Students Difficulties in Learning Electrochemistry

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1 Proc. Natl. Sci. Counc. ROC(D) Vol. 12, No. 3, pp Students Difficulties in Learning Electrochemistry HUANN-SHYANG LIN *, THOMAS C. YANG *, HOUN-LIN CHIU *, AND CHING-YANG CHOU ** *Department of Chemistry National Kaohsiung Normal University Kaohsiung, Taiwan, R.O.C. **Graduate Institute of Science Education National Kaohsiung Normal University Kaohsiung, Taiwan, R.O.C. (Received October 17, 2002; Accepted March 25, 2003) ABSTRACT The purpose of this study was to investigate the difficulties students have in learning electrochemistry. The answers on an open-ended test on electrochemistry obtained from 9th graders, 12th graders, and college chemistry major students were compared and analyzed. The results of frequency analyses of student misconceptions and data from interviews revealed that a larger percentage of college students had misconceptions about the function of electrolytes in an electrochemical cell than did 9th or 12th graders. This study indicates that advanced study in chemistry does not always result in better understanding of certain basic concepts. More importantly, if the concepts are not clearly explained, students misconceptions may become firm and remain during their continued study of chemistry even at the college level. It is also found that the students misconceptions likely result from wrong impression given by pictures and statements in textbooks and improper classroom instructions. Key Words: misconception, electrochemistry, science teaching I. Introduction Electrochemistry has been regarded as one of the most difficult subjects to learn for both students and teachers (Finley, Stewart, & Yarroch, 1982; Johnstone, 1980). As Butts & Smith (1987) indicated, secondary school students find that electrochemical cells and electrolytic cells are very difficult for them to understand since these topics involve concepts about electricity and oxidation-reduction, both of which are very challenging. If chemistry teachers can diagnose students difficulties in learning, and the origins of their misconceptions, then their teaching effectiveness could be greatly improved. In the 1980s, quite a few studies focused on the assessment of student misconceptions in chemistry (Hackling & Garnett, 1985; Gorodetsky & Gussarsky, 1986; Mitchell & Gunstone, 1984). In the 1990s, a number of researchers (Garnett & Treagust, 1992a, 1992b; Ogude & Bradley, 1994; Sanger & Greenbowe, 1997a, 1997b) diagnosed student misconceptions in electrochemistry. It was found that the most common misconceptions included the ideas that electrons flow through a salt bridge and electrolyte solutions to complete an electrical circuit, that anions and cations in the salt bridge and the electrolyte solution transfer electrons from the cathode to the anode, and that the half-cell potential is an intrinsic property that can be used to predict the spontaneity of an individual cell. In addition to the assessment for misconceptions, Sanger & Greenbowe (1997a) reported that proper computer animations aimed at dealing with misconceptions could reduce the number of students who kept them. In a review of the literatures, we found that student misconceptions in electrochemistry are numerous and varied. In general, students start to learn some basic concepts of electrochemistry, including oxidations, reductions, cathodes, anodes, electrolytes, salt bridges, and electrical current etc., at the age of 15 (i.e., 9th grade). They continue to study advanced concepts, including the standard reduction potential, electromotive force, and half-cell reactions in the 12th grade. In college, students are exposed to the principles of electrochemical cells in general chemistry course in greater depth and detail. We were curious about and examined the growth of student understanding of the basic principles of electrochemistry. We also tried to identify the common misconceptions that occur in students of different levels. In addition, we explored the reasons for 100

2 Students Difficulties in Electrochemistry students learning difficulties in electrochemistry. II. Methodology An open-ended paper-pencil test containing four items (see Appendix) was used to assess students understanding in the concepts of oxidation, reduction, and electrochemistry and to assess their explanations. Item 1 asked the students to predict whether there was electric current in a cell containing Zn and Cu electrodes and 1.0 M CuSO 4 as electrolyte. In addition, students were asked to explain the reasons for their prediction. Apparently, the diagram used for Item 1 was different from a typical diagram of a Galvanic cell shown in most high school or college chemistry textbooks. It consisted of a single cell with two electrodes and no salt bridge. The two electrodes in the same beaker were connected by a wire and immersed in the same electrolyte. The correct prediction of this item should be that electrical current flows through this cell since the electrons move from the Zn electrode to the Cu electrode through the external wire and the cations in the solution gain electrons that are plated onto the electrode. The electrolyte conducts electricity within the cell through the action of dissolving ions. The movement of ions completes the circuit and maintains electrical neutrality. Despite the fact that there is only one cell but no salt bridge, the electrical circuit is still complete in such a cell. A voltage meter shown in the external circuit will detect the electromotive force. We conducted the experiment and found that a reading of 1.0 volt was shown on the meter. In addition, the zinc