Ionic Conduction and Electrical Neutrality in Operating Electrochemical Cells Pre-College and College Student Interpretations A. N. Ogude Vista University, Johannesburg J. D. Bradley University of the Witwatersrand, Johannesburg This paper is based on the results of a n investigation on pre-college and college student difficulties regarding the qualitative interpretation of the microsco~icprocesses that take place in-operating electrochemical~ell~. This investigation was prompted by the realization that first-year university chemistry and high school physical science examination auestions set in relation to electrochemical cells ( I ) . Little attention, if any, is given are often to qualitative questions that probe deeper understanding. u his observation has raised ;lot of concern amone science educators especially after several investigations Tevealed t h a t students who Dass examinations sometimes show gross misunderstandings when required to give a qnalitative interpretation of scientific processes. These students, in many cases, master enough to do manipulative tasks often required of them in examinations without necessarily understanding. The aim of this investigation was to determine qualitative rather than quantitative understanding of electrochemical processes. The subjects involved in this study were studying electrochemistry a t high school, or university. The investigation was conducted in four phases that is (1) identification of misconceptions, (2) predominance of the misunderstandings, (3) possible sources or causes of the misunderstandings,and (4) development of a teaching method t o overcome the misunderstandings.
I n a n earlier paper (21, we reported on the results of the first phase that involved the identification of misconcept i o n s k n g the interview method and the pencil and pap& test. I n this first phase, four areas that appeared to present most difficulty to the interviewees were identified. These are (1) conduction in the electrolyte, (2) electrical neutralitv., (3) electrode processes and terminology, (4) aspects relating to cell components, current, and cell emf.
.
On the basis of the interview transcripts and responses to the pencil and paper test, a 20-item questionnaire of multiple-choice. assertion-reason. and true-false onestions was constr;cted. I n this questionnaire, each o i t h e four areas of misunderstanding was investigated bv constructing a t least one question each of the mukiple-choice, assertion-reason, or true-false type. The questionnaire was used in the second and third phases to determine how widespread the misunderstandings identified in the four areas mentioned above were a t different educational levels and to determine the possible causes of the misunderstandings. In this we report on the predominance and possible factors that can contribute to the misunderstandings in two of the four areas of concern-eonduction in
the electrolyte and electrical neutrality. Only the questions that probed understanding in these two areas are discussed in this paper. The results of the analyses of these questions are discussed below. Conduction in the Electrolyte Questions 3,10,and 20 in the questionnaire (see Appendix) tested the re-colleee and colleee students' understanding ofthe micn,scopic processes that take place in the electrolvtc whcn electn~chemicalcells operate. The interviews revealed that many pupils and &dents think that free electmns conduct current in the electrolyte (2.3). The following excerpt from an interview with a f;rsbyLar university student illustrates this point. (In the excerpts quoted, I = Interviewer and S = college Student, P = precollege Pupil and the number refers to the interview number and the line where the excerpt appears.)
-
Set-up: Electrolysis of Aqueous Copper Chloride with Graphite Electrodes (see Fig. 1) 11.32 I :Haw do you think this chlorine gas is formed? 11.33 S : Chlorine ions inside the solution will form gas and then give off electrons. 11.34I :Where do the electmns that are given off go? 11.35 S : From the cathode to the anode. They just move through the solution. One other problem identified from the interviews was the uncertaiity on how the two methods of conduction were related. The excerpts below indicate this point. 3.40S :. . . . I am confused about the movement of the ions and the electrons. I cannot make a connection between the two. I know there are electrons flowing in the wires and the solution. That is why the bulb lights up. But there are also ions in solution. 2.11 P :. . . . Ions are movinghetween the plates in solution and eledrans in the wires hut how the two affect each other I am not sure. I am not sure how they conned up.
C and B are graphite electrodes Figure 1. Electrolysis of aqueous copper chloride with graphite electrodes. Volume 71
Number 1 January 1994
29
Table 1. Percentage of Pupil and Students Choosing Each Alternative in Questions Relating to Ionic and Electronic Conduction Question
Alternative
Science Olympiad
Std. 10 pupils
First-year students
10 (alternativeanalysis)
statement 1 (True)(a + b + C)
statement 2 (True)(a + b + d) 20 True False 8 .
indicates the prevalent misconception(s)
b.
mdicate the correct response CSfandard 10 pupils N = 29 '1st Year students N = 37 The number in brackets indicates the actual number of students who chose the particular alternative.
