Chemical Illustrations of Flattening the Curve

Co-Authored by Dean J. Campbell*, Cassidy Kraft*, Kayla Lippincott*, Emily Rosengarten*, and Thomas S. Kuntzleman**   *Mund-Lagowski Department of Chemistry and Biochemistry, Bradley University, Peoria, Illinois   **Department of Chemistry, Spring Arbor University, Spring Arbor, Michigan 

With the current global COVID-19 pandemic, there has been much discussion of “flattening the curve” by social distancing.^1,2 This paper describes two chemistry demonstrations that can be used to illustrate this concept.

Using a catalyst to decompose hydrogen peroxide to produce foamy oxygen gas is a common demonstration in the chemistry classroom (equation 1). Here we share how we have used it to demonstrate the idea of social distancing to flatten the curve. The rate of hydrogen peroxide catalytically converted to oxygen gas and water by a mixture of iron oxides³ can be illustrated by adding rusty spheres to hydrogen peroxide solutions containing dish soap and measuring the volume of oxygen-containing foam produced.

Equation 1:   2 H₂O_{2 (aq)} → O_{2 (g)} + 2 H₂O _(l)

Decreased hydrogen peroxide concentrations, representing decreased human population concentrations from social distancing, produce oxygen gas foam, representing cases of illness, at a slower rate. See the time-lapse video below for an example of this experiment (video 1).

Video 1: Flatten the Curve: A Chemical Analogy, Tommy Technetium YouTube Channel, (Accessed March 21, 2020)

Similarly, the Diet Coke and Mentos experiment may be used to illustrate these concepts. To do this Mentos candies are added to a bottle of Diet Coke, causing carbon dioxide gas to escape, producing a lot of foam (equation 2).^4,5

Equation 2:   CO_{2 (aq)} → CO_{2 (g)} 

In this case more Mentos represent a greater number of social gatherings. More candies added to a bottle of Diet Coke produces more foam in a shorter period of time. A geyser guide^4,6 connected to the top of each bottle can be used to represent the capacity of the healthcare system to care for people who are ill. See video 2 below for an example of this experiment.

Video 2: Flatten the Curve: An Analogy using Coke and Mentos, Tommy Technetium YouTube Channel, (Accessed March 21, 2020)

EXPERIMENTAL

Rusting the Spheres

Iron rust can have a complex composition and structure, but it can be formed by the oxidation of iron metal, often with the assistance of water (equation 3).⁷

Equation 3:    4 Fe_(s) + 6 H₂O _(l) + 3 O_{2 (g)} → 2 Fe₂O₃ * 3H₂O (or 4Fe(OH)₃) _(s)

A variety of iron oxides could be used to catalyze the decomposition of hydrogen peroxide, but rusted iron BBs can be used over and over again, improving the sustainability of this demonstration. The BBs were obtained from Crossman Corporation, Bloomfield, NY and were composed of iron spheres with copper cladding. The copper cladding does not seem to adhere strongly to the BBs, and the BBs seem to be prone to rusting. We removed the copper cladding by tumbling the spheres in a rock tumbler with water and sand added as an abrasive. The resulting spheres were placed in a single layer in the bottom of a large plastic tub, and were just barely covered with water saturated with sodium chloride.⁸ The water was allowed to evaporate over the course of a few days. The dried salt that formed on the resulting rusty spheres was washed away with water and the spheres were allowed to air dry.

Catalytic Decomposition of Hydrogen Peroxide using Rusty BBs

Successful demonstrations were run using the following materials:

• aqueous H₂O₂ (e.g., 3% can be purchased from local pharmacies or grocery stores)
• aqueous H₂SO₄ (e.g., 4.5 M concentration)
• liquid soap (e.g. Dawn dish soap)
• dye to tint the solution (e.g. red food coloring seems to resist changing color in the presence of H₂O₂ and acid)
• water
• rusted iron BBs
• two transparent containers of equal size (e.g., 50 mL beakers or 100 mL graduated cylinders)