electrode gradually dissolved in the CuSO 4 solution. Solid copper resulting from the reduction of Cu 2+ was observed at the copper electrode and on the bottom of the beaker. The oxidation-reduction reaction lasted for about 20 minutes. Item 2 asked the students what would happen if the electrolyte consisting of 1.0 M CuSO 4 was replaced with electrolyte consisting of 1.0 M ZnSO 4 in the cell shown in Item 1. A prediction that the indicator of the voltage meter would move was considered correct. Explanations that were consistent with the following statements are considered to indicate sound understanding. The ions of Zn 2+ in the electrolyte accept electrons and reduce to Zn. As a result, the cathode becomes coated with Zn, and the current stops. Then, no further oxidation-reduction reaction or electromotive force is observed in the cell since the two electrodes have the same standard reduction potential. When we carried out this experiment, we found that the initial voltage reading was 1.0 V. The reaction went on slowly and lasted for more than 16 hours with a reading of 0.9 V. Item 3 discerned students understanding of the difference in the voltage reading between a typical electrochemical cell with a regular salt bridge and an electrochemical cell with a copper wire used to connect the two half cells. In a study by Ogude & Bradley (1994), a cell containing a copper wire showed no voltage at all. This was mainly because no ions could carry an electrical charge through the wire to complete the circuit. We, however, were surprised by finding a net reading of 0.6 V in a similar experiment. Why we obtained results different from those of Ogude & Bradley (1994) needs further examination. For this item, a reasonable prediction is that the electrochemical cell with a regular salt bridge can provide a higher voltage reading than the cell with a copper wire. A sound explanation would be that the ions can carry electrical charges through the salt bridge to complete the electrical circuit, while the cell with copper wire does not have the same conductive capacity and mechanism. Item 4 discerned students understanding of the difference in the voltage reading between a chemical cell with Zn and Cu electrodes and a cell with two graphite electrodes. It is apparent that there will be no voltage reading for the later cell. This is because no oxidation or reduction will occur. In contrast, the electrochemical cell with Zn and Cu electrodes could have a net voltage reading resulting from the oxidation of Zn and the reduction of Cu 2+. In order to identify the conceptual difficulties that high school and the college students have, the same test was administered to 182 9th graders, 75 college-bound 12th graders, and 49 senior college students whose major was chemistry. The 9th graders were picked from six classes in three high schools. The 12th graders were from two classes in two high schools, who had taken a curriculum based on science or engineering. All the students who participated in this study had learned the concepts that were examined in the test. A longitudinal comparison of 9th, 12th, and senior college students conceptual difficulties can help teachers understand what concepts are often misunderstood by students of different levels. In addition, for the purpose of exploring possible sources of and reasons for student learning difficulties, three students from each of the three levels who made wrong predictions and wrote inappropriate explanations were randomly selected for a semi-structured interview. Each of them was individually interviewed for thirty minutes to solicit further explanations of and reasons for their predictions. All the interviews were audio-taped and transcribed. III. Results Table 1 shows the percentages of students who made wrong predictions or gave wrong explanations for test Items 1 and 2. It can be seen that two major misconceptions were consistently held by the three grade levels of students. The first one concerned the function of a salt bridge. 17.6% 101

3 H.S. Lin et al. Table 1. Percentage Distribution of Students Misconceptions Revealed by Items 1 and 2 (N = 182) (N=75) (N = 49) Students Misconceptions 9th graders 12th graders College students Item 1 wrong explanations 1. There must be a salt bridge to complete the circuit There must be an external battery to force chemical reactions to happen The reaction in the cell releases heat instead of electricity There is no any chemical reaction in the cell at all No explanation Item 2 wrong explanations 1. The direction of the indicator of the voltage meter will be reversed The electrolyte should contain Cu 2+ if Cu is the cathode No explanation Table 2. Percentage Distribution of Students Misconceptions in the Items 3 and 4 (N = 182) (N=75) (N = 49) Students Misconceptions 9th graders 12th graders College students Item 3 wrong explanations 1. The principle and function of the salt bridge and the copper wire are similar Because the copper electrode does not dissociate ions into the solution, the voltage will decrease. 