Research: Science and Education
Conceptual Difficulties Experienced by Prospective Teachers in Electrochemistry: Half-Cell Potential, Cell Potential, and Chemical and Electrochemical Equilibrium in Galvanic Cells Ali Rıza Özkaya Department of Science and Mathematics Education, Atatürk Faculty of Education, Marmara University, 81040 Kadıköy, Istanbul, Turkey;
[email protected] Science educators are paying increasing attention to students’ understanding of scientific concepts. Terms such as preconceptions, children’s science, intuitive beliefs, alternative frameworks, students’ errors, and misconceptions have evolved to describe this understanding. Students at all levels and even science teachers hold misconceptions: conceptual and propositional knowledge that is inconsistent with or different from the scientific consensus and is unable to adequately explain observable scientific phenomena (1, 2). There is growing interest in the field of electrochemical misconceptions. Students and teachers in several countries (The United Kingdom [3], North America [4 ], Australia [5], and The Netherlands [6 ]) ranked electrochemistry as one of the most difficult topics in chemistry. Allsop and George reported that students have difficulty using standard reduction potentials to predict chemical reactions (7). Birss and Truax noted that students who learn electrochemistry from most high-school and first-year university textbooks are likely to experience confusion on this subject (8). They also discussed the most important problems students are likely to encounter. Garnett, Garnett, and Treagust discussed students’ understanding of electrochemistry, with the aim of improving science curricula (9). In subsequent articles, Garnett and Treagust identified common misconceptions about oxidation– reduction reactions, electric circuits, and galvanic and electrolytic cells by using student interviews, and discussed some probable origins of these misconceptions (10, 11). Ogude and Bradley found that although many students can solve the quantitative electrochemical problems on exams, few are able to answer qualitative questions requiring a deeper conceptual knowledge of electrochemistry (12). Sanger and Greenbowe (13) replicated Garnett and Treagust’s interviews (11) on galvanic and electrolytic cells and extended them by addressing student misconceptions about concentration cells. Sanger and Greenbowe analyzed college chemistry textbooks as sources of misconceptions and errors in electrochemistry (14). Overview of the Study Previous studies do not investigate electrochemical concepts such as chemical equilibrium, electrochemical equilibrium, and the instrumental requirements for the measurement of cell potential or electromotive force (emf). Studies of prospective teachers are also scarce (15, 16 ), although there have been many reports on conceptions of students at different levels about various chemistry topics. Therefore a study was designed to identify previously reported and new electrochemical misconceptions of prospective teachers, taking into account the uninvestigated concepts. It was conducted with 92 prospective teachers who were students in the last-year class at Marmara
University, Atatürk Faculty of Education and had received both classroom and laboratory instruction on electrochemistry for about 3.5 months (3 hours per week for both classroom and laboratory instruction). Fifteen volunteers from among the 92 prospective teachers were initially interviewed for 40–45 minutes, using the protocol of Garnett and Treagust (11) to which the following questions were added: 1. How is the emf of a cell measured? Is it possible to use an ordinary voltmeter to measure the emf of a galvanic cell precisely? 2. Under what conditions is an electrochemical equilibrium established in a galvanic cell? 3. Under what conditions is a chemical equilibrium established between the species involving in the cell reaction in a galvanic cell? 4. What happens when a metal is immersed into an electrolyte solution involving its ions?
After the interviews, a test of 27 multiple-choice questions consisting of assertion–reason statements and a set of true–false alternative answers was administrated to all 92 subjects. Some of the assertion–reason statements were designed to test for previously reported misconceptions. The results showed that students from different countries and different levels of electrochemistry have common misconceptions and suggested that concepts were presented to them poorly (unpublished). Other assertion–reason statements were based on the subjects’ scientifically incorrect responses during the interviews. This made possible the identification of conceptual difficulties not previously reported. This paper addresses the prospective teachers’ learning difficulties related to the previously uninvestigated concepts. It illustrates how scientifically incorrect ideas expressed by students during interviews can be used to identify their conceptual difficulties and discusses likely sources of the conceptual difficulties experienced by the subjects. Half-Cell Potential, Cell Potential, and Their Measurement Some questions in the 27-item test were designed to probe the prospective teachers’ understanding of electrode potential, cell emf, and the instrumental requirements for the measurement of these potentials.1 The results from the analysis of these questions are explained below. Question 3 and the percentage of prospective teachers who chose each alternative in this item are given in Box 1. In the course of the interviews, all 15 subjects correctly stated that electrons flow from anode to cathode during the
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Research: Science and Education
Box 1. Question 3 and the Subjects’ Responses to It
Box 2. Previously Unreported Electrochemical Misconceptions a
Assertion Reason In a galvanic cell, the measured Electrons move from a potential difference between a region of high potential point at one electrode and a because to a region of lower popoint at the other electrode is tential in a galvanic cell solely due to differences in the charge density at the points. Statement 1 (Assertion)
Statement 2 (Reason)
a*
True
True
b*
True
True
Alternative
n
%
a
52
56.5
b
18
19.6
c
True
False
c
10
10.9
d
False
True
d
4
4.3
e
False
False
e*
8
8.7
*The difference between alternatives a and b was explained before the test. In both cases statements 1 and 2 are both true, but in a the reason statement correctly explains the assertion and in b it does not.
