problems in teaching the topic of redox reactions: actions and conceptions of chemistry teachers

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JOURNAL OF RESEARCH IN SCIENCE TEACHING VOL. 32, NO. 10, PP. 1097-1110 (1995) Problems in Teaching the Topic of Redox Reactions: Actions and Conceptions of Chemistry Teachers Onno De Jong, Jeannine Acampo, and Adri Verdonk Centre f o r Science and Mathematics Education, Department of Chemical Education, Utrecht University, 3584 CC Utrecht, The Netherlands Abstract Although there is growing interest in studies of teachers’ actions and conceptions, little is known about content-related teaching problems arising in science classrooms. This article presents a case study of problems which can occur when teaching the topic of redox reactions to Grade 11 students. Two chemistry teachers, a senior and a junior teacher, were involved in the study. Their reflective comments on the teaching problems were also investigated. Research data were obtained from classroom observations and audiotaped recordings of classroom practice. After the lessons, we conducted semistructured interviews with the teachers. The teaching problems are reported in terms of teaching activities causing difficulties for students in considering new conceptions to be necessary, intelligible, plausible, or fruitful. Analyses of the teachers’ comments on these teaching activities clarifies a number of reasons why they acted as they did. It can be concluded that teachers’ scientific expertise is an important source of difficulties when teaching redox reactions. Implications for an improvement of current chemistry classroom practice and content- related teacher training are offered. During the last 2 decades, a growing number of studies have focused on learning problems. A review of research in the latter area is given by Driver and Easley (1978) and Pfundt and Duit (1991). Many of these studies attempt to clarify causes of reported learning difficulties in terms of so-called preconceptions or, more broadly speaking, alternative frameworks of students. As yet, studies of teaching problems concerning science subject matter are scarce. Al- though there are a lot of studies concerning teachers’ actions and conceptions, documented by Clark and Peterson (1986) and White and Tisher (1986), most of the descriptions given are not content-related. Thus, little is known about what science teachers do and think and the kind of problems they encounter when teaching specific science concepts, rules, etc. This article presents a study focusing on teaching problems when teaching the topic of electrochemistry: more specifically, redox reactions. This topic was chosen because it appears to be difficult to learn and teach. According to a Dutch survey, students as well as their teachers ranked electrochemistry as one of the most difficult topics in the secondary school chemistry curriculum (De Jong, 1982). This opinion is echoed by students and teachers in several other countries, e.g., the United Kingdom (Bojczuk, 1982), North America (Finley, Stewart, & Yarroch, 1982) and Australia (Butts & Smith, 1987). 0 1995 by the National Association for Research in Science Teaching Published by John Wiley & Sons, Inc. CCC 0022-4308/95/ 101097-14

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Page 1: Problems in Teaching the Topic of Redox Reactions: Actions and Conceptions of Chemistry Teachers

JOURNAL OF RESEARCH IN SCIENCE TEACHING VOL. 32, NO. 10, PP. 1097-1110 (1995)

Problems in Teaching the Topic of Redox Reactions: Actions and Conceptions of Chemistry Teachers

Onno De Jong, Jeannine Acampo, and Adri Verdonk

Centre for Science and Mathematics Education, Department of Chemical Education, Utrecht University, 3584 CC Utrecht, The Netherlands

Abstract

Although there is growing interest in studies of teachers’ actions and conceptions, little is known about content-related teaching problems arising in science classrooms. This article presents a case study of problems which can occur when teaching the topic of redox reactions to Grade 11 students. Two chemistry teachers, a senior and a junior teacher, were involved in the study. Their reflective comments on the teaching problems were also investigated. Research data were obtained from classroom observations and audiotaped recordings of classroom practice. After the lessons, we conducted semistructured interviews with the teachers. The teaching problems are reported in terms of teaching activities causing difficulties for students in considering new conceptions to be necessary, intelligible, plausible, or fruitful. Analyses of the teachers’ comments on these teaching activities clarifies a number of reasons why they acted as they did. It can be concluded that teachers’ scientific expertise is an important source of difficulties when teaching redox reactions. Implications for an improvement of current chemistry classroom practice and content- related teacher training are offered.

During the last 2 decades, a growing number of studies have focused on learning problems. A review of research in the latter area is given by Driver and Easley (1978) and Pfundt and Duit (1991). Many of these studies attempt to clarify causes of reported learning difficulties in terms of so-called preconceptions or, more broadly speaking, alternative frameworks of students.