3. No explanation Item 4 wrong explanations 1. Carbon can conduct electricity, so the cell will show a voltage reading The cell with two carbon rods will create electrolysis No explanation of the 9th graders, 9.3% of the 12th graders, and 12.2% of the senior college students thought that a salt bridge was indispensable for a galvanic for having a close circuit. They believed that without a salt bridge, the cell could not work properly. Another major misconception held by the three groups of students concerned the type of electrolyte used in a Galvanic cell. About 10.8% of the junior high students, 9.3% of the senior high students, and 53.1% of the college students explained that if the electrolyte does not contain the cation of the cathode (i.e., Cu 2+ for the Cu cathode), then the cell will not have any electromotive force. We were surprised that more than one-half of the college students who were majoring in chemistry held the same misconceptions as the secondary school students did. While the reasons they gave were different from those given by the secondary school students, the college students firmly held the same misconceptions. Table 2 presents the percentages of students who had major misconceptions as revealed by test Items 3 and 4. For Item 3, 19.8% of the 9th graders, 8.0% of the 12th graders, and 14.3% of the college students believed that if the salt bridge in an electrochemical cell is replaced with a copper wire, the net voltage will remain unchanged. They thought that the function and principle of the salt bridge are the same as those of the copper wire. In fact, the copper wire does not contain ions dissociated from the electrolyte inside the salt bridge. The ions play the important role of carrying electrical charges to complete the electrical circuit. Although copper is a conductor, it conducts electrons instead of ions. In this case, there are no electrons going through the solution (Ogude & Bradley, 1994). Apparently, the copper wire is not able to transfer ions as the salt bridge does since there is no electrolyte in it. The major misconception of the students revealed by Item 4 was that even if the Zn and Cu electrodes were replaced with two carbon bars, the cell would still create a net voltage. We were surprised by that 36.7% of the college students held this misconception. It should also be noted that 4.9% of the 9th graders, 5.3% of the 12th graders, and 6.1% of the college students did not understand the difference between an electrochemical cell and an electrolytic cell. An electrochemical cell creates electrical current through the chemical reactions of oxidation and reduction. In contrast, an electrolytic cell uses electrical current to decompose compounds into 102

4 Students Difficulties in Electrochemistry elements. In order to understand why the students gave wrong explanations, individual follow-up interviews were conducted after the paper-pencil test. In the interviews, these students gave a variety of reasons. For example, although college students Huang and Hu both believed that the cell with two identical carbon rods in Item 4 could create a voltage reading, Huang explained that some reactions occurred in the half cell while Hu thought that the voltage of the cell had nothing to do with the types of electrodes in this case. College student Huang: In the case where Zn and Cu are used as electrodes, its anode reaction is Zn Zn e, and the cathod reaction is Cu e Cu. If Zn and Cu are replaced with two carbon rods, the anode reaction is OH 1 O 2 + e, while the cathod reaction remains uncharged. Interviewer: Why is it unchanged? College student Huang: The H 2 O solution in the left side of the cell will serve as the anode and discharge electrons while the Cu +2 ions in the right side of the cell will serve as a cathod to receive the electrons. However, the net voltage reading will be smaller than the original cell that uses Zn and Cu as electrodes. College student Hu: The voltage reading will be the same because the voltage has nothing to do with the types of electrodes. It is only influenced by the concentration of the electrolytes. It was found from interviews that student misconceptions about electrochemistry likely stemmed from inappropriate interpretations of textbooks and classroom instructions. For instance, their typical thinking about the salt bridge was greatly influenced by the pictures of electrochemical cells they had seen in textbooks. Interviewer: Why do you think the voltage meter (in Item 1) will not show a reading? Junior high student Wang: Because this picture has only one beaker. The Galvanic cell that I have ever seen always has two half cells and a salt bridge. Interviewer: Why is it necessary to have two cells and a salt bridge? Junior high student Wang: That s what the textbook says. It s a standard format for a Galvanic cell. My physical science teacher also taught the same thing that the textbook said. The following answers of a senior high school student further revealed the typical conception of a Galvanic cell. Interviewer: Don t you think that putting the two electrodes into the same beaker still create electrical current? Senior high student Chen: No, I don t think so. Interviewer: What are your reasons? Senior high student Chen: I have never seen a single Galvanic cell in my junior and senior high school textbooks that uses one cell only. The pictures in textbooks all show that two half cells and one salt bridge are required. It can be seen from the above selection of student explanations that student misconceptions in electrochemistry likely result from their over-simplification and generalization of the information in textbooks and classroom instructions. IV. Discussion and Implications for Chemical Education Sanger & Greenbowe (1997b) reported that student misconceptions about the mechanism of current flow in a cell with a salt bridge and electrolyte are that electrons can either attach themselves to ions in solutions or they can flow by themselves without assistance from ions. This study found two major misconceptions are commonly and consistently held by 9th graders, 12th graders, and college students in Taiwan. (1) A salt bridge is absolutely essential in a Galvanic cell. (2) The electrolyte must contain the cation that corresponds to the electrode in a Galvanic cell (e.g., Cu 