Addressing Misconceptions Related to Mass–Matter Conservation and

Demonstration Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX

pubs.acs.org/jchemeduc

Addressing Misconceptions Related to Mass−Matter Conservation and Bond Energetics with a Modified Gauss Accelerator Robert G. Gullion,† Terry Gullion,*,‡ Michelle Richards-Babb,*,‡ and Mark Schraf‡ †

Morgantown High School, Morgantown, West Virginia 26501, United States C. Eugene Bennett Department of Chemistry, West Virginia University, Morgantown, West Virginia 26506, United States

‡

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S Supporting Information *

ABSTRACT: We describe a demonstration to confront misconceptions associated with mass−matter conservation and bond energetics. This demonstration is based on the Gauss Accelerator and is accompanied by hands-on manipulatives. The demonstration exemplifies mass−matter conservation, bond breaking, and bond making using a chemical-analogue system: a neodymium magnet (M) with two ferromagnetic steel balls (S) attached in an M−S−S (“reactant”) configuration resting on an aluminum track. A third steel ball rolled toward the free side results in the highenergy expulsion of the terminal steel ball and a final S−M−S (“product”) configuration. Discussion of differential “bond” energies (MS−S vs S−M) and observation of the number and type of each “atom” present prepares the audience for the demonstration. In a repeated-measures pre−post design, students (N = 9) performed significantly better (t(8) = 7.234, p < 0.05, d = 2.41) on a conceptual-knowledge assessment of their understanding of mass−matter conservation and bond energetics after engaging with the demonstration. Complete information on how to enable others to construct the track, gather the demonstration components, and safely run the demonstration is provided. This demonstration has been used for audiences of all ages and with different levels of science knowledge. KEYWORDS: General Public, Demonstrations, Analogies/Transfer, Hands-On Learning/Manipulatives, Misconceptions/Discrepant Events, Thermodynamics, Covalent Bonding

H

conversion of adenosine triphosphate (ATP) to adenosine diphosphate (ADP) may be introduced in a simplified manner as the breaking of the “high potential energy phosphate bond” where potential energy is “primarily stored in the bonds between the phosphates”7 with subsequent loss of phosphate and release of energy. Such a simplified description seemingly equates phosphate bond breaking in ATP to energy release. What is easily overlooked by students as they grapple with macroscale understanding of biological processes is that the overall ATP-to-ADP conversion process, which is a hydrolysis reaction, involves more than the breaking of one bond between phosphates.7 In fact, the conversion process involves the breaking of two reactant bonds, the weak, “high potential energy” O−P bond between phosphates in ATP and the H− OH bond in water, and the formation of two product bonds, the H−O phosphate bond in ADP and the HO−phosphate bond of the inorganic pyrophosphate products.7 The overall process is exothermic, meaning that more energy is released when the product bonds are formed than is absorbed when the reactant bonds are broken, and this difference in energy is

eat and work, potential energy and kinetic energy, and bond breaking and bond making are simple topics, yet they are conceptually difficult for many students to understand. Interactive PhET energy simulations,1 interdisciplinary and early introduction of energy concepts including bond energetics,2 and a card game with scoring dependent on bond enthalpies3 are methods used to improve students’ understanding of the relationships among heat, work, and energy. A strong understanding of bond breaking and bond making is essential for advanced topics in chemistry such as exothermic and endothermic processes. Energy is required to break a bond and to separate the atoms that are attracted to one another.4 The stronger the bond, the lower the potential energy of the bonded system, and more energy needed to break the bond. Conversely, energy is released when a bond is formed. The stronger the bond formed, the more energy released upon its formation. However, some students leave their chemistry courses with lingering misconceptions about bond making and bond breaking. For example, a common erroneous belief is that bond breaking releases energy.2,5 Unfortunately, this misconception is sometimes perpetuated in biology, where the emphasis is on macroscale understanding of cell functioning and how chemical reactions in biological systems produce energy for organismal functioning.6 For instance, the © XXXX American Chemical Society and Division of Chemical Education, Inc.