Of the eight first-year university students interviewed individually, only one showed a satisfactory understanding of both electronic and ionic conduction a s well a s how the two are related. The following is a n excerpt of the interview with that student. Set-up :Electrolysis of Aqueous Copper Chloride with Graphite Electrodes (see Fig. 1) 5.6 I : ARer removing one electrode fmm the electrolyte, will the ammeter show a reading? 5.7 S : The ammeter will not show a reading because there is no continuous flow. 5.8 I : What do you mean by continuous flow? 5.9 S : Flow of electrons. But not in solution. The ionic flow in the solution is continuous with electron flow in the wires. Table 1 gives the number of students choosing each alternative in questions 3, 10, and 20.
Question 10 I n question 10, both the assertion and the reason are false. In all the categories of students, less than 30% ofthe subjects could identify both statements a s false. An alternative analysis of the results in question 10 (see Table 1) gives useful clues regarding the apparent explanatory power of the second statement. In this analvsis all those students who chose alternatives a, b, c, by imblication, say that statement 1, that is, the assertion, is true. Likewise, those who chose alternatives a, b, and d by implication say that statement 2, that is, the reason is true. I t is evident from this analysis that statement 2 was considered hv most students a s a n a ~ ~ r o ~ r iexolanation ate and can amount for why electrons shoufd be f o k d in solution. Alarge percentage of all subjects chose alternatives a, b, and d implying that statement 2 was true. Many stu: dents interviewed did not show any dissatisfaction with this reasoning; besides it fit well with their conception of electron conduction in the electrolyte. Studies on how misconceptions can be remediated ( 5 , 6 )indicate that students will not change their views a s long as they feel that there is a good reason for adhering to them. These three items, together with the interviews seem to show a deeply rooted misconception about the nature of conduction in the electrolyte. Further evidence of this was apparent in questions 2 and 7 (see Appendix) that did not seek to find student understanding of ionic conduction directly, but contained alternatives that suggested the presence of electrons in the electrolyte. Students and pupils who held this misconce~tionwere ex~ectedto choose answers consistent with th& reasoning in these questions. To determine whether there was anv clear att tern and consistency in the choice of such alternatives, a n analysis of individual pupil responses to questions 2, 3, 7, 10, and 20 was done. The pupils who held this belief were expected to make t h e following choices: Question 2(a), question 3(e),question 7(b), (el, question 10(a) or (b), and question 20, true. lbenty-nine high school pupils (not those who competed in the Olympiad) answered the five questions above. Ten pupils (35%) chose t h e expected alternatives; whereas, six of them, (20%) did not choose any of these alternatives. These students presumably do not possess this misconception. The remaining 13 pupils, (45%)were inconsistent in their choices. For example, some indicated statement 20 a s true in spite of selecting (c) in question 3. The first-year "slow-stream students" also showed a similar trend with 11of them (28%)choosing the expected altematives and 12 of them (30%) not choosing any of the alternatives. The remaining 17 students, (42%) like the pupils, were inconsistent in their choices. Such inconsistencv is not uncommon in students and pupils who hold misconceptions. Several researchers (7. 8 ) have observed that such learners often are doubtful and that the choice of a correct answer in one instance is not decisive roof of clear understanding. Question 20
Question 3 The results of question 3 indicate that a simificant ~ r o portion of the students and pupils chose aGernative'(e). These students have a picture of free electrons floating in the electrolyte. The resLlts of a larger sample of stud& (Standard 10) who competed in the 25th National Youth Science Olympiad in South Africa in 1989 (N = 6900) also show a similar pattern of results on this question with alternative (e, stiil the most popular c h o i c c ~ ~ h misconcepe tion also u a s noted by Allsop and Geurge f4, among l l C iof the Xulfield A-level srudenrs in the Unitcd Kmgdom.
30
Journal of Chemical Education
Only 35% of the first-year students recognized the statement as false and gave a scientific explanation on why i t was false. Some of the students who r e ~ l i e d"false" to this statement did not necessarily possess the sclcntificconceptlon but ruther showed their belief ~n frcc electnms in the solution. The explanations given by the students were revealing as regards the possible source of the misconception. The following statements by two first-year students illustrate this point: There is a flow of electrons only in the solution, ions are attracted to the anode and the cathode.
Only electrons flowin the solution from one electrode to the other. Ions are attracted to the electmdes. The notion that "ions are attracted to the electrodes" appears to be linked to the misunderstanding of electrode processes and the confusion that arises from designating electrodes as either positive or negative (10,ll). It appears that the students think that ions "stick" to the electrodes and none remain in the solution. They then conclude that electrons conduct current in the electrolyte. We got the impression that the phrase "attraction of ions to the electrodes" evoked the microscopic picture shown in Figure 2.
proposition of one high school pupil to support his argument that both ions and electrons flow in the solution (question 20).