As yet, studies of teaching problems concerning science subject matter are scarce. Al- though there are a lot of studies concerning teachers’ actions and conceptions, documented by Clark and Peterson (1986) and White and Tisher (1986), most of the descriptions given are not content-related. Thus, little is known about what science teachers do and think and the kind of problems they encounter when teaching specific science concepts, rules, etc.

This article presents a study focusing on teaching problems when teaching the topic of electrochemistry: more specifically, redox reactions. This topic was chosen because it appears to be difficult to learn and teach. According to a Dutch survey, students as well as their teachers ranked electrochemistry as one of the most difficult topics in the secondary school chemistry curriculum (De Jong, 1982). This opinion is echoed by students and teachers in several other countries, e.g., the United Kingdom (Bojczuk, 1982), North America (Finley, Stewart, & Yarroch, 1982) and Australia (Butts & Smith, 1987).

0 1995 by the National Association for Research in Science Teaching Published by John Wiley & Sons, Inc. CCC 0022-4308/95/ 101097-14

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1098 DE JONG, ACAMPO, AND VERDONK

Important studies of specific learning problems concerning electrochemistry were reported in the United Kingdom (Allsop & George, 1982), Germany (Sumfleth & Todtenhaupt, 1988), Spain (Barral, Fernandez, & Otero, 1992), and Australia (Garnett & Treagust, 1992a, 1992b). On the topic of redox reactions, these studies together indicate that the main learning problems concern concepts as well as procedures. Concepts evoking learning problems appeared to be the relative strength of oxidizing and reducing agents and the concept of oxidation number. Proce- dures evoking learning problems appeared to be classifying reactions as examples of redox reactions and the balancing of complex redox equations.

Studies of specific teaching problems concerning the topic of electrochemistry have rarely been carried out. The present study is intended to reduce this need by reporting teaching problems that can occur when teaching the topic of redox reactions.

Science teaching activities can be described in general terms as actions to facilitate subject matter learning. According to constructivist ideas of the acquisition of knowledge (see, e.g., Driver, 1989; Osborne & Wittrock, 1983), learning activities require an active attitude on the part of students and involve a change in their conceptions. Viewed from this perspective, teaching activities should create conditions enabling conceptual changes. Some of these condi- tions have been elaborated by Posner, Strike, Hewson, & Gertzog (1982), who developed a model of conceptual change involving four major conditions of learning:

1 . A new conception has to become necessary to students, because they are dissatisfied with existing conceptions. For example, students learn that their existing conceptions are not adequate for describing, explaining, or predicting new phenomena.

2. A new conception has to become intelligible to students. That is to say, they learn that a new conception has a certain meaning and internal coherence which can be expressed by an accompanying terminology.

3. A new conception has to become plausible to students. For example, students learn that a new conception is true within a certain context and consistent with existing conceptions in that context.

4. A new conception has to becomefruitful to students. In other words, learners think that a new conception achieves something of value for them and that its use suggests new possibilities and applications.

The model of conceptual change was described by Posner et al. (1 982) in general terms, thus restricting its value for classroom practice. For example, it does not directly define the specific activities that should be undertaken by students or teachers during classroom transac- tions. Although some other criticism is also possible (see e.g., Hewson & Thorley, 1989), we consider the four conditions to be appropriate for our study, because these conditions can be used as criteria in assessing science teaching. Science teaching activities causing difficulties for students in considering new conceptions to be necessary, intelligible, plausible, or fruitful can be defined as science teaching problems.

In the case of teaching the topic of redox reactions, the following question can be formu- lated: Which content-related teaching problems occur when teaching redox reactions?

Teachers’ activities are related to teachers’ conceptions and vice versa (Clark & Peterson, 1986). Teachers’ conceptions of their teaching role influence their classroom behavior, while reciprocally, their teaching activities influence their conceptions. From this point of view, a second research question can be formulated as follows: Which conceptions of teaching problems do chemistry teachers have when confronted with these content-related problems?

Providing knowledge on content-related teaching problems and teachers’ conceptions of these problems is important, because it can contribute to a better understanding of science

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PROBLEMS IN TEACHING REDOX REACTIONS 1099

teachers’ actions and beliefs, which in turn can contribute to an improvement of current chemis- try classroom practice.

Method

During the winter of 1991- 1992, we launched a research project focusing on teaching redox reactions in Dutch schools for VWO (pre-university education). The VWO curriculum includes 4-year chemistry courses (Grades 9 through 12). The topic of redox reactions is first taught to students of Grade 11 (age about 17).

