A Comparison of University Lecturers' and Pre-service Teachers

Research: Science and Education edited by

Chemical Education Research

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

A Comparison of University Lecturers’ and Pre-service Teachers’ Understanding of a Chemical Reaction at the Particulate Level Kam-Wah Lucille Lee School of Science, Nanyang Technological University, Singapore; [email protected]

Background Recent studies have shown that while chemistry can be taught at (i) the microscopic level; (ii) the macroscopic or sensory level; and (iii) the symbolic level, many chemistry courses concentrate on the symbolic level, neglecting the other two (1). It is true that some teachers do go an extra mile to organize their instructional activities at both macroscopic and symbolic levels; they demonstrate experiments and conduct practical work to show chemical phenomena (the macroscopic level), and explain concepts using chemical equations or symbols (the symbolic level). Nevertheless, often enough they too fail to link the two levels to the microscopic level. It is well established that many students find it difficult to understand chemical concepts. Ebenezer and Erickson (2) describe a typical real-life grade-11 chemistry student, Andrea, who, although she was capable and hard working, felt confused while tackling ionic equations. Neither the detailed notes with explanations supplied by her teacher nor the teacher’s demonstration of the conductivity of various salts predicted by the ionic equations taught seemed to help her much. Andrea’s words reflect the predicament of many chemistry students (2, p 182): I’m trying to make sense of all this balancing stuff (i.e., the symbols for the elements, ions, and their respective states), but visually and mentally it is making me dizzy. I just don’t understand!

One possible explanation of Andrea’s confusion would be that her teacher explained the concept of ionic equations at the symbolic and macroscopic levels only. Another would be that while the lessons might have been taught at all three levels, these were inadequately connected. As a result, the information for each level remained compartmentalized in the long-term memory of Andrea. The literature shows that chemistry instruction that emphasizes all the three levels, but especially the microscopic level, can improve students’ understanding of chemistry concepts (3, 4). Laverty and McGarvey (4) designed and evaluated a scheme of work for teaching elements and compounds to students 13–14 years old. The teaching program incorporated the constructivist teaching sequence of the Children’s Learning in Science (CLIS) project (5) and included an opportunity for the teacher to investigate the students’ (own) ideas at the “atomic” level of (i) the formation of magnesium oxide from the combustion of a given mass of magnesium in air and (ii) the formation of copper oxide from heating copper(II) carbonate. The teacher demonstrated the two experiments macroscopically. The students discussed what was happening and represented diagrammatically the reactions in terms of particles. A number of misconceptions 1008

were detected, but the teachers then followed the CLIS strategies to provide the students with opportunities to abandon these and adopt the scientifically acceptable ideas of elements and compounds. On the other hand, many research studies have also shown that students find it difficult to relate chemistry concepts at the microscopic level (6–10). For instance, the replicated studies conducted by Nurrenbern et al. (6 ), Sawrey (7 ), and Lee et al. (8) found that most students were more able to answer the questions that used symbols and numbers in the traditional test questions than to answer those conceptual questions involving particles. Other studies by Yarroch (9) and Lythcott (10) found that many students were not able to draw the meaning of their balanced chemical equation in terms of “atoms” and “molecules”. In the study of chemical changes, several Swedish studies (11) found that only a minority of the students (20%) thought that a chemical change involved an interaction between the reactants. Many students held other views, such as “it just happens like that”, “the matter just disappears”, “the products must somehow be contained in the reactants”, “the product material is just a modified form of the starting material”, etc. The reactants, for most students, might mix with each other, but the new substances were not formed by chemical rearrangements among these mixed atoms. Furthermore, they did not see that matter was conserved (in atomic terms) when new molecular substances were formed. In the studies of combustion (12, 13), Meheut et al. and Donnelly et al. found that many students perceived oxygen gas as something necessary for combustion but did not regard it as interacting with the combustible materials. One such typical explanation given by the students was that a combustible material is made up of substances that eventually appear as combustion products causing its disappearance. These research studies have all strongly implied the need for chemistry teachers to lay emphasis on the particle method (showing the interaction between atoms and molecules at the microscopic level) so that students can develop scientific understanding of chemistry concepts. In these studies, it has been assumed that teachers can successfully handle the particle model. Is this assumption valid? In contrast to students, university chemistry lecturers and schoolteachers still remain under-studied. This study was an extension of the study by Laverty and McGarvey (4), but focused on teachers instead of students. Specifically, this study sought to ascertain how university chemistry lecturers and pre-service chemistry teachers perceived a chemical reaction at the “atomic” level. Even though the mechanism of the chemical reaction selected for this study,