Received: August 27, 2018 Revised: January 30, 2019

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DOI: 10.1021/acs.jchemed.8b00697 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Demonstration

Figure 1. Simple track design used for the demonstration. Here, the setup is shown without the acrylic safety shield so that track features are more easily seen. One M−S−S assembly is shown and the steel ball that initiates the reaction is shown to the far right sitting beside the low-angle ramp. The meter stick is included for scale.

Figure 2. Close-up view of the demonstration’s M−S−S setup and the low-angle initiation ramp. The meter stick is included for scale.

energy gained by the products.18 Comparisons of the number and type of each object present before and after the demonstration allow observers to investigate atomic theory, including a few of its postulates and mass laws such as the conservation of mass. This largely visual demonstration was developed as part of our annual Children’s Chemistry Show, which is modeled after the Faraday Christmas Lectures.19

available for work via muscle contraction as well as for other processes. Misconceptions also exist related to student understanding of non-nuclear chemical reactions, specifically mass−matter conservation.8 Students learn that mass is conserved and that atoms are not created or destroyed during chemical reactions. However, when applying these concepts, students may display knowledge rather than genuine understanding as they, for instance, indicate that the weight of an iron bar decreases upon rusting8,9 or that the weight of iodine gas weighs less than the same mass of solid iodine.10 Demonstrations11,12 and hands-on manipulatives13,14 aid students in confronting misconceptions such as those described above. Research indicates that optimal learning occurs when demonstrations are accompanied by questioning to promote discussion (student−student and student− instructor)11 and when integrated into existing knowledge.12 Herein, we describe a demonstration with accompanying hands-on manipulatives that has been used for audiences of all ages and with different levels of science knowledge. This demonstration is based on the Gauss Accelerator (or Gauss Rifle), a demonstration commonly used in physics settings to discuss conversion of magnetic energy to kinetic energy.15,16 The initial setup of the Gauss Accelerator in its simplest form is a cylindrical neodymium magnet (M) with two ferromagnetic steel balls (S) attached in an M−S−S configuration resting on an aluminum track. A third steel ball is gently rolled toward the free side of the magnet in the M−S−S configuration. The result is a new S−M−S configuration and the expulsion of the terminal steel ball (i.e., magnetic energy is converted to kinetic energy).17 We have adapted the Gauss Accelerator demonstration to model the basic properties of non-nuclear chemical reactions and to confront misconceptions of the energetics of bond breaking in reactants and bond making in products. Differences in the potential energies of the reactants and products lead to net energy release that is easily recognizable in the kinetic

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DEMONSTRATION COMPONENTS

Materials

A detailed list of suppliers and costs of major materials are listed in Table 1S in the Supporting Information. Steel balls (1 in. diameter) and neodymium magnets were purchased and used as visual analogues of atoms and the forces that hold atoms together in molecules. The sizes of the 1 in. diameter steel balls and magnets allow them to be easily seen from distances of 20 feet or more, as is typical in a large lecture hall. We have fashioned a simple, straight track as shown in Figure 1. Construction of the track requires the following: • Track: approximately 5 feet of aluminum T-slotted framing for the simple track • Base: wooden board for use as a stable base for the aluminum track • Attachments: aluminum, brass, or other nonmagnetic screws to attach the track to the base • Wooden ramps: fashioned with a 2-degree grade and grooved to provide a steel ball with minimal momentum for demo initiation • Safety shield: acrylic sheets 3/8 in. thick, 4 in. wide, and spanning the length of the track Track Construction

Construction of the track is described in greater detail in the Supporting Information. The simple, straight track design shown in Figures 1 and 2 consists of a track made with a straight T-slotted aluminum rail attached by screws to a B

DOI: 10.1021/acs.jchemed.8b00697 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Demonstration