The research was designed as a case study. Research data concerning content-related teaching problems were obtained from observations and audiotaped recordings of chemistry lessons. Research data concerning teachers’ conceptions were obtained from semistructured interviews with chemistry teachers.

Teachers Involved

Two chemistry teachers were involved in the project. These men were selected to represent a certain range of experience: a senior teacher and a junior teacher. The teachers had been teaching chemistry for 13 and 6 years, respectively. Their experience in teaching electrochemis- try was obtained during a period of 12 and 5 years, respectively. The senior teacher was using a common textbook (experienced with it for 12 years), while the junior teacher was using another common textbook (experienced with it for 1 year).

The topic of redox reactions was taught to classes numbering approximately 30 students.

Textbook Content

Regarding the topic of redox reactions, the textbooks used by the teachers contain roughly the same content. Basically, the (identical) sequence of introduction is: transfer of electrons between reactants; definition of oxidizing and reducing agent; balancing and adding (complex) half-equations; verifying and predicting redox reactions by means of a table of relative strength of oxidizing and reducing agents; industrial applications of oxidation and reduction.

One textbook introduces redox reactions (as transfer of electrons) making use of the Daniel1 cell; other electrochemical cells were not discussed in the chapter on redox reactions. The corresponding chapter from the other textbook makes no reference to cells.

Classroom Data Collection

From the beginning of both courses in redox reactions, the classroom activities were observed by one and the same researcher. Drawings and notes on the blackboard were copied into a notebook. Besides, all lessons of the senior as well as the junior teacher (18 and 17 lesson periods of 50 min each, respectively) were recorded on audiotape and transcribed for subsequent protocol analysis.

Analysis of Classroom Protocols

To analyze the classroom protocols, we used the following general procedure. All protocols were independently and tentatively read and analyzed by two of us. When necessary, the copies of notes and drawings on the blackboard were used. The provisional results were compared and

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discussed at length. In the case of differences of opinion, discussions were continued until consensus was reached about the results.

The analysis of classroom protocols focused on statements from the teachers and their students. First, a number of conceptual difficulties experienced by students were deduced from classroom discussions. The students’ main difficulties were classified into the four categories of Posner’s model of conceptual change. Subsequently, and as far as possible, the classified difficulties were related to teaching activities as deduced from classroom discussions and teach- ers’ monologues. Finally, for every category of learning difficulties, the two main categories of teaching activities were determined. These teaching activities were called teaching problems.

Interview Data Collection

Each teacher was interviewed twice, individually, by the researcher who had observed his lessons. The first interview was conducted halfway through the course, the second right after the end of the course. In both cases, the teachers were asked to describe their teaching strategies, explain their choices in the teaching process, and evaluate their classroom practice. To stimulate their reflection, some questions referred to protocols of episodes from their own classroom practice, involving teaching problems. The teachers’ answers were recorded on audiotape and later transcribed.

Analysis of Interview Protocols

To analyze the interview protocols, we used the same general procedure as described before. The results of the classroom analyses were used to examine teachers’ reflective com- ments on their own teaching activities.

Results Concerning Teaching Problems

Before presenting the results concerning teaching problems, we must notice that both teachers appeared to use more or less the same overall teaching strategy. Roughly speaking, they introduced new concepts and procedures by going through illustrative examples taken from the textbook, at the same time asking students several questions about the topic taught. Sometimes, the lectures and discussions alternated with inquiry-oriented and verification-type laboratory experiments, also taken from the textbook. Besides, students had to answer a lot of textbook questions, working in small groups or on their own. The teachers closely followed the textbook and to a large extent, the textbook sequencing of the topics. The observed strategy can be characterized as a daily routine for teachers and students.

Teaching redox reactions appeared to evoke a number of teaching problems. A specification of the main concepts and content-related procedures evoking these difficulties is given in Table 1.

The following sections briefly present the main teaching problems. The reported problems occurred more than once with each of the teachers. In each case, the description of a teaching problem is clarified by an illustrative classroom episode. At the end of the final section, a summary of the main results is given (Table 2).

Teaching Problems Regarding the Necessity of New Conceptions

The two main teaching activities failing to help students to consider new conceptions as necessary will be briefly elaborated on here.

