An Analysis of Undergraduate General Chemistry Students

Research: Science and Education edited by

Diane M. Bunce The Catholic University of America Washington, DC 20064

An Analysis of Undergraduate General Chemistry Students' Misconceptions of the Submicroscopic Level of Precipitation Reactions

Melanie M. Cooper Clemson University Clemson, SC 29634

Resa M. Kelly,* Juliet H. Barrera, and Saheed C. Mohamed Department of Chemistry, San Jos e State University, San Jos e, California 95192 *[email protected]

Chemical equations are important to chemists because they represent events that happen at both macroscopic and submicroscopic levels, and they are useful for quantifying chemical changes. The dilemma is that a chemical equation contains a lot of information condensed into simple numbers and element symbols. Consequently, an instructor is faced with the challenge of teaching the algorithmic nature of the equation as well as what the equation represents macroscopically and submicroscopically. Many instructors choose to emphasize the symbolic nature of equations early in their courses since this is a central focus of general chemistry. However, Johnstone (1) indicates that one reason that students have difficulty learning chemistry is that they do not understand the relationships between the macroscopic, submicroscopic, and symbolic levels, especially when instruction emphasizes the symbolic level without building connections to the other levels. Many investigators (1-5) concur that students have difficulty understanding the submicroscopic nature of matter when instruction focuses on the symbolic and macroscopic levels. This article specifically examines the nature of the misconceptions students exhibited about the submicroscopic level of ionic reaction equations after receiving instruction that emphasized the symbolic and macroscopic levels. Chemical Reactions Several studies indicated that students have a variety of alternative conceptions in their understanding of chemical equations and reactions (6-11). Hinton and Nakhleh (12) noticed that college-level general chemistry students who were able to successfully balance equations demonstrated a flawed submicroscopic-level understanding of reactions and the nature of polyatomic ions involved in the reactions. Boo and Watson (13) suggested that chemical reactions give students difficulty because there are multiple places for misunderstanding. Two possible sources of confusion relevant to this study are

• Students fail to grasp the idea that during a chemical reaction the entities reacting are changed in fundamental ways, which means that the products no longer show the same properties as the reactants. • Students are confused as to the nature of chemical bonds. Bonds are viewed as entities linking atoms together. Thus, a giant lattice of ions, in which no ions are specifically linked to other ions, is difficult for students to interpret as a form of chemical bonding. Weaker bonds such as those associated with the solvation of ions are simply ignored.

_

According to Lee (14), secondary students who struggled with their submicroscopic-level understanding of chemical reaction mechanisms found it especially difficult to visualize the dynamic nature of reactions, specifically, how ions in a lattice interacted or how bonds broke and reformed in the reaction. Background This article focuses on how 21 general chemistry students drew and orally explained their submicroscopic-level understanding of three molecular equations presented to them on worksheets: AgNO3 ðaqÞ þ NaClðaqÞ f AgClðsÞ þ NaNO3 ðaqÞ KNO3 ðaqÞ þ NaClðaqÞ f No Rxn

(1Þ (2Þ

MnCl2 ðaqÞþ2AgNO3 ðaqÞ f 2AgClðsÞþMnðNO3 Þ2 ðaqÞ (3Þ

The worksheets consisted primarily of box frames for drawing submicroscopic-level events for each of three molecular equations. The first equation has only monovalent ions. The second reaction provides an example of where no reaction occurs. The third reaction shows more complicated formulas and coefficients. The specific research question investigated was, “What types of misconceptions are conveyed by students' explanations of the submicroscopic level of precipitation reactions?” Participants Twenty-one general chemistry students of diverse ethnicity (Caucasian (8), Asian (9), Asian Indian (2), and Hispanic (2)) agreed to participate in the study after an announcement was made to three general chemistry sections at a large western university. Two of the sections were taught by instructor A, a professor with 20 years of teaching experience, and the other section was taught by instructor B, a professor with 13 years of teaching experience. The participants consisted of 11 males and 10 females ranging in age from 18 to 44 years. The students were interviewed after the eighth week of classes, after lectures on types of chemical reactions, properties of solutions, solubility, and ionic equations (consisting of molecular, total ionic, and net ionic equations) had been taught. In addition, in the laboratory that accompanied the course, two experiments relevant to the topic were completed. In one experiment, the conductivities of

