In the Classroom
The Chemistry of Self-Heating Food Products An Activity for Classroom Engagement Maria T. Oliver-Hoyo* Department of Chemistry, North Carolina State University, Raleigh, NC 27695; *
[email protected] Gabriel Pinto E.T.S. de Ingenieros Industriales, Universidad Politécnica de Madrid, 28006 Madrid, Spain Juan Antonio Llorens-Molina E.T.S. del Medio Rural y Enología, Universidad Politécnica de Valencia, 46010 Valencia, Spain
Self-heating or self-cooling containers for meals and beverages are excellent examples of chemistry in action for the everyday life of consumers. Such containers consist of dual chambers where the food is usually contained in the internal chamber while the chemical process that would heat or cool the food or beverage occurs in the other. A common cooling system consists of ammonium nitrate and water while a common heating system consists of the reaction of calcium oxide in water. The heating or cooling chamber requires the reagents to be separated until ready to use. These hydration methods are simple and do impart heat effects; however, they present some limitations such as heating times of 5–15 minutes (required to generate the necessary temperature increases) and the need for large amounts of the reagents because of the low heat energy yield, which then must occupy a considerable space in the containers. Emerging technologies are making self-heating foods more accessible to the general public. One such technology is the self-propagating high-temperature synthesis (SHS) that involves the oxidation of a mixture of aluminum and other metals by iron oxide, Fe2O3. The change in enthalpy of this reaction is greater than 3 kJ/g of reactants, making it more than 4 times higher than the heat energy evolved when limestone reacts with water (1). This technology ensures heating a beverage from 2–3 °C to the boiling point in less than 90 seconds and cutting the heating time for food to less than four minutes. In Spain and other European countries self-heating beverages are known as “autocalentables” and are easily found at gas stations, airports, and highway rest stops. A variety of these products are available including different types of coffee (black, with milk, or cappuccino), chocolate, and tea. In the United States commercialization of these food products has been targeted toward outdoors enthusiasts and the military; however, companies such as Starbucks and Wolfgang Puck are advancing into this market. In the 1980s the U.S. Army took the lead to further develop the technology required to enhance the Meals, Ready to Eat or MREs that were used a decade earlier by the U.S. Space Program. One of these advancements included the Flameless Ration Heater (FRH) that allows military troops in combat to have a hot meal. These MREs, which include snacks, main entrees, and desserts, are now sold through a number of online sites and are available individually or in packages of “emergency supply” or “disaster preparedness” units (2).
These products provide an excellent means to promote interest in chemistry. The activity described in this article uses these commercial products to study the chemistry that produces the self-heating mechanism. Concepts such as stoichiometry, enthalpy of reaction, enthalpy of solution, heat transfer, and density of liquids are the core principles involved in these reactions. Creative ways to use these products may also be discussed. For example, the FRH of the MREs have been used in combat situations to warm intravenous fluids before administering them to patients as deployed medical units often do not have means to heat these fluids and by doing so they may prevent hypothermia in patients (3). Methodology We have used this activity with two different methodologies in the classroom: as the foundation for problem-based learning (PBL) and as the framework for inquiry-guided instruction (IGI). Even though this activity could be easily performed in the laboratory, it has been designed, tested, and implemented as a classroom activity where calculations and evaluation of results take precedence to data collection. In the PBL methodology the acquisition of skills and knowledge arise from the need to solve a problem related to the background or experience of the students. The expected learning depends primarily on the collective reflection of a group of students. The role of the instructor is to meet with the group of students primarily as a listener and when necessary pose questions to lead students in the right direction. In PBL the problem is loosely constructed but the goal is clearly stated. PBL has received prominent emphasis in health-related careers (4–6), but its benefits have been documented in a variety of disciplines (7–10) and academic levels of instruction (11–14). IGI adopts a more structured approach where student learning is promoted through the use of questions meant to initiate the curiosity of the students to get them started on finding answers to their own questions (15, 16). Both methodologies rely on group work where students collectively think, reflect, and provide ways by which to solve a problem. The instructor facilitates these processes rather than informing students what needs to be done next. This activity is presented using both methodologies so that instructors may choose the one that better fits their pedagogical goals or teaching styles.