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Table 1 Concepts and Content-Related Procedures Evoking Teaching Problems

Transfer of electrons Identifying reactants as oxidizing or reducing agents Oxidation number and its values Balancing complex redox equations Relative strength of oxidizing and reducing agents

In the first place, the teachers introduced a concept rarely used by the students: the concept of oxidation number. Although this concept was not in the textbooks, because it is not included in the Dutch chemistry curriculum, the teachers did discuss it. They suggested students look on it as a tool which could help to identify reactions as redox reactions and balance complex redox equations. However, they omitted to offer students serious opportunities to explore the necessity of using the new concept. A classroom example will clarify this point:

A teacher defines oxidation number as a kind of formal charge of particles. Accord- ing to him, this concept can be used to identify reactions as redox reactions. Subsequently, he offers his students a number of reactions for identification. Some reactions include just ions or ionic compounds. All other reactions are combustion reactions. In the first case, students compare charges of ions. In the second case, students argue that combustion reactions are a type of redox reactions by definition. In both cases, students do not use the oxidation number but they apply their existing knowledge of previous curriculum con- tents.

The teacher also proposes to use oxidation number as a tool to balance complex redox reactions. Later on, he offers an algorithmic procedure not including the oxidation number (see Teaching Problems Regarding the Fruitfulness of New Conceptions). In this case too, students do not use the oxidation number.

By offering unsuitable problems, the teacher appeared not to stimulate students to consider the new conception as necessary.

Table 2 Summary of Teaching Problems Concerning Redox Reactions

Characteristics of new conceptions Teaching problems

Necessity Offering unsuitable problems Presenting superfluous explanations

Intelligibility

Plausibility

Fruitfulness

Prematurely formulating intended conceptions Using a confusing terminology

Underexposing the importance of contexts Ignoring alternative conceptions of students

Minimally talking about (industrial) applications Overemphasizing procedures of experts

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A second category of teaching problems is presenting superfluous explanations for students. This happened particularly when explaining the concept of transfer of electrons. The teachers failed to trigger a need for the students to change their conceptions. A classroom example illustrates this point:

A teacher has introduced two definitions of redox reaction: a class of reactions between a reactant and oxygen, and a class of reactions including the transfer of electrons. The second meaning is clarified by discussing reactions between metals and halogens by which ionic compounds are formed. Subsequently, the teacher asks if the reaction: 2 H, + 0, -+ 2 H,O is a redox reaction. The students’ answer is yes, because it is a reaction including oxygen as a reactant. The teacher is not satisfied with this answer. He presents an explanation of transfer of electrons in terms of electronic configurations and Lewis structures of H,, 0,, and H,O. After a short while, students report they do not need the given explanation.

To interpret this episode, it is important to know that the concepts of electronic configura- tions and Lewis structures have been taught earlier, but the given explanation was not described in the textbook chapter on redox reactions. The given episode indicates that the teacher’s explanation failed to stimulate students to look around for a new conception of the transfer of electrons.

Teaching Problems Regarding the lntelligibility of New Conceptions

The two main teaching activities that failed to stimulate students to consider new concep- tions as intelligible will be briefly presented next.

First of all, whenever the teachers wanted to introduce a concept by conducting an experi- ment, they prematurely told the students what they were supposed to observe and which explanations corresponded to these observations. This happened especially when the teachers carried out experiments regarding the relative strength of oxidizing and reducing agents. In many of these cases, the teachers did not particularly stimulate their students to observe phe- nomena by themselves and to develop their own explanations.

By prematurely formulating the intended conceptions, the teachers appeared to cause a lot of students’ failure to follow the teachers’ talk, and failed to help students understand new concepts. A clarifying classroom example follows:

A teacher carries out two experiments by which the relative reducing strength of Fe, Cu, and Zn can be determined. First, the teacher puts an iron nail into a solution of copper sulfate and then waits a while. The following classroom discussion takes place:

Teacher: Let us have a look , . . can we see anything . . . and what do you see?

Teacher: The nail becomes brown. In other words, it appears copper is formed. Student 1: Fur on the nail.

For that reason, copper ions are able to react with the iron.

Subsequently, the teacher repeats the experiment using a solution of zinc sulfate instead of copper sulfate.

Teacher: The nail in the zinc sulfate? Student 2: It is smaller. Student 3: Its color seems to be lighter.

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PROBLEMS IN TEACHING REDOX REACTIONS 1103

Teacher: So, nothing happens. For that reason, the zinc two plus ion is not able to react with iron (. . .) We can set up a very beautiful sequence of the metals . . .

With regard to the first experiment, the teacher writes down the half-equations: Cu2+ + 2e- + Cu and Fe + Fe2+ + 2e-. Regarding the second experiment, the teacher only notes a cross on the blackboard. Subsequently, he proposes the row Zn, Fe, Cu as a sequence of reductants.

Students: It is difficult to understand this sequence.