...current is a flow of charge. Electrons can flow in solution because charge is a flaw of electrons and these conduct electricity. This student possesses as the correct knowledge element that "current is a flow of charge" hut does not distinguish between electronic and ionic charge. He. therefore.. a ~ ~ l i e s ttus statement indiscriminately lo theflow of. current in the electrolyte. This intervrctation is consistent aith the work of ~ i e (12) f who observed that novice students usually have a large store of special knowledge elements but often are unaware or uncertain when to apply such knowledge. As a result they do so without much subsequent thinking.
..
(bi Language and Careless Discussion of Electrode Pmcesses Laneuaee. svmbols. and re~resentationsare crucial factors in understanding and interpreting chernioil concepts 113,.The textbook is a maior source of information for the beginning student, yet some textbooks of chemistry and physical science contain obvious mistakes, misleading ideas or representations that can result in the misinterpretation of concepts. Two such statements that can cause confusion and lead to faulty reasoning are quoted below.
-
u Figure 2. Erroneous depiction of "Attractionof ions to the Electrodes".
This aspect will he discussed in greater detail in the misunderstandings relating to electrode processes. Possible Sources of the Misconception The following are possible sources of this misunderstanding or factors that can foster this misconception. (a) Reference to Continuity of Current and Established Belief in the Electmnic Nature of Current Electricity The confusion of considering conduction in the electrolyte as being due to the movement of electrons instead of positive and negative ions in opposite directions could arise from the phrases "continuity of current in a cell" or "continuity of the circuit" often used in standard textbooks. Pupils are introduced to current electricity before doing electrochemistry and by the time they study electrochemistry they are familiar with conduction in metals as being due to the movement of electrons. The phrases "continuity of the circuit" or "continuity of current in the cell" are, thus, misinterpreted to mean that current is electronic through the cell. For example, some pupils erroneously depicted movement of current in the cell as shown in the circuit in Figure 3. In physics courses, current often is implied to he electronic in all parts of the circuit even if there is a battery. It appears that students and pupils extend this to detailed discussion of electrochemical cells. This is evident in the
~
~
(i) "electrons will flow through the cell from the right electrode to the left electrode" (14). (ii) "the direction of flow of electrons always will be such that electrons move through the cell from the negative electrode ta the positive electrode" (14). Although the meaning of these two statements could be clear to senior students and exoerienced chemists. thev arc liable to he rnlslnterpreted'b) btypners i n electrochcmlstrv Both of them can be mlsmti.roretrd as referrme to electr& in solution and reinforce & already existing misconception. Emphasis should be placed on precise use of language, especially when talking or writing for beginners in chemistry. Examples of wrong representation of conduction in the electrolyte from some textbooks are shown in Figure 4. Although the text in these books (14, 16)state that electron exchange occurs a t the electrode surface, the diagrams apDear to illustrate electrons floatine in solution; thus, they reinforce this misconception.
-
-
L ~ o p p eions r receive electrons (19
(14)
Figure 4. Wrong representation of conduction in the electrolyte from textbooks. Electrical Neutrality
Figure 3. Erroneous depiction of conduction in the electrolyte.
The performance in all items relating to this wncept was particularly poor and in none of the items did the percentage of correct responses exceed 40. I n addition to holding Volume 71 Number 1 January 1994
31
Table 2. The Percentage of Precollege Pupils and College Students Choosing Each Alternative in ~ u e s t i o n sRelating to Eiectrical Neutrality Std. 10 school
f
63 e-
-w
electrons-
First-vear studknts N=40
Question
Alternative a bb
ZC
C
da eb ab bb
7d
ca d eb ab
F gure 5. St~dentand pup I
--
dep~cllon~of tne movement of e ectrons an0 ons in a Dane I cel (Q~est~on 7 (a)Sluaent Pup D agramj.
11 (alternative analysis)
Most of the diagrams given in response to question 7 (a), displayed a lot of doubt, with seven of the 28 high school pupils (25%) and eight of the 40 first-year students (21%) indicating electrons in solution. Seventy-one percent of the high school pupils and (72%) of the first-vear students shiwed wrong direction of movement of eithkr electrons or iuns or hoth. The remaining - students d ~ dnot respond to this question.