_

r 2009 American Chemical Society and Division of Chemical Education, Inc. pubs.acs.org/jchemeduc Vol. 87 No. 1 January 2010 10.1021/ed800011a Published on Web 12/18/2009

_

Journal of Chemical Education

113

Research: Science and Education

various aqueous solutions were tested to determine the strength of the electrolytes in the solution. In the second experiment, students identified aqueous salt solutions by studying the formation of precipitates, using solubility and dissociation rules. Instruction The instruction for the course was primarily symbolic as determined from interviews with both instructors and the student participants. Both instructors assumed that the concepts associated with precipitation reactions had been covered in students' previous chemistry courses. Regardless, they still spent part of approximately three 50 min lectures on the material. Initially, the instructors focused on solubility and dissociation rules and discussed salt dissolution. They addressed how to represent acids, bases, and salts in water. Then they discussed the concepts of solubility (soluble versus insoluble) and dissociation before introducing solubility rules for acids, bases, and salts and dissociation rules for acids. Next, the instructors introduced chemical equations by having the students predict products of reactions by “switching the ions” to balance the molecular equation. They applied solubility and dissociation rules to construct total ionic equations, and they derived net ionic equations from total ionic equations while addressing the nature of spectator ions. Neither instructor emphasized the required textbook, Chemistry: The Central Science (15); however, students did have access to the textbook illustrations of salt dissolution and precipitation reactions. The textbook illustrations showed a static submicroscopic-level depiction of a sodium chloride lattice being dissolved in water and static macroscopic, microscopic, and symbolic representations of the reaction of aqueous potassium iodide reacting with aqueous lead(II) nitrate to form a precipitate of lead(II) iodide and a solution of potassium nitrate. The submicroscopic-level depiction of sodium chloride dissolution emphasized the role of water molecules in the process; however, the depiction of the reaction did not illustrate any water molecules. The reaction illustrated the reactants as separate ions for both aqueous solutions. The products consisted of a twodimensional lattice arrangement of lead(II) iodide with separated spectator ions of potassium and nitrate floating above the solid. Setting The interviews took place outside of class time in a conference room where individual students were presented with a packet that consisted of one page of instructions and three worksheets formatted as previously mentioned. One typed chemical equation appeared at the top of each page, and underneath it were four blank boxes for students' drawings. Additional pages, with blank boxes, were available at the back of the packet. In reference to students' drawn and oral explanations, pseudonyms were substituted for the students' names to protect their anonymity. In this article, the students will be referred to by the letter “S” and a number. Theoretical Framework Constructivism was used as the theoretical framework guiding this study. Crotty (16) defines constructivism as the view that what we know and understand depends on our experiences interacting with other human beings and the world. He indicates that knowledge is developed and transmitted within 114


_

Vol. 87 No. 1 January 2010

_

an essentially social context. According to Fergusen (17), “Constructivism is best suited for studies that focus on sense- or meaning-making, concept construction, or elucidation of alternative concepts.” The goal of this study was to identify the misconceptions that students illustrated to explain three chemical equations. Students drew and orally explained their understanding of the chemical equations and a semistructured interview was used to gain further insight into their understanding. In constructivism the aim of learning is to understand and reconstruct. Data Analysis Students' drawings depicting the molecular nature of each of three equations were analyzed to ascertain how the students communicated molecular-level understandings of the given equations. Special attention was paid to students' illustrated perceptions and apparent misconceptions about precipitation reactions. In addition, the students were interviewed to better understand their drawings and to address specific questions about the nature of the aqueous solutions involved in the reactions. The semistructured interviews were audiorecorded and transcribed. The worksheet drawings and transcribed student interviews were read using a constant comparison method of analysis (18). Auditing was performed, in which two other researchers checked the interpretation and credibility of the coded data (19). Results and Discussion The analysis of students' graphics revealed that the details of the students' drawings ranged in complexity. Approximately half of the students (10) chose to draw a very simplistic representation showing only the simplest whole number ratio of species reacting to form products, while eight students provided a detailed submicroscopic-level depiction showing populations of ions reacting. Three students were admittedly unable to imagine how the equation would look submicroscopically and, instead, rewrote the equation as their submicroscopic explanations. The investigation revealed that students illustrated features of molecular equations, total ionic equations, and often both (Table 1) in their drawings, seeming to map their submicroscopic-level view onto these equations. Interestingly, no students wrote or included a depiction of the net ionic equation for the reaction between aqueous sodium chloride and silver nitrate, and only two students wrote a net ionic equation for the reaction involving aqueous manganese chloride and silver nitrate. More than half of the students included molecular equation features in their drawings. These references to the equations suggest that students who are taught about precipitation reactions with a symbolic emphasis may have difficulty understanding how the molecular equation relates to the total ionic equations at the submicroscopic level. Possibly as a result, many students incorrectly incorporated features of both the molecular equation and the total ionic equation in their drawn depictions, suggesting that they view the total ionic as an intermediate step in the reaction, giving a summary of the process (Figure 1). Several misconceptions that may stem from students' misunderstanding of the relationship between the symbolic equations and the submicroscopic level they represent were coded (Table 2). One of the most common misconceptions was depicting aqueous reactants of all three equations as molecular pairs prior to mixing. Some of these students rationalized that