This classroom episode indicates that students’ observations were overruled by the teacher’s statements. Differences between the observations were not discussed. Besides, students did not get the opportunity to interpret the observed phenomena in terms of half-reactions. The teacher at once gave his explanations and conclusions. For the students, this teaching approach caused difficulties in seeing the proposed sequence of the metals as reducing agents. The teacher’s activities did not help the students to learn the meaning of a new conception concerning reducing agents by trying to develop this meaning on their own.

A second category of teaching problems consists of using a set of words and expressions which are insufficiently precise for students. This was particularly the case when describing oxidizing or reducing agents. In many cases, the teachers talked about substances while mean- ing particles (and vice versa). Besides, they often used the name of a certain oxidizing (or reducing) agent without specifying that agent, either as a substance or as a (charged) particle.

Teachers’ imprecise terminology caused a lot of confusion among students and did not facilitate learning processes concerning the identification of reactants as oxidizing or reducing agents. An illustrative classroom episode follows:

A teacher has just defined oxidants and reductants in terms of the transfer of elec- trons. Subsequently, he carries out an experiment involving the Daniel1 cell (galvanic cell: Cu2+ should gain electrons and Zn loses them). The following classroom discussion takes place:

Teacher: It appears that copper is formed and zinc is dissolved. Well, which is the oxidant, Marit?

Teacher: Are gained, therefore . . .

Teacher: Copper is the oxidant . . . and therefore, zinc is . . .

Marit: Uh . , . they are gained . . . they are gained . . .

Marit: Therefore . . . zinc . . . copper . . .

Marit: Reductant. [Later on]

Marit: But why, why is copper the oxidant at this point? And not the reverse? Teacher: Zinc prefers to lose its electrons; it appears that copper does not do so.

Copper appears to prefer gaining electrons.

In this classroom episode, the teacher used the name copper in several statements. By saying “copper is formed,” the teacher refers implicitly to the formation of a certain substance or certain type of atoms. However, the later statements “copper is the oxidant” and “copper appears to prefer gaining electrons” imply that “copper” is used in the sense of a certain type of positive ion. For the student, this implicit difference in meaning of the term “copper” did not help her to identify this reactant as an oxidizing agent. When these teachers talked about oxidizing and reducing agents, they used expressions such as “the oxidant is reduced” and “the

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reductant is oxidized.” The linguistic complexity of these statements also appeared to cause students confusion regarding the meaning of both kinds of reactants.

Teaching Problems Regarding the Plausibility of New Conceptions

The two main teaching activities that failed to facilitate learning concerning the plausibility of new conceptions will now be dealt with briefly.

First, the teachers introduced concepts without sufficiently exposing their meaning within the particular context in which they were used. This occurred particularly when discussing the concept of oxidation number. Students considered these numbers to be values of ion charges, instead of numbers with a formal meaning, which can be used to balance redox equations. According to them, the formal meaning was not plausible with nonionic compounds. For that reason, they experienced difficulties when balancing redox equations including molecular com- pounds such as SO, or PH,.

By underemphasizing the importance of a particular context to the formation of the meaning of a concept, the teachers appeared not to facilitate learning processes concerning values of the oxidation number of a specific entity. The following classroom episode will better clarify this point:

Talking about H,O, a teacher claims that the oxidation number of H is + 1 and 0 is -2. Students say they agree with these values, because they see a strong similarity to charges of H+ and 02- ions.

Subsequently, the teacher discusses H,O, and claims a changed value of the oxida- tion number of 0, namely - 1 (H remains + I ) . Some students disagree with these changes of value, supposing that both oxidation numbers have fixed values, like the values of the ion charges of H and 0. Other students say that a different change of value is also possible: the oxidation number of H changes to +2 (0 remains -2).

This classroom episode indicates that students became confused by the teacher’s proposi- tions about the values of the oxidation number of hydrogen and oxygen. Claiming these values without convincing arguments did not help students to accept the values in question.

A second category of teaching problems is ignoring student proposals that are plausible to them, but not to the teacher. This often happened in the case of predicting redox reactions by using the relative strength of oxidants and reductants. The teachers used the answers and suggestions of some students without paying attention to those of others.

Ignoring alternative conceptions of students appeared not to stimulate students to discuss their own conceptions to learn about the plausibility of these conceptions. An illustrative classroom episode follows:

A teacher asks his students to predict the reaction which will occur when a piece of iron is added to a solution of copper sulfate (the experiment in question is lacking). Some students yell that a redox reaction between Fe and Cuz+ will occur, by which Fez+ and Cu are formed. Other students yell that Fe3+ will be formed instead of Fez+.