Placement of a second ball a few inches away from the bound ball allows it to roll and bind to the first ball, creating the M− S−S system. Prior to performing the demonstration, the demonstrator leads an introductory discussion on the chemical-analogue reactant system (S + M−S−S) and how the magnet (M), steel balls (S), and M−S−S are loosely analogous to macroscopic representations of matter, atoms, and molecules, respectively. The discussion should include observations of the number and type of each “atom” present as well as a generalized discussion of the overall mass of the chemical-analogue reactant system. After the demonstration, a similar discussion about the chemical-analogue product system (S−M−S + S) along with discussion prompts (e.g., “Did the number of atoms of each type change?”, “Were any atoms created or destroyed?”, “Did the overall mass of the system change?”, and “How is the chemical-analogue product system different from the chemical-analogue reactant system?”) leads audience members to conclude that mass is conserved as are the numbers and types of each atom present but that the atoms are recombined in a different configuration during the chemical-analogue reaction. Further evidence of mass conservation can be obtained by weighing and comparing the total mass of the chemical-analogue reactant system prior to the demonstration to that of the chemical-analogue product system after the demonstration. The demonstrator should discuss with students the number of reactant and product molecules and how, even though the number of molecules remains the same in this demonstration, the number of molecules is not necessarily conserved during non-nuclear chemical reactions. An introductory discussion of the magnet and ball arrangement plus allowing a select few students or audience members to carefully handle the manipulatives provides insight into the physical properties and bond energetics of the magnet−ball M−S−S system. Ask a mature volunteer to remove the outermost steel ball from the magnet. This task is not difficult and requires about 8 pounds of force to break the MS−S “bond”. However, the volunteer’s attempt to remove the remaining ball from the magnet will be noticeably more difficult as about 18 pounds of force is needed to break the M− S “bond”. The demonstrator, through questioning to promote discussion (e.g., “Does bond breaking absorb or release energy?” and “Is the amount of energy absorbed independent of the bond type?”), leads the audience to the observations that (i) bond breaking requires (absorbs) energy and is endothermic, and (ii) the amount of energy absorbed depends on the bond strength (i.e., the stronger the bond, the more energy is required to break it). Asking the volunteer to carefully reassemble the magnet−ball M−S−S system with observation of the energetics (e.g., “Does reassembly require an input of energy?”, “Is reassembly accompanied by energy release?”, and “How do you know energy is released?”) highlights the fact that bond formation releases energy and is exothermic. Clearly, the M−S bond energy (defined as positive), BEM−S, is greater than the MS−S bond energy, BEMS−S. A discussion of the energy threshold required to break bonds, differential bond energies for MS−S versus M−S, and energy comparisons of bond breaking versus bond formation prepares the audience for the demonstration.11 To initiate the demonstration, place and release a lone steel ball on the ramp. Prior to its release, audience members should be asked to predict what will result when the ball is released (i.e., “What will be the final configuration and why?”). The

wooden board. The T-slotted rail is used to guide the steel balls and align the magnet−ball assembly. The track is 50 in. long with a ramp located at the far end to allow objects to smoothly exit the track. A small, low-angle, grooved, wooden ramp is located at the beginning of the track to allow a free steel ball to roll slowly toward the first magnet−ball assembly.

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HAZARDS The neodymium magnets used in this demonstration have a reported pull force of 97.90 pounds per magnet.20 These magnets are not safe for use by children and can cause injury to appendages (e.g., fingers) if caught between magnets and magnetic objects (e.g., steel balls) or especially between two magnets. We recommend wearing leather gloves when handling magnets of this strength. Neodymium magnets that come together are very difficult to separate. Thus, avoid bringing the magnets into direct contact. We house each magnet individually in a wooden container, as shown in Figure 3. Chipping of the magnets may occur after repeated use, so

Figure 3. Wooden storage container for a single neodymium magnet. A magnet and a steel ball are shown for size comparison. Only the magnet is placed in the hole of the wood base. After the magnet is placed in the hole, the wood top is aligned with the wood base via the brass rods and held in place by brass wing nuts.

acrylic safety shields are attached to the tracks to act as safety barriers to retain any chips. Eye protection should be worn by the person performing the demonstrations and leather gloves should be worn by students or audience members who volunteer to handle the magnet and ball arrangements.