pubs.acs.org/jchemeduc

_

r 2009 American Chemical Society and Division of Chemical Education, Inc.


upon mixing the “molecules” break apart in order for them to “change partners”. In the case of the first and third equations, six and eight students, respectively, thought that once the ions switched partners some “molecules” formed the precipitate while the other “molecules” remained dissolved in the water as molecular pairs. Only two to three students thought that the aqueous product was present as separate ions in solution, while the precipitate remained a molecular pair. In fact, the precipitate was commonly (17 out of 21) illustrated as molecular pairs, with Table 1. References to Ionic and Molecular Equations Conveyed in Students' Drawings of the Three Ionic Equationsa Number of Students Drawn References to Ionic Equations

Eq 1

Eq 2

Eq 3

Molecular equations

15

14

16

Total ionic equations

16

18

18

Both molecular and total ionic equations

10

11

13

Net ionic equations a

0

2

There were a total of 21students.

one student representing the precipitate as separated ions and three students omitting a submicroscopic-level depiction. Three students drew covalent bond lines to represent a single bond between the atoms of their product molecules. No students represented the precipitate as an aggregate of more than the simplest ratio of ions. In the case of the equation that does not result in a reaction, 13 students illustrated the aqueous reactants as molecular pairs prior to mixing. Interestingly, nine of these students indicated that upon mixing the reactant molecules broke apart. Two students indicated that atoms that make up compounds must become charged ions before they are able to separate, believing that prior to becoming charged they are neutral. Five of the nine students depicted the switching or changing of partners and drew the formation of the molecular product molecules potassium chloride and sodium nitrate; however, two of these students, S7 and S2, indicated that the product molecules dissociated into separate ions, while students S6 and S8 indicated that the product molecules remained as molecules that were also soluble. In essence, S6 and S8 appeared to think that a reaction occurred since they drew the atoms rearranged; however, macroscopically

Figure 1. Example of one student's (S8) depiction that illustrates both molecular and total ionic equation features.


_


_


_


115

Research: Science and Education Table 2. Misconceptions Conveyed in Students' Drawings of the Three Ionic Equationsa Number of Students Misconceptions

Eq 1

Eq 2

Eq 3

Aqueous reactants are molecular pairs prior to mixing

12

10

13

Upon mixing reactant molecules break apart

10

8

Precipitate represented as molecular pair(s)

17

Aqueous product not represented as free ions (made of molecular pairs)

8

Incorrect formulas

2

10 0

Reactant NaCl does not break apart, it is not soluble

3

Products KCl and NaNO3 are formed

5

Product molecules that formed then dissociate

5

3

Covalent bond line drawn between ions

3

Ions separate then form molecules for both products

6

Compounds become charged ions to separate

3

3 8

2

Ions separate, form molecular pairs, aqueous solution separates into ions

2

Both pairs of reactant ions switch or change partners a

11 17

14

3 14

14

There were a total of 21 students.