Journal of Chemical Education • Vol. 76 No. 7 July 1999 • JChemEd.chem.wisc.edu

Research: Science and Education

namely the combustion of magnesium in air, has not yet been reported in the literature, this simple reaction in fact is commonly taught in high schools. The method used to collect data was to ask the teachers to draw diagrams depicting the reaction in terms of particles. In this study, how the majority of the lecturers viewed this reaction would be recommended as a model for the school chemistry instruction. Furthermore, a better understanding of teachers’ particulate views of the reactions affords teacher trainers and teachers important insights for developing more appropriate instructions and materials for high school chemistry curriculum. The Study Ten chemistry lecturers from a university in Singapore made up the first group. The second group had 88 pre-service chemistry teachers (referred to as student teachers from here onward) from three year cohorts of chemistry graduates who took the chemistry teaching method course in the one-year Post Graduate Diploma Program of the National Institute of Education. The instructional mode for the chemistry teaching method course was usually lecture with tutorial on various pedagogical methods that may be used to teach chemistry at the high-school level. The tutorial activities were conducted in groups. One representative of each group reported to the whole class about the results of their activities at the end of the tutorial session. The overall feedback from the peers and lecturer followed. To collect data, since the chemistry lecturers were all familiar with the reaction, they were approached individually and invited to make a diagrammatic representation of particles to show the reaction mechanism. They were advised to use circles (s) or shaded circles (d), etc., to present their diagrams. The researcher interviewed seven of these lecturers to seek clarification about responses that appeared ambiguous. For student teachers, the chemical experiment was demonstrated by the researcher in one of the lecture-with-tutorial sessions. The topic for this lecture was “the use of constructivist approach for teaching chemistry”. The student teachers were then divided into 18 groups of 4–6 members each to discuss the reaction mechanism involved in terms of particles. Like the chemistry lecturers, they were asked to use circles or shaded circles to represent particles to show the reaction mechanism on an overhead transparency. A representative from each group was invited to explain their diagrams after the discussion. Results

The Forms of the Reactants Two main views about the forms of the two reactants: magnesium and oxygen gas, were indicated by the lecturers and student teachers. Eight of the 10 lecturers and 5 of 18 groups of student teachers thought that the molecules of oxygen (or nitrogen) sat on top of the magnesium lattice atoms before the reaction took place (Fig. 1). On the other hand, one lecturer and 7 of the 18 groups of student teachers thought that heat caused both magnesium atoms in the lattice and oxygen atoms of its molecules to vibrate and finally turn into free atoms and/or ions. Electrons were lost, gained, or equally divided between atoms in this process. The atomic particles may be formed as in Figure 2. Two groups of student teachers, however, suggested that, instead of two reactants, only one be broken into free particles. Between them, one group thought that the oxygen molecules would break into atoms, whereas the other group thought that the magnesium atoms were ionized before they interacted with the other intact reactant to form magnesium oxide. The Steps in the Mechanism The following three views about the mechanism of the reaction were indicated in the lecturers’ and student teachers’ diagrams: 1. Formation of intermediates, 2. Direct interaction of free atoms and/or ions, and 3. Combination of 1 and 2.

O2 O2

Mg

Figure 1. Oxygen gas molecules sitting on top of magnesium lattice atoms.

(i) (ii) (iii) (iv) Mg (v)

The 28 resulting diagrams were examined for common characteristics in the particulate representations of the reaction mechanism. The commonalties were identified and classified under the following three areas: 1. The forms of the reactants,

2+

2-

,

+

, ,

-

Mg2+ + 2e–

(oxygen ions) (oxygen ions) (oxygen atoms) (magnesium ions)

Mg atoms

Mg lattice

Figure 2. Types of free particles formed during combustion.

O2

O

O

Mg

Mg

O

O

Mg

Mg

2. The steps in the mechanism, and 3. The forms of the products.

“The forms of reactants” refers to the pre-reaction stage of the reactants. “The steps in the mechanism” concerns the steps leading the reactant particles to a reaction. “The forms of the products” concerns the arrangement of particles in the products formed.

Mg

Intermediate (i)

Intermediate (ii)

MgO lattice

Figure 3. Diagrammatic representation of intermediates produced during combustion.

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2 Mg –

O2 + 4 e

or

2 Mg2+ + 4 e– 2 O2– +

–

or

MgO

x x

O2–

Mg2+

Figure 5. A student–teacher group’s diagrammatic representations of ionic bonding of MgO.

Figure 4. A lecturer’s diagrammatic representation using the intermediates mechanism.