One combination that worked particularly well, shown in Figure 1, involved the combination of 100 mL of 3% H₂O₂ solution with 10 mL of 4.5 M H₂SO₄ solution. A drop of red food coloring and a drop or so of Dawn dish soap were mixed in. 40 mL of this mixture was added to one of two identical 100 mL graduated cylinders. 40 mL of this mixture was combined with 40 mL of water and added to the other graduated cylinder. The dye color in the diluted solution cylinder was noticeably lighter. About 35 g of rusty BBs were added to each cylinder. Immediately the H₂O₂ began to react at the iron oxide surface of the spheres, producing O₂ gas foam which rose up in the remaining space in the cylinders, and the solution color began to fade. In the more concentrated solution, the foam rose at a faster rate (in most cases spilling down the side of its graduated cylinder after several minutes), the dye decolorized more quickly, and the solution became warmer. It is interesting to note that the rate of foam production increased as the reaction progressed. It is hypothesized that this might have been due to the reaction system becoming warmer as the reaction progressed. The rate of reaction from the more dilute solution was much slower, and even though there was much less available space in the graduated cylinder, the foam did not tend to spill down the side of its graduated cylinder. The cylinder with the more concentrated solution can be marked with tape or a marker to indicate what volume would be equivalent to that of the headspace of the cylinder with the less concentrated, more voluminous solution. Halving the concentration of the hydrogen peroxide produced a much more dramatic difference in foam volume than halving the concentration of sulfuric acid (data not shown). Therefore, a facile preparation for this demonstration involves combining peroxide, acid, dye, and soap at a particular concentration, and saving some of the concentrated mixture for use. A dilute mixture can be obtained by adding water to the desired amount of concentrate.

graduated_cylinders.png

Figure 1: (LEFT) Graduated cylinders containing aqueous solutions of H₂O₂, acid, dish soap, and red dye. The solution in the cylinder at right has been diluted with water. (RIGHT) The response of the solutions to adding rusty BBs to each cylinder. In several minutes, the foam produced by the more concentrated solution had a larger volume than that produced by the less concentrated solution.

Demonstration using the Diet Coke and Mentos Experiment

To prepare for the Diet Coke and Mentos demonstration, hot glue was used to connect together ten Mentos candies in as straight a line as possible. A geyser guide (also known as a Coke collector) was prepared as described previously.^4,6 Two separate 2L bottles of Diet Coke at room temperature were opened and a geyser guide was attached to the top of each bottle. A PVC pipe was used to direct the addition of one Mentos candy into the first bottle, and then the PVC pipe was used to direct the addition of the assembly of ten Mentos candies into the second bottle. Upon addition of one Mentos candy, foam production reached a maximum of roughly 1.5 L in eight seconds, and this approximate foam volume was maintained for about one minute. Foam was produced over the course of two minutes, with the rate of bubble popping balancing the rate of bubble formation so the foam generated was entirely contained in the geyser guide throughout the entire experiment. On the other hand, the foam generated upon addition of 10 Mentos candies could not be contained and almost immediately shot out of the geyser guide for about five seconds. The foam continued to spill out of the geyser guide for 15-20 seconds, and foam production ceased after roughly 45 seconds (video 2).

If a geyser guide is not available, the demonstration can be conducted in the traditional manner,¹⁰ by alternatively dropping either a single or several Mentos into separate, freshly opened 2 L bottles of Diet Coke. If the bottles are placed next to a wall, the capacity of the health care system to care for patients can be illustrated by taping or drawing a line on the wall roughly 1-1.5 m high above the bottles. Addition of several (7-11) Mentos to a 2 L of diet carbonated beverages at room temperature generally produces fountains that reach over 2 m high.¹⁰ On the other hand, addition of a single Mentos in the same experiment rarely reaches 1 m in height. See video 3 below for an example of doing the experiment in this manner.

Video 3: Mentos and Coke: 1 Mentos vs. 7 Mentos, Tommy Technetium YouTube Channel, (Accessed March 21, 2020)

DISCUSSION

These demonstrations convey the basic idea that the rate of many processes – chemical or otherwise – can be altered by changing various conditions. It is obvious that principles of chemical kinetics are strongly related to controlling rates of change. When studying chemical kinetics, it is recognized that changes in temperature or concentration of reactants impacts how fast chemical reactions proceed. In terms of collision theory, increasing the number of collisions between reactant molecules increases the rate of chemical reactions. By analogy to the spread of a viral infection, increasing human-to-human interaction increases the rate at which a disease spreads. Both of the demonstrations presented here attempt to use a chemical system to model, with limitations, human interactions. One reviewer has suggested that the demonstrations could be shown to students with a minimum of explanations, and then the students could be prompted to describe, with guidance, the strengths and limitations of these analogies. Even if spillover were to occur for both the faster and slower processes, the difference in rates of reaction are obvious, and can generate discussion about which case resulted in more overflow.