they indicated that both the reactants and products were soluble, meaning no reaction occurred. Three students indicated that there was no reaction because sodium chloride is insoluble and does not dissociate. This explanation could be attributed to the students' recalling the solubility rule that some chlorides are insoluble. Once again, students S9, S14, and S8 drew single covalent bond lines between atoms in their drawings, suggesting a lack of understanding of ionic structures. When learning about total ionic equations, students are often taught that the ions that remain spectator ions can be “canceled out” to aid them in deriving the net ionic equation from the total ionic equation. Some students may be unaware that “canceling out” is inappropriate for describing submicroscopic processes, because matter is not canceling or getting rid of other matter. In this study, three students explained that there was no reaction because the reactants and products canceled each other out. Perhaps this is less of a misconception and more of a vocabulary issue, since orally they conveyed that they understood that the species were still present in the solution. Ben-Zvi et al. (8) reported that students hold a static view of reactions; however, in this study it was noticed that approximately half of the students indicated movement via their misconception that reactant molecules break into ions. Another way that students represented movement was through their depiction of ions switching partners. While it is not surprising that students talked about switching partners since they were taught this method for writing equations, it should raise concern that students infer that the movement of ions in solution is directed or that ions somehow know where to go. Another trend revealed was that 19 of 21 students omitted water molecules. Ten students provided an iconic, macroscopic representation of water at the surface level, or as wavy lines in the solution. This is not surprising because the instruction they received focused only on the reacting species involved in the ionic equations. However, it is important to recognize that students hold a very simplified view of the submicroscopic nature of solutions. Only two students gave water a more prominent presence. One student labeled the solvent region with two H2O formulas. Another student, S14, drew water molecules mixed in with the 116


_


_

other reaction species. When asked why she drew the water molecules as circles surrounding the formula, she replied, “to show that the H2O is its own molecule and doesn't break up or anything.” One participant, S13, indicated that while he knew that water molecules were present and surrounded the ions, he was uncertain of the exact arrangement. He acknowledged that he did not feel that they were necessary at this level of chemistry. What Does aq Mean? During the semistructured interview students were asked what the aq in parentheses in the balanced equations meant. Thirteen students stated that aq stood for aqueous, while the other students did not specifically state what the abbreviation meant. Upon analysis of the definitions of aqueous, it appears that students tried to describe some of the properties of aqueous solutions when they attempted a definition. Nine students focused on the macroscopic properties and referred to aqueous as being the liquid state of a traditional solution. Twelve students focused their description on the presence of solute, ions, molecules, or “something” in the solution. Three of the twelve students who focused their descriptions on the presence of solute, ions, molecules, or “something” explained that one would be unable to see the solute in the solution. Interestingly, fourteen students did not mention water in their definition of aqueous, and two admitted that they were uncertain whether water was the solvent. Five students thought that aqueous was a transitional state between a solid and a liquid. Only one student, S10, was unable to define what aqueous meant and did not even attempt an explanation. These results are not surprising, since previous research by Kelly and Jones (20) reported that even when students viewed animations of salt dissolution that emphasized the nature of an aqueous sodium chloride solution, they were unable to relate that process to the same solution used as a reactant in a precipitation reaction. Summary and Implications When students are taught ionic equations from a symbolic perspective a question that chemistry instructors need to consider


_



is, “How are students making sense of that information?” Studies (6-9) have indicated that students have misconceptions about the nature of ionic reactions. This article suggests that when instruction emphasizes the symbolic nature of matter, students tend to fit their submicroscopic-level ideas to the equations, developing or reinforcing misconceptions that may be difficult to correct. This research indicates that students have an ill-formed sense of what aqueous solutions are and do not have a clear understanding of what the symbol aq represents. Thus, when they are introduced to molecular equations, where formulas of the reactants and products are represented without indicating their ionic character, students seem to conclude that the formulas are representing molecules. When students are introduced to total ionic equations, where all of the soluble, strong electrolytes are represented as ions, the students fail to realize that this is simply another way to symbolically convey the submicroscopic nature of the reaction. Instead, some students consider the total ionic equation as an intermediate step in the reaction. They believe that the molecules break apart into ions and then pair up to form products. This misconception was especially pronounced in the depictions of students S4 and S16, who showed compounds becoming charged ions before they separated, suggesting that these students did not interpret the formulas correctly and did not realize that the compounds were initially composed of ions. Further evidence of the strong influence the molecular equation has on students' conceptions is noted in students' depictions of the simplest whole number ratio of reactants and products and by representing the product as molecular pairs.

connection between the equations that they have been taught and the submicroscopic nature of the equations, either through instructor-modeled drawings and textbook images or with animations that emphasize molecular nature. In addition, instructors need to help students understand the strengths and limitations of their submicroscopic-level representations; for example, instructors can indicate when they are excluding water molecules in depictions to focus on the reacting species.