Formation of Intermediates Before the formation of magnesium oxide, at the points of contact, oxygen molecules and magnesium atoms of the lattice attract each other and form intermediates as shown in Figure 3. In the intermediate stages (i and ii), the attraction between oxygen and magnesium is getting stronger while the attraction both between the oxygen atoms of the oxygen molecules and between the magnesium atoms in the lattice is getting weaker. The dissociation of these intra-elemental bonds is complete when magnesium oxide is formed. Figure 4 is a lecturer’s diagrammatic representation showing this mechanism. Seven of the 10 lecturers shared a similar view of intermediates being formed in the transition stage of the reaction, but only 4 of 18 groups of student teachers had this view. Among the lecturers, two suggested that both nitrogen and oxygen gas were active, the final product being a mixture of magnesium oxide and nitride. They emphasized the role of energy supply as a determinant of the reaction. “If the energy is sufficient, MgO will be formed from the intermediate.” Although one lecturer and one student-teacher group suggested that the molecules of oxygen (or nitrogen) and the magnesium lattice came into contact with each other physically (Fig. 1), neither showed in the diagram how the particles of one reactant interacted with those of the other. When the lecturer was interviewed, he said, “It just happened.” Direct Interaction of Free Atoms and/or Ions The free particles of the reactants (e.g., oxygen ions, oxygen atoms, magnesium ions, or magnesium atoms) are produced at the initial stage (Fig. 2). When the free particles of magnesium and oxygen come into contact with each other, they directly combine and form magnesium oxide. The reaction is interatomic between individual free atoms and/or ions. Only one lecturer and 7 of the 18 groups of student teachers held this view. The lecturer’s diagram showed the formation of gaseous atoms of magnesium and oxygen in the first step, and then the gaseous ions of magnesium and oxygen in the second step. The free gaseous ions then combined to form 1010

Figure 6. A student-teacher group’s diagrammatic representation using the free particles mechanism.

Figure 7. A student-teacher group’s diagrammatic representation using combined mechanism.

magnesium oxide in the third step. Of those sharing this view among the student teachers, 3 groups showed the ionic bonding of magnesium oxide in their diagrams. They either used ionic equations at the symbolic level or circles with “+” and “–” signs in them, or with electrons on the circles (see Fig. 5). A diagrammatic representation of a student teacher’s group is shown in Figure 6.



MgO

Figure 8. Closely packed representation of magnesium oxide.

Figure 9. Loosely packed representation of magnesium oxide.

Combination of Intermediate-Formation and DirectInteraction Mechanisms A minority of participants used a combination of the intermediate-formation and direct-interaction views to explain the reaction. One lecturer suggested that the product could be formed through the formation of intermediates and direct interaction between the free particles but it all depended on the specific conditions. The lecturer stressed that in the normal laboratory situation, the reaction mostly took place through intermediates. Two groups of student teachers also used this combination of views to explain the reaction, but they differed on which reactant broke into free particles. One group thought that the oxygen molecules broke into atoms before combining with magnesium, while the other group thought that before joining oxygen molecules, the magnesium atoms (were) ionized and formed first an intermediate and then a compound. A diagram of a student-teacher group is shown in Figure 7 to illustrate this view.

The life span of the intermediates is so short that they are unlikely to break up into free particles, such as atoms or ions, under laboratory heating condition. In contrast, 50% of the student teachers thought that the magnesium lattice and oxygen gas molecules broke into either free atoms or ions before they reacted with each other. What accounts for this common alternative view so different from that held by most of the chemistry lecturers? From the same group of lecturers, two were interviewed to find out how they explained the student teachers’ alternative view. These two, one an inorganic chemistry specialist, the other a structural chemistry specialist, have some teaching experience in high schools. They felt there could be two possible influences at work: the fundamental principle of ionic bonding and the concept of the Born–Haber cycle. The fundamental principle of ionic bonding (14 ) is part of the Singapore– Cambridge O-level chemistry curriculum and the Born– Haber Cycle (15) is part of the A-level chemistry curriculum. The discussion of these two influences follows. The Influence of the Principle of Ionic Bonding Ionic bonds are formed between metals and nonmetals, when the electrons are transferred from metals to nonmetals to get the electronic configuration of a noble gas. As a result, metal atoms form metal ions (positively charged), and nonmetal atoms form nonmetal ions (negatively charged). What holds them together is the force of attraction between the oppositely charged ions. This force is called an ionic bond. The principle of ionic bonding probably underlay the student teachers’ alternative view of the attraction of the free particles. Magnesium oxide is presented in school as an ionic compound in which two electrons are transferred from a magnesium atom to an oxygen atom to form magnesium oxide, a compound consisting of divalent magnesium and oxide ions. Their school-time experience of the formation of magnesium oxide in a spectacular burning reaction probably still haunts them. They probably interpreted this heating as responsible for turning the oxygen molecules and magnesium lattice atoms directly into positive and negative ions, which attracted each other and hence formed the ionic compound. Free particles were prominent in the findings of Laverty et al. (4), in that a number of students aged 13–14 thought that magnesium and oxygen atoms broke apart and mixed to form magnesium oxide; others thought that only magnesium atoms separated before magnesium oxide was formed. The Influence of the Concept of the Born–Haber Cycle The second possible influence could be the concept of the Born–Haber cycle. This is usually used to calculate the lattice energy of a crystal, which is defined as the energy liberated when one mole of the substance is formed from its constituent gas-phase ions. In theory, lattice energies can be deduced from