Subsequently, the teacher writes a reaction equation on the blackboard corresponding to the first given answer, without paying attention to the alternative suggestion. The students who suggested Fe3+ instead of Fez+ do not comment on this action of the teacher.

To interpret this episode correctly, it is relevant to know that a list of relative strengths of oxidants and reductants (format: paired half-equations) is only offered some lessons later. For

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PROBLEMS IN TEACHING REDOX REACTIONS 1105

that reason, students would be unable to select the right combination out of the two possibilities: Fe/Fe2+ or Fe/Fe3+. This classroom episode indicates that the teacher did not help students to determine the plausibility of their conceptions by providing relevant data.

Teaching Problems Regarding the Fruitjiulness of New Conceptions

The two main teaching activities that do not help students to consider new conceptions as fruitful will now be described briefly.

First, the teachers tended to neglect important applications of electrochemical concepts in the field of technology and society, such as the industrial production of steel. This category of applications is described in the textbooks, although very much simplified. In many cases, the teachers paid attention only to the chemical core of a textbook section and hardly discussed the value of knowing about the redox reactions. An illustrative classroom episode follows:

A teacher says to his students that to a large extent, the textbook section on the industrial applications of redox reactions can be skipped. This section describes the production of iron in blast furnaces, as well as the large-scale production of sulfuric acid using sulfidic ores as one of the main raw materials. The teacher merely points out some formulas and reaction equations. Textbook questions accompanying the section content are also skipped.

Some lessons later, students remark that they fail to see the importance of most of the electrochemical concepts they have to learn.

Only minimally talking about (industrial) applications of electrochemical concepts ap- peared not to have stimulated students to explore the wider sense of conceptions of redox reactions.

A second category of teaching problems is overemphasizing the importance of using (algorithmic) procedures as found in textbooks, specifically those with which to balance com- plex redox half-equations. The teachers informed their students that to a large extent, the textbook procedures correspond to their own favorite ways of problem solving. They prescribed these procedures to their students.

Overemphasizing procedures of experts, such as teachers or textbook authors, appeared not to convince students of the value of the prescribed procedures. Again, an illustrative episode follows:

A teacher prescribes the following algorithm for balancing complex half-equations, such as Mn0,- + 8H+ + 5e- -+ MnZ+ + 4H20. He tells his students that the first step is writing down the skeleton half-equation (Mn0,- + MnZ+). The second step is balancing the 0 by adding H,O. The third step is balancing the H by adding H+ . The fourth step is balancing the charge by adding e-. In case of neutral or basic solution, there is a fifth step: adding OH- (which combines with the H+ to H20).

According to students, adaptations may make this procedure more attractive. One adaptation is to put an extra step right after step 1, namely, balancing the charge by adding e- (Mn0,- Mn2+ + 3e-). Subsequently, all of the other steps, including the original Step 4, are executed. Another adaptation is taking together Step 5 (adding OH-) and Step 3 (adding H+) to form a single step (adding OH- or H+). Both student procedures lead to success, but are not adopted by the teacher.

This episode indicates that students preferred to use their own ways of balancing complex half-equations. Both present students’ procedures involve a differing set of steps. One of the

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procedures focuses on balancing charge (by adding e-) at a much earlier stage. Implicitly, this setup strongly emphasizes one of the central characteristics of redox reactions: the transfer of electrons. The other student procedure involves one single step concerning characteristics of the solution in question (acid, neutral, basic), which requires two steps in the teacher’s approach. Implicitly, the students’ approach strongly emphasizes the necessity of a coherent view of the reaction system. The mechanistic and administrative procedure of the teacher was less attractive to students. The teacher’s prescription was not considered to be a fruitful approach to balancing complex redox half-equations.

Results Concerning Teachers’ Conceptions of Teaching Problems

The Necessity of New Conceptions

Concerning the problems that occurred regarding the necessity of new conceptions, teach- ers’ comments on their own teaching activities can be characterized as ambivalent.

Regarding setting students unsuitable problem-solving tasks, the teachers made two kinds of remarks. On the one hand, they said they introduce oxidation numbers because they consider this concept to be a useful tool in identifying redox reactions and balancing redox equations. On the other hand, they said it is artificial to use the concept in the case of problems which can also be solved without this.

Regarding the problem of presenting superfluous explanations, the teachers also made two kinds of remarks. On the one hand, they claimed it is necessary to teach the subject matter in such a way so that students gain insight into concepts on a level as scientifically high as possible in secondary-school chemistry classes. Such a level implies the use of formal scientific models such as electronic configurations and Lewis structures. On the other hand, they recognized students’ difficulties in accepting the necessity of using concepts at this intended level. The teachers said that helping students with this aspect is not easy for them.

