A New Road to Reactions Part 3. Teaching the Heat Effect of Reactions Wobbe de Vos and Adri H. Verdonk Department of Chemical Education. University of Utrecht, P.O.Box 80.083.Utrecht, the Netherlands
Some of the major problems in teaching and learning chemistry are encountered in the very first stages of an elementary chemistry course. This seems t o he a paradox since obviously one aims a t teaching the simplest things first. hut in fact it is not. he knowledge gap between teacher and student is a t its widest when the student is relativelv voune and knows next to nothing about chemistry. The teicger, trained as a chemist, has adopted concepts, facts, models, rules, and theories that govern his or her thoughts and perception. I t is difficult for the teacher to cast aside this structure even for the purpose of understanding a beginner's learning prohlems. T o understand chemistry even a t an elementary level, one needs to he familiar with some fundamental hut certainly not very simple concepts, like "suhstance" and "chemical reaction". All too often in hieh school chemistrv textbooks the first of these two concep;s is taken tor granied and the second is introduced more or less in vassine. The difficultv of crossing the threshold of chemistrGis of& underestimaied. This series of articles summarires our work on the concept of chprnical reactions as a teaching and learning problem. Our goal is t o extend the reaction concept heyona the point of giving a correct definition, either in the vocabulary of official chemistry or in the language of daily life. We also seek to do more than merely demonstrate anumber of examples of chemical reactions and other processes, hoping that students will discover the essence of the concept by themselves. Instead. we aim at the eradual develovment of a reaction concept that starts from a very primitke intuitive basis. We confront our students with ever-increasing dosages of experience with the reaction phenomenon, encouraging them to enrich and readiust their~oriainal - ideas as their knowledge matures. The first step in this sequence occurs as the student is confronted with chemical reactions in laboratory work where he or she observes unexpected, unusual, unaccountable, and unpredictahle changes. The second step is for the student to describe in his or her own words the formationof a new substance.' Stev three involves a closer analvsis of the reaction by the stu'dent who, on the hasis of asprimitive chemical theory, explores the level of direct observation to a first case of predictahi1ity.z The main theme of this third article in the series is the heat effect of chemical reactions. This includes the recognition of exothermic and endothermic processes in laboratory work and in daily life. Heat Effects
-
-
Classifvinc reactions as exothermic or endothermic is not as easy as i t seems. For instance, defining an endothermic reaction as one that uses uv heat made some of our students incorrectly call the burning of a candle endothermic since, as thev vointed out. heat was needed to lieht it. T o define hur%ng as exothermic, one requires a ~ e i i ~ u a n t i t s t i apve proach in which the amount of heat needed for lighting the candle is compared with the amount of heat produced by the candle as i t hums. Not all students are able to make that comparison. Not only are students often unable to estimate 972
Journal of Chemical Education
and compare amounts of heat (Does i t matter how long the candle burns?), they also often fail to distinguish between heat and temperature. Once a candle is burning, the temperature of the flame remains constant, incorrectly suggesting to some students that no more heat is being produced. A steady state is incorrectly taken as static. In another example, almost all students classed the reaction of copper with oxygen as endothermic since copper oxide is only formed when a copper sheet is heated in a gas flame. In fact, the reaction is exothermic. Recognizing the learning difficultiesthat exist, we investigated the teaching problems associated with reactions in which activation energy plays no role and a spontaneous rise or fall in temperature can be observed. We encouraged our students to discover and analyze the heat effect as a feature of the reaction phenomenon. Once again, our main teaching tools were laboratory experiments together with a series of oven-ended ouestions. In this aonroach we amee with Phans k e that ~ experiment shouid not just i n k u c t students on how to imitate a chemist working in a lahoratory hut that it should he "more like adialog between the observer and the natural world around the observer". Furthermore, in a classroom situation we consider the social dimension an important aspect of the learning process of individual students. Therefore, our students carry out the experiments and answer the questions in small groups of three or four. Some of the experiments are in fact designed to foster cooperation and allocation of tasks among students. Most of the questions are desiened to stimulat~discussionsin which students formulate intuitive ideas, actively use new words in description and argumentation, and listen to each other.
-
An Experiment Flrsl From an educational point of view, chemical knowledge can he expressed schematically in terms of questions (Q), answers (A), and experiments (E). However, nothing has yet been said about who asks the questions and who is supposed to answer them. With this in mind, there are at least three possible ways to use practical (laboratory) work in chemistry lessons. (Reality is, of course, more complicated since several questions and answers are involved in one chemistry lesson; nevertheless. the wav in which an exveriment is used in actual teaching can often be recognized as one of these three possibilities. Other possible permutations of A, E, and Q are left out of consideration in this article.) In the first and most traditional approach, an experiment is introduced after a question has been asked and an answer has been provided: the Q-A-E sequence. The exveriment serves to eonfirm or to ill-ustrate the answer. I t shows that the teacher who provided it was right. This proves his expertise and underlines his authority. I t also shows that chemistry is a solid body of reliable knowledge, unless, of course, the experiment "fails", which, in this approach, should he avoided at all costs.
' De Vos, W.; Verdonk, A. H. J. Chem. Educ. 1985, 62,238.
De Vos, W.;Verdonk, A, H. J. Chem. Educ. 1985, 62,648. Phanstiel. 0.J. Chem. Educ. 1985, 62. 523.