The Forms of the Products In the particulate representation of the end product, magnesium oxide, two kinds of arrangement were indicated. Eight lecturers and 8 groups of student teachers showed explicitly that the end product of magnesium oxide was arranged in a compact order (see Fig. 8). However, 8 groups of student teachers (but no lecturer) drew the pictorial representation of magnesium oxide suggestive of a loose order (see Fig. 9). Discussion This study shows some significant differences between the two groups of teachers in their views of how magnesium and air interact with each other at the microscopic level. These differences pertain to two aspects: reactions involving intermediates vs free particles, and the arrangement of particles in magnesium oxide.

Reactions Involving Intermediates vs Free Particles Eight of the 10 lecturers held the intermediates view or a combination of the intermediates and free particles views, whereas half the student teachers held the free particles view or a combination of the intermediates and free particles views about the reaction of burning magnesium in air. Even though the mechanism of this reaction has not been reported, the majority of the lecturers agreed that the reaction between magnesium lattice atoms and oxygen gas molecules involves intermediates which denote the transition stages between the reactants and the product. The interaction between the tworeactant particles is intramolecular rather than interatomic. The dissociation and formation of bonds occur spontaneously.

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Conclusion Mg(s) + 1/2 O2(s) ∆H2 Mg(g) ∆H3 Mg2+(g)

∆H1

MgO(s)

∆H4 O(g) ∆H5

∆H6 (lattice energy)

O2–(g)

Figure 10. Born–Haber cycle of the combustion of magnesium in air.

heats of formation by use of suitable energy cycles, commonly called “Born–Haber cycle”. In the calculation of the lattice energy of magnesium oxide, the following Born–Haber cycle reactions of Figure 10 may be used. All the student teachers learned this concept at A-level chemistry and also during the first year of their university chemistry courses. The concept of the cycle of heats of formation probably also accounts for the student teachers’ alternative view that magnesium lattice and oxygen gas turn into free particles, such as magnesium gaseous atoms and ions, oxygen gaseous atoms and ions, prior to the formation of magnesium oxide lattice. During the interview, the two lecturers emphasized that under ordinary heating conditions, the free particles such as gaseous atoms and ions did not go through the different stages shown in Figure 10. They said that the cycle was used just for calculating the reaction energy.

Formation of Loosely Packed Magnesium Oxide Molecules Again, nearly half the student teachers (like many of the students of Laverty et al.) did not arrange the magnesium oxide particles in a closely packed manner (Fig. 8). It was somewhat surprising to see that so many future teachers, all chemistry majors, did not hold the scientific concept of the arrangement of particles in a solid. This is yet another telling example of the conceptual detachment that those quite successful students demonstrate toward the science knowledge they acquire.

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This study found that well-educated teachers have views that differ from those currently accepted in the science they are teaching (16, 17 ). Some insights into the possible sources of their alternative views have been provided with regard to a simple reaction. A model of the particulate behavior of the combustion reaction of magnesium has also been identified and highlighted. The bond formation and bond breaking between the magnesium lattice atoms and oxygen gas molecules continue spontaneously throughout. Discrete free particles of magnesium and oxygen are unlikely to exist under ordinary laboratory heating condition. This intermediate model for explaining such a simple combustion reaction should be included in the high school chemistry course to improve students’ understanding at the microscopic level. Student teachers of chemistry, in particular, need help if they are to link their instruction at the microscopic level to the macroscopic and symbolic levels. The ability to pass confidently between these levels should be an important goal for student teachers to ensure that they will not pass conceptual misunderstandings on to their students. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8.

9. 10. 11. 12. 13. 14. 15. 16. 17.

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A Comparison of University Lecturers' and Pre-service Teachers

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