Recommendations for Instruction

Recommendation 4: Address Misconceptions Directly Instruction should be designed specifically to address misconceptions such as those revealed by this research. For example, many students tend to represent a formula by drawing the simplest whole number ratio of ions as a molecular pair. The instructor could start with this representation and then ask the students to consider whether aqueous molecules would conduct electricity. This approach could lead to helpful discussions about the nature of strong and weak electrolytes. Next, to help students think about the number of ions that are present in solutions the instructor could ask students how many ions would need to be present for them to be able to see a precipitate. Inviting the students to recognize the importance of the quantities of ions involved in a reaction may help students recognize the limitation of a drawing that only shows the simplest ratio of species reacting.

Recommendation 1: Connect Simplified Views to Complex Views of the Submicroscopic Nature of Reactions The findings of this research reveal that introductory chemistry students tend to make obvious simplifications when illustrating their understanding of chemical equations. When instruction focuses on the stoichiometry of the equation, students tend to focus on the reacting species and the products that are formed using the simplest whole number ratio of ions and formula units reacting. This simplistic view can hinder students' ability to learn from visualizations that convey a more accurate representation of the large numbers of ions and water molecules that are interacting. Kelly (21) reported that some students viewing animations of salt dissolution were overwhelmed and confused by what they were viewing, indicating that “it just looks like a bunch of balls moving around”. Instructors could help students make a connection to a more complex submicroscopic view of reactions by asking them to first draw their understanding of the submicroscopic nature of a reaction. Then the instructor could model his or her own detailed understanding of the submicroscopic nature of a reaction or show an animation of its submicroscopic nature and ask the students to compare and contrast their drawings to the instructor's or to the animation. Through discussion, the instructor should emphasize how the symbolic formulas relate to events at the submicroscopic level. Recommendation 2: Address the Connection between the Submicroscopic Level and the Symbolic Nature of the Chemical Equation Introductory chemistry students, admittedly, do not often visualize atoms when picturing equations. Instead, they fixate on the symbolic nature of the equation. Students need to see a


_

Recommendation 3: Address the Connection between the Submicroscopic and Macroscopic Levels Students incorporate macroscopic references in their drawings in an attempt to relate submicroscopic events to first-hand macroscopic observations. It is important that the instructor also incorporate a more realistic relation between these levels to foster the natural connection that students try to make. One way to address the relation would be to present the students with an image of a typical macroscopic representation such as a beaker or flask with a marker leading from the solution to a picture that depicts the submicroscopic representation of the aqueous solution. Instructors could also make a connection by demonstrating a reaction, showing a video clip of a demonstration or having the students perform a reaction, followed by asking students to draw what they think is happening at the submicroscopic level. It might also be helpful for students to analyze how an animation of the submicroscopic-level reaction translates to what they would see macroscopically.

Recommendation 5: Address the Responsibilities of the Students Students can be reminded to read carefully and completely. In this study, it appears that students focused on the formula (e.g., NaCl) while ignoring the designated state, (e.g., aq or s) of the formula. It is possible that students drew incorrect depictions simply because they did not read carefully. In addition, instructors can help students to recognize the importance of the details of symbolic representations, and how omitting something as simple as a charge on an ion can have serious consequences in conveying the meaning of the chemical event. Students can also be encouraged to examine the illustrations in their textbook or other sources and to consider the limitations of the models. Conclusions The goal of this research was to learn the nature of submicroscopic-level misconceptions that students demonstrate