In the second approach, the experiment is carried out after a question has been asked. The experiment is supposed to provide the answer: Q-E-A. A possible question might he: "What is this dark solid on the surface of a copper sheet?"or "How much acetic acid does this bottle of vinegar contain?" Experiments will probably show thatthe darksolid is copper oxide and that the vineear contains 4% of acetic acid. In fact the whole of chemical Galysis, qualitative as well as quantitative. is an a~nlicationof the Q-E-A anoroach to chemistrv. .. ". and it'is presented as such in cGemical education.' In another example of the Q-E-A approach, and experiment might show which of two possible explanations of an observed phenomenon is the better one. As a general result of this second approach, chemistry is to students as a steadily advancing science and a chemist as a specialist who knows how to findout thines. Althoueh we nrefir this second approach to the first one, w;? donottcink iiprovides the best conditions for learning new chemical concepts. Rather a third approach to teaching chemistry which starts in the lahoratorv "is referr . red. In this case. a teacher or a curriculum developer has designed an experiment that does not answer auestions but raises them: E-Q-A. . . or mavhe just E-Q or E,-Q:E~-A. An introductory experiment should not just produce unexpected or surprising results. Astonishment alone does not fool a student into learning. The experiment should challenge students, intellectually as well as emotionally. And, secondly, questions provoked by the experiment should be meaningful within the context of chemical education. Thev shouldnot iust occuov students' minds but also help themto learn somkthing w&hwhile. The formula E-Q-A does not mean that certain ouestions are asked by students automatically after they carried out certain experiments. This would require alevel of manipulation of students that we have not achieved and would not wish to achieve. I t does mean, however, that after students perform the experiment, they recognize certain questions as relevant and interesting. And indeed, thev do occasionallv ask these questions themselves. This approach draws the attention of students to one of the moreadventurous sides of practicing chemistry. I t shows that chemists do not just apply routine procedures to all sorts of standard nroblems b u t t h a t thevcan become confused and fascina'ted and even change 'their views in the course of their work. We have developed our experiments after analyzing a particular concept from the chemical point of view and carefully listening to students discussing problems relating to that concept. Manv of the useful ideas in our work stem from our students, even if theywere unaware of their contribution while they made it. Changing Temperatures Before carrying out the experiments described below, students have already developed some basic understanding of chemical reactions. They know that the name chemical reaction is given t o processes in which substances change into other substances. Thev will recoenize reactions involvine fire, like the combustion of magne&un or the decomposition of ammonium dichromate, as chemical reactions. either as a result of comparing the initial and the final situation or simply because of the spectacular nature of the process. However, heat effects and reactions with fire have not been dealt with in the course so far. As an introductory experiment, we choose the reaction of steel wwl in a copper sulfate solution. About 0.5 g of steel wool is pushed into a test tube containing 5 mL of 0.5 M copper sulfate solution. (Care should he taken not to push the steel wool too far down into the test tube; it should be
Pardue, H. L.; and Woo, J. J. Chem. Educ. 1984, 61,409.
uniformly distributed in the solution.) This reaction was chosen because it not only shows a temperature change, hut because i t also includes a number of familiar characteristics of a chemical reaction: the disappearance of the blue color and the formation of a brown . ~ r e c.i ~ i t a tStudents e. recoenize this as an example of a chemical reaction because they can easilv draw the oarallel to orevious reactions in which substanc& have be& converted into other substances, but a t the same time they feel the test tube becoming warmer. No thermometer is needed since the temperature rises by more than 10 OC. We prefer that students feel the change with their own hands, as this is a more direct experience. Thermometers should he available for those who do not believe their skin. We do not a t this point announce t o our students, "This ex~erimentshows that chemical reactions are accomoanied b;a heat effect." T o them, the experiment does ndt show this. Thev are not lookine for a eeneral statement. and thev have no reason to general&e ah&t chemical reactibns on thk basis of one particular experiment. First of all, they are interested in what this temperature rise has to do with the chemical reaction. When asked for their opinion on this matter, some groups of students call the rise in temperature a result or a product of the reaction whereas others seem to consider it as part of the reaction itself. Almost every group accepts that there is a relationship between the temperature rise and the reaction. Acceptance of this relationship is important, since in the next experiment the temperature change is not accompanied by any of the familiar signs of a reaction. This experiment, which is of crucial importance a t this stage of the course, involves the consecutive addition of eight 2-mL portions of 2 M hydrochloric acid to 10 mL of 2 M sodium hydroxide solution in a small flask. (As the chemical names tend to divert students' attention away from the experiment, the solutions are labelled "Solution of P" and "Solution of Q", respectively.) We warn our students not to touch or spill any of the solutions. Students use a thermometer to ascertain that both solutions are a t room temperature. When the first portion of hydrochloric acid is added, the temperature rises by 3 or 4 OC. No other chanees are observed. This sur~risesmost students and often persuades them to examine the solution in the flask carefullv. The subseauent additions of 2-mL portions of hydrochl&ic acid p r o d k e similar but somewhat smaller temperature increases until 10 mL has been added. When the sixth portion is added, the temperature drops suddenly. This seems to surprise most students just as much as the initial rise in temperature did. After carrying out the experiment, students are asked to collect their results in a tahle and to draw a diagram showing the temperature changes. A typical student's result is presented (see table and figure). When asked to explain such a result, most of our students, after some discussion. associate the temDerature chanees with a chemical reaction. But they may make this assoc
