Asia-Pacific Forum on Science Learning and Teaching, Volume 17, Issue 1, Article 8 (Jun., 2016)
Anita Yung Li BONG and Tien Tien LEE
Form four students’ misconceptions in electrolysis of molten compounds and aqueous solutions

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Results and Discussion

The discussion on the misconceptions in the electrolysis of molten compounds and aqueous solutions has been grouped into three categories. The categories are misconceptions in identifying cathode and anode, about the reaction in the electrolysis of molten compounds and aqueous solutions, and lastly in writing the chemical equations. Then, the factors contributing to the students’ misconceptions in the electrolysis of molten compound and aqueous solution are discussed.

Identifying Cathode and Anode

The results (Appendix B) showed that three respondents were having misconceptions in identifying the anode and cathode in the electrolytic cell. They assigned the anode to the negative terminal of the battery while the cathode was assigned to the positive terminal of the battery. This finding is parallel with what Acar and Tarhans’ found in 2007. In their study, the respondents assigned the anode to the negative terminal because it was negatively charged while the cathode was assigned to the positive terminal because it was positively charged.

Five respondents (8.3%) were having misconceptions in explaining how they assigned the anode and cathode. They gave explanation such as:

The findings of this study are compatible with the findings in Nordin and Chong’s study (2010). In their study, they stated that students tend to memorize the position of the anode and cathode drawn by the teacher and in the textbook. The anode is always drawn on the left of the cell notation. Hence, students just memorized the position of the anode and cathode without understanding the concept.

Another seven respondents (11.7%) were having the misconception that the polarity of batteries does not affect the position of the anode and cathode in the electrolytic cells. This matched the findings of Özkaya, Üce and Sahin whom in their study in 2003 discovered that their respondents also misconceived that the polarity of batteries has no effect on the position of the anode and cathode in the electrolytic cell.

Reaction in the Electrolysis of Molten Compounds

In question 1(e)(i), respondents were asked to state the ions which moved to electrode X and electrode Y and give their explanations in question 1(e)(ii). These questions were created to test students’ understanding of the electrolysis of molten compounds. In the situation for the electrolysis of molten potassium iodide, potassium ions (K+ ions) were selectively discharged at the cathode (electrode Y) while iodide ions (I- ions) were selectively discharged at the anode (electrode X). A total of 11 respondents were having misconceptions about this question where they gave the answers below:

The result of our study is supported by Bakar and Mukhtar’s (2011) findings in their study whereby their respondents were having problems in identifying the ions which were selectively discharged at the anode and cathode during the electrolysis of molten silver bromide.

A total of six respondents (10.0%) were having misconceptions in answering question 1(e)(ii). Three of them explained that potassium ions (K+ ions) were selectively discharged at the anode while iodide ions (I- ions) were selectively discharged at the cathode because potassium ions (K+ ions) and iodide ions (I- ions) were the only ions present in the electrolysis process. Another three respondents stated that hydroxide ions (OH- ions) and hydrogen ions (H+ ions) were present in the solution; thus, the ions were attracted to the anode and cathode due to their lower position in the electrochemical series. This showed that these respondents were having misconceptions by involving water molecules in the electrolysis of molten compounds.

To ensure that the students can differentiate between molten compounds and aqueous solutions, we created question 1(g)(i) whereby the electrolysis of molten potassium iodide was replaced with aqueous potassium iodide. A total of 11 respondents (18.3%) were having misconceptions in answering this question and we found that this result is similar to the findings of Acar and Tarhan (2007). They discovered that students cannot identify products formed in the electrolysis of both aqueous and molten situation of salt. From the 11 respondents who were having misconceptions, seven respondents stated that only potassium ions (K+ ions) and iodide ions (I- ions) were present in the aqueous solutions. The result is parallel with the findings of Sanger and Greenbowe’s study (1997). In their study, they interviewed students to find out whether water was involved in the electrolysis of aqueous solution. A particular student said “The water shouldn’t do anything, so there may be a little reaction, but it’s not going to be measurable. Water will not enter into the equation”. Another two respondents stated that oxygen was the ions involved in this reaction. In fact, hydroxide ions (OH- ions) were present in this electrolysis and it formed oxygen as the end product. Thus, oxygen cannot be considered as the ions involved in this electrolysis. Another two respondents were having misconceptions in writing the chemical symbol, whereby one of them wrote potassium ions as sodium ions (Na+ ions) and iodide ions as I2- ions. In Bakar and Mukhtar’s study (2011), they discovered that students cannot remember the chemical symbol and oxidation states of ions and this finding is parallel with the result of this study.

Question 1(g)(ii) asked the respondents to select which ions were selectively discharged to the anode and cathode. In this question, hydroxide ions were selectively discharged to the anode while hydrogen ions were selectively discharged to the cathode. A total of 15 respondents (25.0%) were having misconceptions in selecting the ions which were selectively discharged at the anode and cathode. They gave answers such as:

The result of this study is compatible with Sanger and Greenbowe’s study (1997) whereby their respondents were having difficulties in identifying which ions were selectively discharged at the anode and cathode when there were more than two ions or more possible oxidation or reduction of half reactions.