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RUNNING THE DEMONSTRATION

Consequences of Bond Breaking and Bond Making and Basic Properties of Chemical Reactions

This demonstration models bond energetics, bond breaking and bond making, and the basic properties of non-nuclear chemical reactions. Postulates of atomic theory,4 such as (i) atoms in substances rearrange to form new substances and (ii) atoms are not created or destroyed during chemical reactions, are addressed with this demonstration and the analogous discussion.4 The discussion can be extended to include insights into the laws of conservation of mass and energy. Use of the simple, straight track and one M−S−S chemical-analogue reactant system best supports student learning as this limits distractions due to extraneous structures and allows observers to focus on the most important aspects of the demonstration.21 Preparation of the M−S−S system begins by placing a magnet on the track. Next, a steel ball is placed a few inches from the magnet; it will roll toward and bind to the magnet. C

DOI: 10.1021/acs.jchemed.8b00697 J. Chem. Educ. XXXX, XXX, XXX−XXX

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Demonstration

questions that display item functioning threats (i.e., correct student response supported by incorrect explanation).23 Prior to the demonstration, the average student answered 1.1 of 6 questions correct (18%). Immediately following the demonstration, the average student answered 3.6 of 6 questions correct (59%). A repeated-measures parametric t test indicated that students (N = 9) performed significantly better on a conceptual assessment of their chemistry understanding of conservation of mass−matter and bond energetics after engaging in the demonstration (t(8) = 7.234, p < 0.05, d = 2.41). Further, students displayed larger gains in their understanding of bond energetics than in their understanding of conservation of mass−matter (71 and 25% possible gain, respectively).

released steel ball will accelerate toward the nearby magnet, soon forming a strong bond to the magnet with the attendant release of energy equivalent to the negative of the S−M bond energy (−BES−M). In turn, the much weaker terminal MS−S bond is broken with an attendant absorption of energy equivalent to the MS−S bond energy (+BEMS−S). For the analogue chemical reaction in eq 1, the change in energy (as approximated by the change in enthalpy) is negative (exothermic) as predicted by summing the respective bond energies of bond breaking and making.4 For a single S + M− S−S system, the reaction is summarized by eqs 2−4: S + M−S−S → S−M−S + S ΔH =

∑ BE (bonds broken in reactants) − ∑ BE (bonds made in products)

ΔH = BEMS − S − BES − M

(1)

Other Uses: Useful Work Obtained from Reactions (2)

Other uses for this demonstration platform can be envisioned. In fact, author R.G.G. used the platform as a loose analogy for how the chemical energy of an exothermic reaction is converted to work. This use was in response to a highschool-physics-class challenge: to propel a model car the farthest. A model car was parked adjacent to the terminal steel ball. The reaction was initiated by a lone steel ball placed on the ramp near the first of three M−S−S arrangements. Some of the excess energy from the chemical-analogue reaction was transferred to the model car. The car accelerated and speedily exited the track because of its gain in kinetic energy. Work was done on the car by the reaction. A discussion of initial and final potential and kinetic energies and how useful work is obtained from a chemical reaction could aid an audience in understanding the connection of work with chemical reactions.

(3)

where BES − M > BEMS − S

and

ΔH < 0

(4)

In this demonstration, less energy is required to break the MS−S reactant bond than is released when the S−M product bond is formed. The excess energy is transferred as kinetic energy to the ejected steel ball (S), which is seen to have a high velocity. After the demonstration, prepare one each of the chemical-analogue reactant (M−S−S) and chemical-analogue product (S−M−S). Ask a mature volunteer to remove the steel balls from each chemical analogue, reporting out to the class the contrasting product- and reactant-bond strengths and which configuration is less stable and has the greatest potential energy. On the basis of the relative differences in reactant and product potential energies, ask the class to explain why the reaction is exothermic and why the steel ball exits at a higher velocity (and higher kinetic energy) than the initial steel ball. The demonstrator may find it useful to draw a reaction diagram (potential energy vs reaction progress) for the chemical-analogue system to aid audience members in determining the origin of the product steel ball’s kinetic energy. Discussion should include initial and final potential and kinetic energies of the chemical-analogue system, the energetics of bonds (those broken in reactants and those formed in products), and the concept of energy conservation.