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 86 No. 11 November 2009 • Journal of Chemical Education
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The design of this activity fulfills the common elements these methodologies share along with what Duch identifies as practical criteria for a “good” problem (17):
in Figure 1. If the methodology adopted is IGI, a homework set may be distributed that includes the following guiding questions:
• “hooking” the student to spark interest and motivation preferably within a real-life context
• requiring reflection as students recognize and explain how to proceed, discerning which information is relevant and necessary at each stage of the solving process
• relying on group thinking rather than dissecting tasks among group members
b) Suggest a procedure by which the instructor was able to determine the masses given.
In addition we designed this activity to depend on analysis of data, evaluation of results, and extension of concepts of a real-life chemistry application. Activity The same principles and rationale apply to the two examples provided: self-heating beverage and MRE (or the FRH system). Instructors may use the example that students are more familiar with so that chemistry principles become more tangible to them. The aim is to propose a real-life chemistry problem for which students will need to calculate the heat produced by the chemical reaction or the dissolution process, the accompanying theoretical change in temperature, and finally compare the theoretical change to the temperature observed. With either methodology, PBL or IGI, the instructor shows the class the container, carries out a demonstration, and gives necessary data or appropriate information resources. The activity is designed to use five minutes of class time and allow students to work in groups, outside the classroom, to solve the problem posed. Self-Heating Beverage In class, the instructor asks a student to follow the directions on the label of the self-heating chocolate beverage, while the instructor does the demonstration. The temperature reached inside the beverage container is recorded. The beverage is passed around the classroom for students to feel the warmed container. Students are given the masses of the different substances. In this particular example these are CaCl2 (54.31 g), water (60.45 g), chocolate (93.68 g and a volume of 75 mL), container (21.22 g polypropylene and 8.39 g Al). The instructor may choose to adopt PBL methodology when presenting this activity to the students and the problem could be similar to the one presented
a) Draw a scheme that describes the container (materials and design). Based on this design could you propose the chemical process involved in heating the beverage?
c) Because the dissolution of CaCl2 in water is an exothermic process, how would you estimate the heat (in kJ) liberated during the process and the temperature the beverage should reach? Does the estimated temperature match the temperature claimed by the manufacturer of the container? Provide explanations and state possible assumptions.
d) Explain the rationale for the following recommendations given in the product label: i) Shake upside down for 40 seconds. ii) Do not perforate or cut the container. iii) Self-heating will occur only once. iv) Do not warm the container by any other means such as microwave or oven.
e) Comment on the advantages or disadvantages of this type of container and suggest improvements.
f ) Decide whether this type of container could be used to cool a beverage rather than to heat it. Propose a chemical process that could achieve that.
g) With the data provided in this exercise, could you compute the density of the chocolate? Using values of the enthalpy of salt dissolution in water and other thermochemical data, could you calculate the lattice energy of the salt?
The proposed solutions to each question can be found in the online material. It is worth pointing out the rationale for including each of these questions in the homework is because the methodology is as important as the exercise. When students are required to examine the information provided by manufacturers they realize that without a chemical equation to describe the process they
Product A: Self-Heating Beverage
Figure 1. Scheme of the self-heating beverage container and PBL problem.
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PBL Problem: Currently there are commercial products that claim to heat its contents based on the dissolution process of a salt, in our case, calcium chloride. We need to warm the 76 mL of the beverage in this container to 60 °C. To do so we need to know how much salt and water are required in this container.