*Correct answer.
3e. In a galvanic cell, electrons move from a region of high potential to a region of lower potential. 3g. An ordinary voltmeter can be used to measure the emf of a galvanic cell precisely. 3h. There is no relation between the emf of a galvanic cell and maximum capacity of the cell to do electrical work. 9d. When an electrode is immersed into an electrolyte, an electrical double layer does not form at the interface between the electrolyte and the electrode immersed in it. 17a. There is not any difference between chemical and electrochemical equilibrium established in a galvanic cell. 17b. A current flow might be permitted through a galvanic cell in which a pair of half-cells is connected by a potentiometer. 17c. When a pair of half-cells is connected by a very high resistance voltmeter, a chemical equilibrium is established between the species involved in cell reaction. 17d. E ° values given in the table of standard reduction potentials do not refer to an electrochemical equilibrium.b 17e. There is not any relation between the chemical and electrochemical equilibrium. aThis system for numbering the misconceptions is consistent with that used by Garnett and Treagust (11) and Sanger and Greenbowe (13).
passage of a current through the external circuit in a galvanic cell. However, five of the 15 claimed that since mass transport occurs from a region of high concentration to a region of lower concentration and heat flows from a point of high temperature to a point of lower temperature, electrons also must move from a region of high potential to a region of lower potential. This scientifically incorrect idea was included in question 3 as the reason statement (Box 1). The results from the analysis of this question were consistent with the prospective teachers’ responses during the interviews. A significant proportion of the subjects marked either a or b. This led to the identification of a misconception not previously reported (3e, Box 2). It is evident that the prospective teachers had difficulty understanding the flow of electrons from a region of low potential to one of higher potential to make the potential difference between two halfcells equal. The assertion statement in question 3 was a previously reported misconception (10, 11). It also appeared as a misconception among the subjects of this study. They were not aware that the measured potential difference between a point at one electrode and a point at the other electrode or between two half-cells in a galvanic cell is due not only to the charge density at the points, but also to the potential differences between two metal–solution interfaces. It can be concluded that one of the difficulties experienced by the subjects of this study involved the concept of different potentials. Of the 15 subjects interviewed, six stated that an ordinary voltmeter can be used to measure the emf of a galvanic cell precisely. Therefore, a question with appropriate assertion– reason statements was designed on the basis of this alternative idea. The reason statement, “The emf of a galvanic cell is the potential difference between the electrodes under the conditions of no current flow”, was the scientific definition of cell emf. All but one of the 92 prospective teachers recognized this statement as true. However, 32 (35%) thought the assertion statement, “An ordinary voltmeter should not be used to measure the emf of a galvanic cell precisely”, was false; they held 736
b It was expected that a significant proportion of prospective teachers would demonstrate this misconception, but they did not.
misconception 3g (Box 2). Although during their practical work in electrochemistry laboratory they had used a potentiometer to measure the emf of a galvanic cell but an ordinary voltmeter to read the applied voltage between the electrodes of an electrolytic cell, 32 subjects appeared to be unaware of the difference in the function of these instruments in a cell circuit. Most of the university chemistry textbooks used by the subjects do not explain this difference. Moreover, these textbooks generally use the term “voltmeter” when discussing electric circuits related to the galvanic cells. This finding is consistent with results of previous research on electrochemical misconceptions (9, 12, 14), which show that a major source of student misconceptions is imprecise, insufficient, and inappropriate explanation of electrochemical concepts in textbooks. The third edition of a general chemistry textbook (17) contains the statement “The potentiometer or voltmeter measures the difference in electrical potential between the two electrodes” (italics added) in Figure 19-4, which illustrates the measurement of the emf of a cell. However, the sixth edition of the same textbook (18) uses only the term “voltmeter”, and Figure 21-3 states, “For precise measurements the amount of electric current drawn from the cell must be kept very small by using either a specially designed voltmeter or a device called a potentiometer.” This interesting example shows that it is important for textbook authors and instructors to use carefully chosen terminology to explain electrochemical processes. Use of the term “voltmeter” in cell circuits illustrating the measurement of cell emf may mislead students into believing that an ordinary voltmeter can be used for precise measurements of the cell emf. Therefore, it is strongly recommended the term potentiometer be used for circuits related to the measurement of cell emf.