In Malaysia’s Form Four Chemistry syllabus, all electrolyses of molten compounds are carried out by using inert electrodes such as carbon electrodes and platinum electrodes. Students are taught that carbon or platinum electrodes are suitable to be used in the electrolysis of molten compounds because they are inert and do not take place in reaction. In this study, a total of 15 respondents (25.0%) were having misconceptions in that they answered that carbon electrodes took place in the reaction of electrolysis because of some of the reasons below:

Acar and Tarhan (2007) in their study also revealed similar finding in that their students were also having misconceptions in explaining why carbon electrodes cannot take place in the reaction of electrolysis. However, the finding of their research was different than that of this study. Their respondents answered that inert electrodes can take place in the process of electrolysis because they can be oxidized or reduced.

Reaction in the Electrolysis of Aqueous Solutions

Question 2(c) asked respondents “Does water take place in the electrolysis of aqueous copper (II) chloride solution?” Nine respondents (15.0%) were found to have misconceptions in that they answered water was not involved in this electrolysis. The result of this finding is compatible with Sanger and Greenbowe’s study (1997). In their study, the interviewed respondents said that water will not be included in the equation for the electrolysis of aqueous solution.

Question 2(b)(i) asked the respondents to list out the ions present in the aqueous copper (II) chloride solution. The ions which were supposed to be present in the electrolysis of aqueous copper (II) chloride solution are copper (II) ions (Cu2+ ions), chloride ions (Cl- ions), hydrogen ions (H+ ions) and hydroxide ions (OH- ions). About 26.7% of the respondents, namely 16 of them, were having misconceptions in listing out the ions present in the aqueous copper (II) chloride solutions. Out of these 16 respondents, nine of them listed copper (II) ions (Cu2+ ions) and chloride ions (Cl- ions) were the only ions present in the electrolysis of aqueous copper (II) chloride solution. They did not include water in the electrolysis of aqueous copper (II) chloride solution. This finding is supported by the findings of Acar and Tarhan’s (2007) study, whereby their students also stated the same ions were produced in both the aqueous and molten situation of salt electrolysis. Another seven respondents (11.7%) in our study gave the wrong chemical symbol for the ions present in copper (II) chloride solution. Copper ion was written as Ca2+ ions in this study. This finding is similar to that of Bakar and Tay’s study (2010) in which their respondents could not remember the correct symbol for the chemical ions.

Question 2(b)(ii) requested the respondents to state the sources of ions in copper (II) chloride solutions. A total of 36 respondents (60.0%) gave incomplete answers. Out of these, five of them (8.3%) listed the sources for copper (II) ions (Cu2+ ions) and chlorides ions (Cl- ions) only and another three respondents (5.0%) listed sources for water only. 28 of the respondents (46.7%) stated that ions came from the copper (II) chloride solution without classifying where each ion came from.

Question 2(d)(i) requested the respondents to list out the ions that were moved to the anode and cathode for electrolysis of aqueous copper (II) chloride solution. Two respondents (3.3%) were having misconceptions with this question. One respondent wrote copper (II) ions (Cu2+ ions) which were the cations discharged at the anode. This respondent was confused with the concept of the anode attracting the anions and cathode attracting the cations. This finding is parallel with Acar and Tarhan’s (2007) findings, whereby their respondents stated that the cathode attracts anions because it was positively charged. The other respondent was having misconceptions by giving the wrong chemical symbol. The respondent wrote chloride ions (Cl- ions) as I- ions and this finding is similar to Bakar and Tay’s study (2010). In their study, their respondents could not remember the correct chemical symbol for the ions present in the electrolyte.

Question 2(d)(ii) requested the students to explain how they identified which ions were moved to the anode and cathode. Four respondents (6.7%) were having misconceptions with this question. Two of them stated that copper (II) ions which were the cations are attracted to the anode because the ions were positively charged. Another two respondents answered that chloride ions which were the anions are attracted to the cathode because the ions were negatively charged. The finding is compatible with Bakar and Mustafa’s study in 2010 whereby they stated their respondents were having difficulties in analysing which ions were selectively discharged at the anode and cathode during the electrolysis process.

Question 2(e)(ii) required the respondents to explain how they chose which ions were selectively discharged at the anode and cathode. A total of 20 respondents (33.3%) were having misconception with this question whereby 18 of them answered chloride ions (Cl- ions) were discharged at the anode due to the higher concentration in the solutions. Based on the answers given, respondents were having problem in identifying the concentrated electrolyte in the electrolysis of an aqueous compound. Another two answered that hydrogen ions (H+ ions) were selectively discharged at the cathode instead of copper (II) ions (Cu2+ ions) because they misconceived that hydrogen ions are placed in the lower position of the electrochemical series as compared to the copper ions. This clearly showed that they did not understand the electrochemical series. This finding matches the findings of Bakar and Mustafa (2010). In their study, their students had difficulties in analysing which ions were selectively discharged at the anode and cathode because the students were unable to determine the position of ions in the electrochemical series.