Statement 1 (True)(a + b + c)
(a) Unbalanced Charges in the Electrolyte
b
11
C
d ea
Statement 2 (True) 18
True
Falsea indicates the correct response indicates the prevalent misconception(s) Cstandard 10 pupils N = 28 dstandard10 pupils N = 28 The number in brackets indicates the actual number of students who chase the particular response. a
inappropriate knowledge about the salt bridge, some of the mistakes made by students included: (a) Unbalanced charges in the electrolyte (b) Presence of electrons in the solution. incorrect direction of movement of anions and cations in the electrolyte, and wrong movement of electrons in the external wires. Table 2 shows the responses to the four questions (2, 7 (b),11and 18--see Appendix), while Figure 5 shows some of the responses to question 7 (a) on how neutrality is maintained in a Daniel1 cell. Question 2, 7, and 18 Question 2 was included to test the understanding of electrical neutrality in electrolytic cells while question 7 (b) investigated the same concept in galvanic cells. The responses to these auestions were scattered throu~houtall options that implied widespread unccrtainty abo"i thc mtcluscovic events that take lace. Thts dificultv also is CVIdent in the diagrams shown in Table 3. ~ndeed,not one of the high school pupils gave a correct representation. The first-year students also performed poorly in this question.
32
Journal of Chemical Education
The choice of (b) in question 2 and either (a) or (b) in question 7 (b), was illustrative of disregard for the need for balanced charges a t all times in the electrolyte. This dificulty seems to arise from the interpretation of the charge distribution in the two half-cells. There were two interpretations: (i) One half-cell could he positive with cations only while the other could he negative with an equal number of anions only. In this way the whole cell was considered to be eketrically neutral. This misunderstanding is evident in question 18 where 66%of the high school pupils and 50% of the first-year students indicated this statement as true. c i i Unrqcml diatrihutiurt uf pusrtivc a d negntwr wns as long as there wereequal numhersnfchnrgr~ovrrnllin thewll. It is ofconcern that the ubove&~oss misunderstanding of elcrtrical neutralitv tauedtion 71Ibt alternatives (a1and ib, attracted 53% andh0% of the high school pupils and firstyear students, respectively. Comparison with question 2 (a) shows that fewer pupils and students experienced this problem in relation to electrolytic cells. I t seems that the interpretation of the charge distribution in the two halfcells could be the source of this problem. One other possibility, especially in the case of alternative (a) question 7 (b) is that the students arrive a t the response from misinterpreting the introductory section to this question. It appears to them that events stop a t the point where halfcell G is ~ o s i t i v eand half-cell H is negative a s i m ~ l i e dbv the description of zinc dissolvingand introducing positive ions in C: while comer Ions are d c ~ l c t c din half-cell 11. I'he oredominance oithis misconcep&on was unexpected because the subjects interviewed in this study could write correct equations to represent dissociation of electrolytes such as copper chloride but failed to transfer this knowledge of electrical neutrality to questions 2 and 7. McDermott (9)
Questions Relating to Electrical Neutrality Question 2 (Multiple-Choice) In the electmlysis of aqueous sodium chloride (NaCl), if a chloride ion moves out of a small volume of solution toward the anode a s shown below, which of the following visual representations best depicts the process by which electrical neutrality will be maintahed? Note: In the following diagrams, a cation is symbolized as +; an anion is symbolized as -and a n electron is symbolized a s e-.
Question 11 (Assertion-Reason) In this question, the two statements refer ta the diagram shown.
(a) Either Aar D (b) B anly (c) Acombination of C and D (dl Acambination of C and E (el E anly.
Question 7 (Multiple-Choice) I n the galvanic cell shown below, as the cell operates, oxidation of the zinc introduces additional zinc ions into half-cell G, and reduction of copper ions leaves a n excess negative charge in half-cell
Assertion The ammeter will show a reading
because
H.
Reason There will he a wntinuous flow of current since graphite wnducts electricity
The alternatives offered are: 1st statement 2nd statement (Assertion) (Reason)
G
H
(a) Indicate the movement of all the ions and the electrons. (b) Which one(s) of the following series of diagrams below depict the change in each half-cell as the reactions proceed? Note: In the following diagrams, a cation is symbolized as +and anions as An electron is symbolized as e-.
-.
(a) Either C and D (b) E only (c) B only (dl Either B and E (el F only
34
Journal of Chemical Education
(a)
True
True
(b)
True
True
(c) (dl (el
True False False
False True False
second statement is a correct explanation of the first second statement is not a correct explanation of the first
Question 18 (True-False) The salt hridge maintain clectncal neutrahty by ensunng that onr half-cell is posrtlve and the nthcr is nrgnlivr