Journal of Chemical Education • Vol. 79 No. 6 June 2002 • JChemEd.chem.wisc.edu
Research: Science and Education
Box 3. Questions 19 and 21 and the Subjects’ Responses to Them Question 19 Assertion There is not any difference between the chemical and because electrochemical equilibrium established in a galvanic cell
Reason Net reaction occurring in a galvanic cell is obtained by adding half-cell reactions which involve transfer of electron(s)
Question 21
Reason These values were determined under the conditions of no current flow by the use of a potentiometer
Assertion E ° values given in the table of standard reduction potentials refer to because an electrochemical equilibrium in which all reactants and products are in their standard states
For both questions, the multiple choice alternatives are the same as for question 3. Question 19 Alternative
n
%
Question 21 Alternative
n
%
a
33
35.9
a*
36
39.0
b
14
15.2
b
33
35.9
c d* e
0
c
17
18.5
43
0
46.7
d
3
3.3
2
2.2
e
3
3.3
*Correct answer.
Responses to another question showed that all prospective teachers were aware that when current flows, the potential difference between two half-cells is less than emf of the cell. Although nearly all of them were able to define emf correctly, 28% could not establish the relationship between the emf of a galvanic cell and the maximum capacity of the cell to do electrical work; they recognized as false the statement “Only the emf of a galvanic cell, measured under the conditions of no current flow, gives an indication of the maximum capacity of a cell to do electrical work.” This led to the identification of another misconception not previously reported (3h, Box 2). Half of the prospective teachers believed that when an electrode is immersed into an electrolyte, an electrical double layer does not form at the interface between the electrolyte and the electrode. Of the 46 who held this misconception (9d), 30 thought that it is not possible to measure a half-cell potential without using another half-cell. However, they had the unscientific idea that “If an electrical double layer had been formed at the interface between the electrode and electrolyte it would have been possible to measure a half-cell potential without using another half-cell.” These subjects appeared to have difficulty in understanding that the half-cell potential talked about in electrochemistry is the potential difference between the solution and the electrode, and this potential difference cannot be measured but the difference between two differences, or the potential difference between two half-cells, can. These findings suggest that textbooks and instructions in electrochemistry should offer some explanation of the origin of half-cell potentials (the interactions between the metal atoms on the electrode and the metal ions in solution, the electrical double layer, and the formation of a potential difference at two metal–solution interfaces during approach to equilibrium).
Chemical and Electrochemical Equilibrium Four questions were constructed to test the understanding of chemical and electrochemical equilibrium in galvanic cells. Two questions (19 and 21) and the percentage of the prospective teachers who chose each alternative in these items are given in Box 3. In the interviews, five of the 15 subjects thought there should be no difference between chemical and electrochemical equilibrium because the net reaction in a galvanic cell does not involve electrons and is obtained by adding the electrochemical half-cell reactions. Subsequently, 47 of the 92 subjects demonstrated misconception 17a (Box 2) by choosing either a or b in response to question 19. Birss and Truax stated (8) that a source of confusion for students arises because many texts show the half-reactions in the table of standard reduction potentials not as equilibria, but as net reactions. The half-cell reactions must shown as equilibria, as suggested by Birss and Truax, and this is the scientific consensus. However, according to the responses to question 21 (Box 3), the presentation of these reactions as net reactions was not a source of confusion in this study, since 86 of the 92 prospective teachers believed that E ° values given in the table of standard reduction potentials refer to an electrochemical equilibrium. The main difficulty experienced by the subjects in the concept of equilibrium was in distinguishing between the chemical and electrochemical equilibria. After the test, some subjects stated that they were informed about the concepts of electrochemical and chemical equilibrium in electrochemical cells, but about not the distinction between these concepts. During the interviews, there was widespread uncertainty about the relationships between the use of a potentiometer to measure the cell emf, the current flow in a cell, and chemical and electrochemical equilibrium. The results of the test were in accordance with the responses in the interviews. In one test question, 71 subjects implied that a current flow might be permitted through a galvanic cell in which a pair of half-cells is connected by a potentiometer (holding misconception 17b), although nearly all subjects were able to define the cell emf correctly. Comparison of the results for different questions indicated that the subjects had simply memorized the definition of emf. Seventy-five percent of