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CONCLUSIONS We have described a demonstration suitable for large audiences that utilizes hands-on manipulatives. This demonstration engages audience members and improves their understanding of mass−matter conservation in non-nuclear chemical reactions and chemical-reaction energetics, particularly bond breaking and bond making. The chemical-analogue manipulatives (e.g., M−S and MS−S) reinforce to students that bond breaking requires energy and bond making releases energy and that chemical reactions can be used for work.

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ASSOCIATED CONTENT

S Supporting Information *

Results of Conceptual Pre- and Postassessments

To demonstrate students’ improved understanding of nonnuclear chemical reactions and bond energetics, one of us (M.S.) presented the demonstration to a group of students (N = 9) enrolled in an undergraduate preparatory chemistry course. As posited by Nehring and Busch, the demonstration was arranged to maximize student learning.21 For instance, the track was set up to run from left to right and extraneous features were limited through the use of the simple, straight track. A six-question multiple-choice conceptual assessment, provided as Supporting Information, was administered to students prior to the demonstration and immediately following the demonstration in a repeated-measures pre−post design. This design was granted an exemption by our Institutional Review Board (protocol no. 1811348947). The assessment contained three questions each from the content areas of (i) the conservation of mass−matter and (ii) bond energetics. All questions were obtained from Chemical Concepts Inventories published in the literature,9,10,22 but we avoided the use of

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.8b00697.

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Description of materials, construction of chemicalanalogue track, and pre- and postassessment details (PDF, DOCX)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (T.G.). *E-mail: [email protected] (M.R.-B.). ORCID

Terry Gullion: 0000-0002-1899-6884 Michelle Richards-Babb: 0000-0002-0487-566X Notes

The authors declare no competing financial interest. D

DOI: 10.1021/acs.jchemed.8b00697 J. Chem. Educ. XXXX, XXX, XXX−XXX

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Demonstration

(18) Elliott, L. A.; Sippola, E.; Watkins, J. Modeling Chemical Reactions with the Gaussian Gun. J. Chem. Educ. 2019, 96 (1), 100− 103. (19) History of the Christmas Lectures. The Royal Institution. http:// www.rigb.org/christmas-lectures/history (accessed Jan 2019). (20) 1′′ × 1′′ Cylinder − Neodymium Magnet. APEX Magnets. https://www.apexmagnets.com/1-x-1-cylinder (accessed Jan 2019). (21) Nehring, A.; Busch, S. Chemistry Demonstrations and Visual Attention: Does the Setup Matter? Evidence from a Double-Blinded Eye-Tracking Study. J. Chem. Educ. 2018, 95, 1724−1735. (22) Eggen, P.-O.; Persson, J.; Egholm Jacobsen, E.; Hafskjold, B. Development of an Inventory for Alternative Conception Among Students in Chemistry. LUMAT: Int. J. Math Sci. Technol. Educ. 2017, 5 (1), 1−11. (23) Schwartz, P.; Barbera, J. Evaluating the Content and Response Process Validity of Data from the Chemical Concepts Inventory. J. Chem. Educ. 2014, 91, 630−640.

ACKNOWLEDGMENTS This work was supported by grant CHE-1608149 from the National Science Foundation. The authors graciously acknowledge support from Allen Burns (Lab Instrument Specialist Supervisor) and Randall Eaglen (Senior Lab Instrument Specialist) from the West Virginia University C. Eugene Bennett Department of Chemistry Machine Shop for aid in designing and fashioning the track and Buzz Meade for some of the photographs.

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REFERENCES

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DOI: 10.1021/acs.jchemed.8b00697 J. Chem. Educ. XXXX, XXX, XXX−XXX