Journal of Chemical Education • Vol. 86 No. 11 November 2009 • www.JCE.DivCHED.org • © Division of Chemical Education
In the Classroom
will not be able to start their investigation. This emphasizes the idea that important information is provided to consumers and connects chemistry to real-life situations. The instructor’s actions model the fact that experimentation is part of any chemical investigation and this becomes noticeable when students are prompted to figure out the source of the data provided to them. For the information not explicitly given in the problem, students must first realize they need the enthalpies for the process and specific heats of involved substances, and then must search appropriate resources for the values not provided. In addition, as the values calculated are per mole, they must realize that the values must be adjusted for the quantity of salt used. These guiding questions promote reflection as an integral part of the problem-solving process. In class, the chocolate beverage with an initial temperature of 25 °C reached a temperature of 62 °C, which is less than a 10 °C difference from the calculated temperature. A number of approximations may have caused this difference. Students could discuss the following assumptions: (a) the salt solution and the beverage had the same specific heat as water, (b) the system is perfectly isolated from the environment (which is clearly not the case as the students can feel the warmth from the container), and (c) there is no variation of the enthalpies or heat capacities with temperature, among others. A temperature gradient exists and the effects could be part of the discussion for more advanced students. Reflecting and comparing calculated values to the ones obtained in the classroom brings a sense of real chemistry to otherwise mechanical calculations. As students are asked to explain the reasons for the manufacturer’s recommendations, their evaluations of these instructions connect common sense to the chemical process under investigation making the chemical knowledge useful and “tangible”. In addition, these questions bring different chemical concepts into one problem so that connections can be facilitated and concepts emphasized as they are revisited. Extension of ideas is promoted via the questions for this activity. Students’ discussion of advantages and disadvantages may be framed in technological and social context issues such as the selection of materials, design of the container, and environmental impact among others. From what students learn in this activity, they can evaluate the enthalpy of the dissolution process of other salts. MRE In class, the instructor shows the different components of the MRE and prepares it as directed in the instruction pack-
age by adding ~20 ml of water to the FRH plastic bag. In the case of an MRE containing a beef patty, after 10–15 minutes, a meat thermometer is inserted into the MRE beef package and the temperature of the meat is read. The instructor must point out that FRH system indicates that the maximum mass of the magnesium is 8 g per FRH (18). Students are given the masses of the two plastic bags, one that contains the meat (13.82 g) and the other that holds all the other components (6.73 g). The cardboard containers (package and FRH) add up to 26.24 g. The instructor may choose to adopt PBL methodology when presenting this activity to the students and the problem could be similar to the one presented in Figure 2. For the IGI approach the following questions are part of the homework:
a) Could you propose the chemical process involved in heating MREs?
b) Based on your observations in class, which components of the MRE must you consider in the heat calculations? What data do you need to start your calculations?
c) Justify or disprove the claim of the manufacturer. Provide clear and compelling statements.
d) The meat thermometer reached 150 °F. How does this compare to your calculations? Provide explanations and state possible assumptions.
The questions are the springboard for important discussion points intended to promote reflection, evaluation, and extension of ideas. For example, in this case, students might consider the formation of the metal oxide instead of the formation of the metal hydroxide, which opens the door for discussing the hygroscopic properties of MgO, the reversibility of this reaction, and the use of cold water versus steam in manipulating the formation of the hydroxide or oxide, respectively. The demonstration in class and data provided by the instructor show students that magnesium, meat, water, plastic, and cardboard are essential in the calculations. Once students think about these components they must realize that conduction is the primary mode of heat transfer and attempt to relate the components using the heat equation (Q = mcΔT). Thermal properties of materials or specific heats for each component become necessary. As in the first problem, the instructor may provide these values; however, it is best for students to find the values because searching for these data requires exposure to different discipline-based resources (19–21) and most likely to demand practice in unit conversions. Solutions to these questions are found in the online material.
Product B: MRE Figure 2. Contents of a typical MRE package and PBL problem. MREs usually contain an entree, side dish, dessert, dehydrated beverage, condiments, and accessory packet.
PBL Problem: The reaction of magnesium with water is responsible for the heat energy that allows MREs to offer hot meals to military personnel without the need for outside heat sources. Manufacturers claim that this heating system can warm 12 oz of food to about 190 °F (19). Is this possible or is it a misleading advertisement?