In the hydrogen peroxide demonstration, reactant concentration is used as an analogy for social interactions. This comparison is fairly straightforward: large congregations of people (which occurs in cities, in schools, and at sporting events) relates to high hydrogen peroxide concentration. Larger congregations result in more human interactions, much like higher concentration results in more collisions; both contribute to faster rates. Therefore, decreasing human interactions through social distancing should decrease the rate of spread of disease.

The analogy to the Diet Coke and Mentos experiment is slightly more involved. Mentos candies cause degassing from carbonated beverages because dissolved CO₂ enters pre-existing air bubbles trapped in non-wettable cavities on the surface of Mentos candies.⁹ These pre-existing air bubbles are the well-known nucleation sites that catalyze the degassing in this experiment. The nucleation sites may therefore be viewed as “meeting places” that attract CO₂ – much like schools, sports arenas, and places of worship are meeting places that attract people. The video for this demonstration has been used as a remote-session learning experience in a science course for non-majors. In this particular class the minute details of nucleation were not discussed, being beyond the scope of the course. Rather, Mentos candies were simply used to represent social interactions that would occur at meeting places that remained open during an outbreak.

SAFETY

All reagent containers should be clearly labeled. Proper personal protective equipment such as goggles should be used, ESPECIALLY considering vertically rising foams can be produced. Avoid spilling reagents on clothing. Avoid skin contact and wear nitrile gloves while working with hydrogen peroxide. Always wash your hands after completing the demonstrations. A tray or plastic sheet can be placed under the demonstrations to help contain spilled materials.

ACKNOWLEDGMENTS

This work was supported by Bradley University and the Mund-Lagowski Department of Chemistry and Biochemistry with additional support from the Illinois Heartland Section of the American Chemical Society and the Illinois Space Grant Consortium. We also want to give a HUGE thank you to Deanna Cullen and the reviewers of the manuscript for their great suggestions and lightning fast work on this article, which deals with a very important and time-sensitive issue.

REFERENCES

1. Centers for Disease Control and Prevention. Interim Pre-pandemic Planning Guidance: Community Strategy for Pandemic Influenza Mitigation in the United States. (accessed Mar 2020).
2. Hatchett, R. J.; Mecher, C. E.; Lipsitch, M. Centers Public health interventions and epidemic intensity during the 1918 influenza pandemic. Proc. Natl. Acad. Sci. [Online], 2007, 104, 7582-7587. (accessed Mar 2020).
3. Lin, S. S.; Gurol, M. D.; Catalytic Decomposition of Hydrogen Peroxide on Iron Oxide: Kinetics, Mechanism, and Implications. Environ. Sci. Technol. [Online], 1998, 32, 1417-1423. (accessed Mar 2020).
4. Kuntzleman et. al, Kinetic Modeling of and Effect of Candy Additives on the Candy-Cola Soda Geyser: Experiments for Elementary School Science through Physical Chemistry J. Chem. Educ., 2020, 97, 283–288.
5. Kuntzleman, T. S.; Davenport, L. S.; Cothran, V. I.; Kuntzleman, J. T.; Campbell, D. New Demonstrations and New Insights on the Mechanism of the Candy-Cola Soda Geyser J. Chem. Educ. 2017, 94, 569–576.
6. Kuntzleman, T. S.; Nydegger, M. W.; Shadley, B.; Doctor, N.; Campbell, D. J. Tribonucleation: A New Mechanism for Generating the Soda Geyser. J. Chem. Educ., 2018, 95, 1345-1349.
7. Tro, N. J. Chemistry: A Molecular Approach, 4th ed. Pearson Education, Inc.: Boston, 2017.
8. Malel, E.; Shalev, D. E.; Determining the Effect of Environmental Conditions on Iron Corrosion by Atomic Absorption. J. Chem. Educ. [Online], 2013, 90, 490-494. (accessed Mar 2020).
9. Kuntzleman, T. S.; Johnson, R. Probing the Mechanism of Bubble Nucleation in and the Effect of Atmospheric Pressure on the Candy–Cola Soda Geyser J. Chem. Educ., 2020, 97.
10. Kuntzleman, T. S.; Davenport, L. S.; Cothran, V. I.; Kuntzleman, J. T.; Campbell, D. J. New Demonstrations and New Insights on the Mechanism of the Candy-Cola Soda Geyser, J. Chem. Educ., 2017, 94, 569–576.

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