the subjects answered that “A chemical equilibrium is established between the species involved in a cell reaction when a pair of half-cells is connected by a potentiometer.” This was regarded as misconception 17c. In response to another question, 75 subjects correctly stated that when a current is allowed to pass through a galvanic cell until the cell voltage is zero (i.e., when a chemical equilibrium is established), current no longer flows. However, 33 subjects assumed that chemical equilibrium is not a special instance of electrochemical equilibrium in galvanic cells. They seemed to believe that “There is no relationship between the chemical and electrochemical equilibrium” (misconception 17e). Therefore, another difficulty experienced by a significant proportion of the prospective teachers was in establishing the relationship between chemical and electrochemical equilibrium. The electrochemical misconceptions identified for the first time in this study are listed in Box 2. Garnett et al. (9–11) and Sanger and Greenbowe (13) discussed some probable origins of student misconceptions based on their electrochemistry interview
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studies. These include (i) inadequate prerequisite knowledge; (ii) misuse of everyday language in chemical situations; (iii) use of multiple definitions and models, and rote application of algorithms; (iv) ignorance of the relative nature of electrochemical potentials; and (v) imprecise and inappropriate language used by textbooks in explaining electrochemical concepts. Several reports state that chemistry and electrochemistry courses and all or most of the assessments of these courses in various countries are based on quantitative problem-solving (12, 19–21). The situation in Turkey is no exception. According to Sanger and Greenbowe (13), students believe that conceptual knowledge is not important or even necessary for success in chemistry. If instructors believe that conceptual knowledge is important, they need to teach and assess conceptual knowledge as well as problem-solving abilities. Nearly all prospective teachers had the same reaction to the assertion–reason questions with true–false alternatives in the 27-item test: these questions were considerably different from those they had faced in their electrochemistry course and exams. They stated that they were familiar with quantitative questions requiring problem-solving ability rather than with the assertion–reason-type questions. The failure to acquire conceptual knowledge of electrochemistry seems to be the main reason for learning difficulties. Since students from different countries and different levels of electrochemistry hold common misconceptions and concepts seem to have been poorly presented to them, the lack of conceptual knowledge appears to be worldwide. Because of these findings, I plan to apply a conceptualchange method to teach about galvanic and electrolytic cells in a university general chemistry course. An experimental group will be told about the known misconceptions and why these statements are considered incorrect. The students in this group will be asked some true–false questions constructed from statements based on the electrochemical misconceptions, to test their conceptual understanding rather than their problemsolving ability. A control group will be taught in the traditional manner. The achievement of the two groups in qualitative understanding of electrochemical processes will be compared statistically, to assess the ability of this instructional method to prevent and overcome misconceptions. Several reports (20, 22) have shown that when students are taught chemical processes conceptually and assessed accordingly their conceptual knowledge improves considerably. Summary A study of prospective teachers showed that students from different countries and different levels of electrochemistry hold common misconceptions and that concepts were poorly presented to them. Also, it identified new misconceptions by considering the electrochemical concepts of chemical equilibrium, electrochemical equilibrium, and the instrumental requirements for the measurement of cell potential, which were ignored in the relevant literature. This paper reports how scientifically incorrect ideas stated by prospective teachers during interviews were modified into assertion–reason-type questions and how these questions were used to identify
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previously unreported conceptual difficulties related to previously ignored concepts. The origins of the learning difficulties were attributed mainly to failure to acquire adequate conceptual knowledge about electrochemistry, and to the insufficient explanation of the relevant concepts in textbooks. The results of this study are consistent with the constructivist model of learning, which suggests that students construct new knowledge through their existing experiences and knowledge, and that some misconceptions may appear quite logical to them. A study of a teaching method to overcome the conceptual difficulties experienced by prospective teachers is planned. Note 1. Additional information is available from the author by request.
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Journal of Chemical Education • Vol. 79 No. 6 June 2002 • JChemEd.chem.wisc.edu