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 86 No. 11 November 2009 • Journal of Chemical Education
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Conclusions Consumer chemistry and its real-life relevance is the basis for the interest shown by students to solve the chemistry problems presented in this activity. The two examples have motivated students to work and think about chemistry principles and provide instructors with an activity that promotes cognitive skills such as comprehension, application, and evaluation as well as research skills that include literature searches and resource handling. In addition, because each self-heating beverage or MRE package contains different items, the masses and temperature changes vary. This keeps the data “fresh” and may deter students from simply copying classmates’ calculations. The context of these activities connects a variety of chemistry topics and provides the opportunity to practice common operations such as unit conversion and algorithmic problem solving within the context of a real-lfe scenario. Discussion of this kind may encourage students to explore topics in more depth. For example, students can be asked about the difference between a chemical reaction and dissolution of salts in water. Engineering students have shown special interest in the design of the components in the self-heating products and the environmental impact of such designs. The key element is the evaluation process that students must conduct at every stage, which includes examining food products promotion information. Acknowledgments The authors would like to gratefully recognize the support provided by the Universidad Politécnica de Madrid under Projects IE070535020 and IE08053505 and to Víctor Manuel Díaz for drawing Figure 1. The National Science Foundation via CAREER Award No. REC-0346906 has also made this collaboration possible.
6. Guerrero, A. P. Acad. Med. 2001, 76 (4), 385–389. 7. Levia, D. F.; Quiring, S. M. J. Geography Higher Educ. 2008, 32 (2), 217–231. 8. Dods, R. F. J. Chem. Educ. 1996, 73, 225–228. 9. Neville, D. O.; Britt, D. W. Foreign Language Annals 2007, 40 (2), 226–246. 10. Gurses, A.; Acikyildiz, M.; Dogar, C. Res. Sci. Technol. Educ. 2007, 25 (1), 99–113. 11. Tarhan, L.; Ayar-Kayali, H.; Urek, R. O. Res. Sci. Educ. 2008, 38 (3), 285–300. 12. Senocak, E.; Taskesenligil, Y.; Sozbilir, M. Res. Sci. Educ. 2007, 37 (3), 279–290. 13. Mierson, S. Am. J. Physiol. 1998, 275 (6), S16–S27. 14. Ram, P. J. Chem. Educ. 1999, 76 (8), 1122–1126. 15. Oliver-Hoyo, M.; Anderson, M.; Allen, D. D. J. Coll. Sci. Teach. 2004, 33 (6), 20–24. 16. Martin-Hansen, L. Science Teacher 2002, 69 (2), 34–37. 17. Duch, B. Problems: A Key Factor in PBL. http://www.udel.edu/ pbl/cte/spr96-phys.html (accessed Jun 2009). 18. FRH MSDS Sheet. http://www.theenergykit.com/FRH%20 MSDS-1.pdf (accessed Jun 2009). 19. Handbook of Physical Testing of Paper, 2nd ed.; Borch, J., Lyne, B. M., Mark, R. M., Habeger, C., Murakami, K., Eds.; Marcel Dekker, Inc: New York, 2002. 20. Food and Foodstuff – Specific Heat Capacities. http://www. engineeringtoolbox.com/specific-heat-capacity-food-d_295.html (accessed Jun 2009). 21. Handbook of Chemistry and Physics, 88th ed.; Lide, D. R., Ed.; CRC Press/Taylor and Francis: Boca Raton, FL, 2008; Internet Version.
Supporting JCE Online Material http://www.jce.divched.org/Journal/Issues/2009/Nov/abs1277.html
Literature Cited
Abstract and keywords
1. Gluch, G.; Mandler, D. Chemical Innovation 2001, 31 (9), 24–28. 2. (a) Mountain House Food Review. http://www.FreezDriedFood. com. (b) APack Ready Meals. http://www.readymeal.com (both accessed Jun 2009). 3. Modesto, V. L.; García, G. D.; Lee, K. T. Military Medicine 2000, 165 (12), 903–904. 4. Fyrenius, A.; Silen, C.; Wirell, S. Adv. Physiol. Educ. 2007, 31 (4), 364–369. 5. Ozturk, C.; Muslu, G. K.; Dicle, A. Nurse Educ. Today 2008, 28 (5), 627–632.
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JCE Classroom Activity for November 2009
This article is discussed in the JCE Classroom Activity for this issue (Enjoy a Hot Drink, Thanks to Chemistry!). See p 1280A for details.
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