_


_


117


in their drawn and oral explanations of precipitation reactions. The findings indicate that students are unclear of the definition of an aqueous solution and of the nature of ionic compounds. Students represented aqueous ionic reactants as molecular pairs prior to mixing, perhaps modeling their drawings on the molecular equations. Interestingly, some students incorporated an intermediate step of free ions that may indicate how they interpret total ionic equations. All 21 students indicated that once ions are free they attract others to form molecules of precipitate, again perhaps modeling their representations on the molecular equations. Some students thought the aqueous product formed molecules, while others knew that the aqueous product consisted of free ions. In addition, the findings suggest that students depict a simplified view of chemical equations by leaving out water molecules and by illustrating the simplest whole number ratio of chemical species. Since instruction for learning about chemical reactions tends initially to spotlight the symbolic and macroscopic nature of chemical reactions, many students struggle to conceptualize the submicroscopic level. Instructors need to recognize that when course content emphasizes the symbolic nature of equations, students have difficulty comprehending how molecular equations and total ionic equations can represent the same submicroscopic events while differing in form. Developing instructional strategies such as pictures that are designed to address the needs identified by this research is recommended to help students gain a more comprehensive understanding of the submicroscopic nature of ionic equations. Acknowledgment The authors acknowledge support from the National Science Foundation (NSF) award number REC-0440103, Design Principles for Effective Molecular Animations. They also thank Loretta Jones, University of Northern Colorado, for helpful comments on the manuscript. Literature Cited 1. Johnstone, Alex H. J. Chem. Educ. 1993, 70, 701–704. 2. Nurrenbern, Susan C.; Pickering, Miles. J. Chem. Educ. 1987, 64, 508–509. 3. Pickering, Miles. J. Chem. Educ. 1990, 67, 254–255.

118


_


_

4. Sawrey, Barbara. J. Chem. Educ. 1990, 67, 253–254. 5. Nakhleh, Mary. J. Chem. Educ. 1993, 70, 52–55. 6. Ahtee, Maija; Varjola, Irma. Int. J. Sci. Educ. 1998, 20, 305– 316. 7. Garnett, Patrick J.; Garnett, Pamela J.; Hackling, Mark. W. Stud. Sci. Educ. 1995, 25, 69–95. 8. Ben-Zvi, Ruth; Eylon, Bat Sheva; Silberstein, Judith Educ. Chem. 1987, 24, 117–120. 9. Yarroch, William L. J. Res. Sci. Teaching 1985, 22, 449–459. 10. Garnett, P. J.; Hackling, M. W.; Vogiatzakis, L.; Wallace, T. Year 10 Students' Understandings of Chemical Equations. Paper presented at the 9th National Convention, Royal Australian Chemical Institute, Melbourne, 1992. 11. Hesse, Joseph J.; Anderson, Charles W. J. Res. Sci. Teaching 1992, 29, 277–299. 12. Hinton, Michael E.; Nakhleh, Mary B. Chem. Educ 1999, 4, 158–167. 13. Boo, Hong-Kwen; Watson, J. R. Sci. Educ. 2001, 85, 568–585. 14. Lee, Kam-Wah L. Res. Sci. Educ. 1999, 29, 401–415. 15. Brown, T. L.; Lemay, H. E., Jr.; Bursten, B. E.; Burdge, J. R. Chemistry the Central Science, 10th ed.; Pearson Education, Inc.: Upper Saddle River, NJ, 2006. 16. Crotty, M. The Foundation of Social Research: Meaning and Perspective in the Research Process; Sage Publications: London, 1998. 17. Ferguson, R. L. Constructivism and Social Constructivism. In Theoretical Frameworks for Research in Chemistry/Science Education; Pearson Education, Inc.: Upper Saddle River, NJ, 2007. 18. Merriam, S. B. Qualitative Research and Case Study Applications in Education; Jossey-Bass Publishers: San Francisco, CA, 2001. 19. Lincoln, Y. S.; Guba, E. G. Naturalistic Inquiry; SAGE Publications: London, 1985. 20. Kelly, Resa M.; Jones, Loretta. L. J. Chem. Educ. 2008, 85, 303– 309. 21. Kelly, Resa M. Exploring How Animations of Sodium Chloride Dissolution Affect Students' Explanations. Ph.D. Dissertation, University of Northern Colorado, Greeley, CO, 2005.

Supporting Information Available Additional examples of student drawings. This material is available via the Internet at http://pubs.acs.org.


_


An Analysis of Undergraduate General Chemistry Students

Recommend Documents