2+ for a Cu cathode). A comparison of the percentages of students holding the two major misconceptions reveals that more advanced study in chemistry does not necessarily result in better understanding of some particular basic concepts. In fact, if these concepts are not thoroughly clarified, students misconceptions may become more firm and widespread as they progress in their studies in chemistry from secondary school to college. This speculation is supported by the finding that about one-half of the college chemistry majors in this study held a misconception, while only about onetenth of the high school students had the same misconception. This finding indicates that chemistry teachers should give multiple examples and explanations when they teach. For instance, when the functions of a salt bridge and electrolytes are taught, examples showing what types of electrolytes (e.g., CuSO 4 and ZnSO 4 ) are appropriate (or not) for a specific electrode (e.g., Cu) and why should be given. Using a single example in teaching is likely to result in student misconceptions as those found in this study. Further research could develop more test items to investigate students understanding of electrodes and electrolytes in electrochemical cells. 103

5 H.S. Lin et al. The finding that students typically believed that a salt bridge is necessary in a Galvanic cell should be noted by chemistry teachers. About one-sixth of the ninth graders, one-tenth of the 12th graders, and one-eighth of the college chemistry majors believed that a salt bridge is absolutely necessary in a Galvanic cell to create electromotive force. The follow-up interviews revealed that these students conceptions were likely influenced by textbooks and classroom instructions. All the students who were interviewed expressed that their explanations regarding the Galvanic cell were based on the pictures and statements they had seen and read, respectively, in textbooks and on their teachers instructions. It seems that textbook writers and chemistry teachers should be particularly careful when explaining the structure and principles of a Galvanic cell. After introducing examples of Galvanic cells, open discussion and further practice in designing a variety of Galvanic cells might help students construct their own understanding and to try out their own hypotheses. In addition, introducing the students to Volta s 1799 experiment may reduce the number of students who hold the typical misconceptions since Volta did not use any salt bridge in his demonstration of the electrochemical cell. Further studies on applying conceptual change teaching approaches are encouraged. neutrality in operating electrochemical cells. Journal of Chemical Education, 71(1), Sanger, M. J., & Greenbowe, T. J. (1997a). Common student misconceptions in electrochemistry: Galvanic, electrolytic, and concentration cells. Journal of Research in Science Teaching, 34(4), Sanger, M. J., & Greenbowe, T. J. (1997b). Students misconceptions in electrochemistry: Current flow in electrolyte solutions and the salt bridge. Journal of Chemical Education, 74(7), Appendix The Open-Ended Test Items 1 to 4 1. In the following diagram, Zn and Cu electrodes are connected at one end by an electrical wire and a voltage meter. The other end of each electrodes is immersed into a solution consisting of 1.0 M CuSO 4. Do you think the voltage meter will show a reading or not? Please explain as well as you can. Acknowledgment The authors sincerely thank the National Science Council, R.O.C., for financial support through project NSC S References Butts, B., & Smith, R. (1987). What do students perceive as difficult in H.S.C. chemistry? Australian Science Teachers Journal, 32(4), Finley, F. N., Stewart, J., & Yarroch, W. L. (1982). Teachers perceptions of important and difficult science content. Science Education, 66(4), Garnett, P. J., & Treagust, D. F. (1992a). Conceptual difficulties experienced by senior high school students of electrochemistry: Electric circuits and oxidation-reduction reactions. Journal of Research in Science Teaching, 29(2), Garnett, P. J., & Treagust, D. F. (1992b). Conceptual difficulties experienced by senior high school students of electrochemistry: Electrochemical (galvanic) and electrolytic cells. Journal of Research in Science Teaching, 29(10), Gorodetsky, M., & Gussarsky, E. (1986). Misconceptions of the chemical equilibrium concept as revealed by different evaluation methods. European Journal of Science Education, 8(4), Hackling, M. W., & Garnett, P. J. (1985). Misconceptions of chemical equilibrium. European Journal of Science Education, 7(2), Johnstone, A. H. (1980). Chemical education research: Facts, findings, and consequences. Chemistry Society Review, 9, Mitchell, I. J., & Gunstone, G. F. (1984). Some student conceptions brought to the study of stoichiometry. Research in Science Education, 14, Ogude, A. N., & Bradley, J. D. (1994). Ionic conduction and electrical 2. In the above diagram, if the electrolyte is replaced with a 1.0 M ZnSO 4 solution, do you think that the voltage meter will show a reading or not? Please explain as well as you can. 3. If the salt bridge in the following diagram is replaced with a copper wire, do you think that the voltage reading will be different from the voltage reading obtained when a salt bridge is used? Please explain in your own words. 4. In the above diagram, if the Zn and Cu electrodes are replaced with two similar carbon rods, do you think that the voltage reading will be different from the voltage reading obtained when the original Zn and Cu electrodes are used? Please explain in your own words. 104

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