The Intelligibility of New Conceptions

Comments on teaching problems regarding the intelligibility of new conceptions can be summarized thus: Some teaching activities are developed into self-evident routines without much reflection.

On the problem of prematurely formulating teacher conceptions, the teachers said they have made it a habit to offer students their own observations and explanations. According to the teachers, this teaching activity prevents students from developing conceptions that differ from those intended by the teachers.

Regarding the problem of using confusing terminology, the teachers remarked that describ- ing concepts and procedures in a shortened way is a daily routine for them, because this is usual for the experts they consider themselves to be. They also said that they should have to use more precise terminology.

The Plausibility of New Conceptions

The comments on teaching problems arising with respect to the plausibility of new concep- tions can be described thus: There are circumstances beyond the teachers’ control.

On the problem of underemphasizing the importance of contexts to the meaning of con- cepts, the teachers blamed a lack of time. Besides, they said, it is often difficult for them to

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PROBLEMS IN TEACHING REDOX REACTIONS 1107

formulate proper meanings of concepts, because these meanings should be scientifically valid as well as acceptable to their students.

Concerning the problem of ignoring alternative conceptions put forward by students, they also blamed a lack of time for being unable to respond seriously to every proposal from every student.

Fruitjiulness of New Conceptions

The comments on teaching problems arising on the fruitfulness of new conceptions re- flected two kinds of arguments.

Regarding the problem of hardly talking about applications of electrochemical concepts, the teachers gave their opinion that chemical facts as described in the textbooks are the hard core. As a consequence, these facts are more important to learn than, for example, domestic or industrial applications. They also pointed out that it is mainly knowledge of chemical content which is required when solving problems in tests-for example, in the final examination.

On the problem of overemphasizing expert procedures, they argued that these procedures are important to teach, because they can function as standard algorithms and give students something to hold on to. According to the teachers, by starting to teach expert procedures at an early stage, fruitful habit formation can be developed.

Discussion

The present results confirm that the topic of redox reactions is difficult not only to learn, but also to teach. The study reveals a number of specific teaching problems arising in the lessons of two chemistry teachers. These problems together represent a wide variety of teaching activities. These activities can be classified into four categories by relating them to the four conditions of learning postulated by Posner et al. (1982). Some teaching activities do not facilitate learning about the necessity of new conceptions. Other teaching activities do not facilitate learning about new conceptions’ intelligibility, plausibility, or fruitfulness. It is remarkable that not every reported teaching problem is recognized as a problem by the teachers. Their comments indicate that they are largely unaware of teaching problems in nearly half the cases. Specifically, they are not very conscious of problems in the case of prematurely formulating their own conceptions, hardly talking about applications and overemphasizing expert procedures. Therefore, not every teaching problem is a teacher problem. This conclusion underlines the need for content-related teacher training to make teachers become conscious of the reported teaching problems.

It is also remarkable that one of the teaching problems is in teaching a concept that is supposed not to be taught, namely, oxidation numbers. More than 10 years ago, this concept was removed from the Dutch secondary curriculum (and textbooks), because according to the curriculum development agency, it was superfluous. Nevertheless, both teachers still introduced this concept into their classrooms. Their teaching decision could be explained by their need to teach subject matter at a level conceived by them to be academic and at the same time appropri- ate to pre-university education. However, in this case it appears that they underestimated difficulties in teaching oxidation numbers, specifically the difficulty in clarifying the need to use this number and the plausibility of its values for specific entities.

The incidence of the reported teaching problems can be explained in several ways. As far as the teachers are aware of any teaching problems, they have pointed out a number of causes. Their explanations concern external causes such as a lack of time, and internal causes such as a certain lack of competence to help their students to an intended level of insight.

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In our opinion, an important explanation also covering difficulties not noticed by the teachers refers to teachers’ subject matter expertise. Based on their expertise in chemistry, the teachers tended to teach the topic of redox reaction at as high a level as possible. Based on their expertise in school chemistry, i.e., chemistry as described in curricula and textbooks, the teachers tended to pay little attention to students’ suggestions on the meanings of concepts and students’ procedures in solving problems. To the teachers, concepts such as the transfer of electrons, or oxidation numbers, or a procedure such as balancing redox equations have become self-evident. Their knowledge of concepts and experience with procedures has accumulated during a long period of learning (school) chemistry. However, this does not apply to students. As novices, they reason and act within another frame of reference. To be conscious of novices’ conceptions is not something that comes easily to experts. For that reason, teachers’ subject matter expertise can be considered an important source of difficulty in teaching science (De Jong, 1992a).