Using Concentrated Electrolyte in Electrolysis of Aqueous Solution

In question 2(f)(i), we replaced the electrolyte with a more concentrated copper (II) chloride solution, then asked the respondents to predict the observations at the anode of the electrolytic cell. The correct scientific answer should be that green gas is released at the anode and there is a pungent smell. Ten respondents (16.7%) were found to have misconceptions by giving their observations as follows:

Question 2(f)(ii) required the respondents to give explanations based on their observations. Nine respondents (15.0%) were found to be having misconceptions when they gave explanations such as:

Lee and Osman (2012) found that the respondents in their study were having misconceptions in solving questions which involved the use of concentrated electrolyte in the electrolysis of concentrated copper (II) chloride solution. In their study, they found that only 7.9% of their respondents could explain correctly why chlorine gas was formed at the anode during electrolysis of concentrated copper (II) chloride solution.

Using Active Electrode in Electrolysis of Aqueous Solution

In question 2(g), we replaced the carbon electrodes with copper electrodes and asked the respondents about the physical appearance of the two electrodes at the end of the electrolysis. Four respondents (6.7%) were having misconceptions whereby two of them stated the anode becomes thicker while the cathode becomes thinner and another two of them stated there are no changes on the electrodes. The finding is compatible with Acar and Tarhan’s study (2007) whereby they stated that their respondents answered that metal electrodes only act as an electron carrier and there will be no physical changes in the electrodes’ physical structures.

Out of the 43 respondents who were able to give correct observations for question 2(g)(i), only 12 respondents (20.0%) were able to give correct scientific explanations for their observation in questions 2(g)(ii). Five respondents (8.3%) were having misconceptions whereby they gave answers such as:

Writing Chemical Equation

The topic of electrolysis involves many chemical equations to represent the reactions that took place. In this study, it was found that many students were having misconceptions in writing the chemical equations. In question 2(e)(i), respondents were required to write a balanced equation for the reaction of hydroxide ions (OH- ions) at the anode. Instead of writing the correct balanced equation, the respondents wrote 2OH- → O2 + H2O + 2e- to represent the process at the anode. The chemical equation written was not balanced; the correct balanced equation should be 4OH- → O2 + 2H2O + 4e-. Similarly, this is supported by the findings of Bakar and Tay (2010) who stated that their respondents could not write balanced chemical equations because they were weak in mathematics.

Additionally, some of the respondents were having misconceptions by writing the wrong oxidation states of elements. As an example, for the equation in question 2(g)(iii), one of the respondents wrote the equation Cu → Cu+ + e- because of his misconception. The balanced equation should be Cu → Cu2+ + 2e-, where one copper atom was oxidized to produce one copper (II) ions and two electrons. This respondent gave the wrong oxidation state for copper (II) ions (Cu2+ ions). This finding is supported by Bakar and Mukhtar’s finding (2011) whereby in their study, their respondents could not remember the oxidation states of ion. According to Bakar and Mukhtar, when students cannot remember the oxidation states of an ion they will have problems in writing the correct chemical equation.

In addition, there were also four respondents who remembered the wrong chemical symbols. Three of them wrote copper (II) ions (Cu2+ ions) as Ca2+ ions in question 2(d)(i) and one of them wrote potassium ions (K+ ions) as Na+ ions in question 1(f). This finding is parallel with Bakar and Mukhtar’s finding (2011) whereby their respondents could not write the correct chemical symbol for lead (II) ions.

Furthermore, we found that the respondents could not write proper chemical equations. In question 1(g)(ii), the equation was written as Cu2+ + 2e- = Cu. The respondents replaced the arrow in the equation with an equal sign.

Factors Contributing to Students’ Misconceptions in the Electrolysis of the Molten Compounds and Aqueous Solutions

An interview was conducted to find out the reasons contributing to the students’ misconception. The findings of the interview showed that the lack of basic knowledge in the topic of electrochemistry resulted in the respondents not being able to proceed in solving the questions in the open-ended electrochemistry assessment. We discovered that many of the students did not have the habit of doing revision after learning a new concept in the topic of electrochemistry. Some of the respondents were able to answer the question in the interview after they did their revision. Bakar and Mukhtar (2011) also found that students who lacked basic knowledge were unable to write out the correct chemical symbol for the ions present in the electrolyte.

In addition, there were three respondents who were having language problems in studying electrochemistry. This was the reason which contributed to the students’ misconceptions in answering the questions. Only one student tried to search for the meaning of words when she could not understand the language in the text book. Language obstacles contributed to students’ misconceptions in this study. According to Eilks and Hofstein (2013), the linguistic ability of students affects their ability to learn Chemistry. Hence, students who have a problem in understanding the terms will influence their learning in electrochemistry (Bakar & Mukhtar, 2011).

We also discovered that three students in this study applied rote learning whereby they memorized the concepts in electrochemistry without understanding the concepts in this topic. They tend to write anything that they could remember in answering the open-ended electrochemistry assessment given and this caused misconceptions. The interview results correspond to those of Bakar and Tay’s study (2010) whereby their respondents also applied rote learning in electrochemistry and this contributed to their respondents’ misconceptions. In their study, they discovered that students were able to list out the electrolyte and non-electrolyte compounds but failed to give an explanation for their answers.

 

 

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