backward reactions actually take place, they would immediately conclude that these reactions would have to occur with equal rates because “Otherwise, the color would have to change.” When asked for an explanation in corpuscular terms (see above), students usually ended up with notions of moving and colliding particles: “The molecules still move, so they can still touch each other.” Occasionally, however, students would incorporate statistical notions in their explanation: for instance, “I think all molecules react, but new molecules are being created all the time…it’s not that the same molecules remain intact, but they are recreated.” When discussing the consequences of the dynamic view, many students still demonstrated a tendency to connect the occurrence of chemical reactions with the color changes they had observed earlier. Specifically, many students appeared to separate the two opposite reactions in either space or time. Bergquist and Heikkinen (1) reported a similar finding. Conclusions and Implications for Teaching From the study of students’ reasoning during chemistry classes, we concluded that the most important goals of our educational design were attained. That is, the experiments on reversibility and incomplete conversion intrigued most students, stimulating them to search for an adequate explanation. Moreover, the dynamic concept of chemical equilibrium appealed to most students as an explanatory model to account for the results of these experiments. Only very few students perceived a state of chemical equilibrium as static (cf. 8, 9). Most students would adopt the dynamic conception using arguments that revolved around the macroscopic conditions for the chemical reactions to take place. Alternatively, some students preferred to apply corpuscular notions. The idea that in a state of equilibrium chemical reactions take place simultaneously appeared to be problematic for many students, some of whom, for example, would reason in terms of oscillating reactions. To anticipate such difficulties, teach-
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