This study is the first exploration into the unknown field of problems in teaching redox reactions. Although the research was designed to be a case study involving two Dutch chemistry teachers, there is some evidence supporting the external validity of the results. First, it appears that the concepts and content-related procedures evoking teaching problems correspond largely with difficult concepts and procedures reported in studies from other countries (see the introduc- tory section). Second, it appears that most of the present comments of both teachers fit teachers’ conceptions of science teaching as reported in other studies. For example, in a case study of classroom practice of two British science teachers, Johnston (199 I ) found both teachers support- ing the notion that for meaningful learning to occur, students must think through the issues for themselves. However, there were indications in their actions that they did not always seem to value the students’ own ideas, and saw these as barriers on the way to correct knowledge. In a questionnaire-based study of the conceptions of about 350 Portuguese science teachers, Sequira, Leite, and Duarte (1993) found that about one half of this group had ambiguous views on the issue of students’ preconceptions and their importance to their teaching.

The results of these studies largely fit our report of (ambivalent) conceptions of the two teachers regarding their own classroom practices, especially the activities of presenting super- fluous explanations, ignoring alternative conceptions of students, and prematurely formulating their own conceptions. The close correspondence outlined before suggests that the teaching problems and teachers’ comments reported in the present study can be conceived of as represen- tative examples of chemistry teachers’ activities and beliefs.

Implications and Recommendations

The present study underlines the importance of improving current chemistry school practice and accompanying content-related teacher training. Regarding current classroom practice, the following guidelines can be recommended:

1. Teachers should omit to use the concept of oxidation number (as far as possible). Insofar as the concept of oxidation numbers is not present in national curricula, such as in the Dutch curriculum, teachers should no longer use this concept. In the case of teaching the identification of reactions as redox reactions, this requires no more than a set of adequate reactions, such as combustion reactions or reactions including ions or ionic compounds in which ion charges change. In the case of teaching a procedure to balance redox equations, this requires an adequate set of rules involving the principles of conservation of charge and conservation of element.

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In many countries, the chemistry curriculum does include oxidation numbers. In that case, we recommend removing this concept from the curriculum. As the Dutch situation has pointed out, core electrochemical concepts and procedures can be taught without using oxidation numbers.

2. Teachers should know much more about students’ conceptions of the transfer of electrons and relative strength of oxidants and reductants. Teachers should observe students’ learning activities and listen to students’ statements with an open mind, as much as possible. Listening and observing students can be facilitated by putting students into small working groups. Teachers should use the acquired knowledge to adapt their existing teaching activities, to improve the clarification of necessity, intel- ligibility, and fruitfulness of core conceptions.

3. Teachers should encourage students to develop their own ways of balancing redox equations. Teachers should pay much more attention to students’ procedures in solving problems, especially ways of balancing redox (half-) equations. To students, it will be much easier to understand the fruitfulness of a procedure they themselves have devel- oped instead of one prescribed by the teacher. Thus, students should be encouraged to develop their own procedures. To that purpose, teachers should organize discussions in small groups, followed by a final plenary session.

4. Teachers should pay ample attention to the industrial applications of redox reactions. This activity can contribute to clarify the fruitfulness of electrochemical concepts and procedures. One teaching option is to start off the topic of redox reactions by discuss- ing industrial processes, such as the production of steel, or by carrying out experiments involving a process such as galvanizing iron objects.

5. Teachers should become (more) aware of their own teaching beliefs and practices regarding teaching redox reactions. Teachers should reflect on their own conceptions as well as their own practice. This reflection can be facilitated by writing down important experiences and problems concerning one’s own lessons and discussing notes with colleagues.

To help teachers carry out these recommendations, it is important to offer them an appro- priately designed inservice course in teaching redox reactions. Such teacher training should take into account the main results of this study. Besides, the course should exclude a directive training approach in favor of active teacher participation. This means that the course should have the characteristics of so-called interactive teacher training (De Jong, 1992b).

It is important not only to develop a course for chemistry teachers as described before, but also to investigate the impact of such a course on teachers. Specifically, research can focus on teachers’ learning processes in changing some teaching conceptions and reconstructing a num- ber of teaching activities. The results of this research could contribute to the improvement of teaching redox reactions.

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Received April 11, 1994 Revised June 5, 1995 Accepted June 7, 1995