Throughout this year, the underlying theme for all our experiments was chemical reactions and the importance of energy. Generally, in a traditional chemistry classroom, there are five basic types of reactions that are discussed and tangentially performed: single replacement, double replacement, combustion, synthesis and decomposition. In most curricula, the importance is placed on the names and details regarding the reactions without emphasis on the underlying reasons for why the reactions proceed. In essence, chemistry is the study of transformations and energy is a tremendous guiding force in ensuring reactions occur spontaneously yet energy is usually not discussed in this unit.
Calorimetry studies the energy generated in chemical reactions but is usually only reserved for AP Chemistry classes yet the general ideas behind this concept can be taught to virtually any age. In short, it is the study of energy in the form of heat. The process for conducting a calorimetry experiment is very simple: when a known amount of chemical is added to a known amount of water, the chemical may ionize (“come apart”) depending on its bonding and release energy into the water changing the heat of the solution. This change in temperature is directly proportional to the heat produced by the reaction of the added chemicals with water. To measure the energy produced, students can build a simple apparatus called a calorimeter using a beaker, test tube, or even a styrofoam cup, a lid or cap, and a lab thermometer though far more complex designs are possible. For an added math element, the students can actually calculate the heat in Joules produced by multiplying the change in temperature of the water (in Celsius), the mass of the water plus added chemical, and the specific heat capacity, which is a constant. For an added lesson in metric system skills, the students can convert the heat in joules to kilojoules (kJ), a more common unit in the reporting of energy or to kilocalories for a real world link to foods and energy. In addition, as either an introduction or extension activity, students can actually experimentally determine the specific heat capacity of water which is 4.18 Joules / (grams * Celsius).
In the Acera lab, we spent several weeks on this project. First, the students were given a class period of two hours to design and troubleshoot a calorimeter with common items they found in the lab or school. Some of the problems that the students faced in their calorimeter design was water leakage, how to add the chemical in an effective manner, and the biggest problem of how to avoid heat loss to the surrounding environment. Finally, given the tremendous amount of choice regarding size of hardware and limitation of reaction materials, the students need to be cognizant of choosing the best equipment for the job and for the reaction materials required. In order to begin, the students needed a cogent design but adapted the design if a problem arose as troubleshooting is a routine part of every experiment.
Once they feel they have a final optimized design, I gave them a variety of chemicals in which to experiment. This included ammonium nitrate, which endothermically reacts with water and lowers the temperature of the liquid. I also gave them dextrose, which does not ionize in water due to its bonding and thus, does not modulate the temperature. I also included the exothermically reacting chemicals lithium chloride, sodium chloride, hydrochloric acid, and sodium hydroxide. Finally, with my assistance, the students also reacted very small amounts (0.1 – 0.3 grams) of elemental sodium and lithium under a chemical extraction arm (or hood) to collect the gases.
Safety note: It is advised not to let the students react the elemental sodium and lithium unsupervised as they react with water very strongly. In addition, the sodium must be sliced very thin as spherical pieces can result in flash of light and a loud popping noise. While harmless, the reaction can be very startling. You could leave these out entirely but the very rapid reaction of these reactions can provide a lot of insight into the nature of the first column of the period table.
Once the the students have gathered all their data from the various experiments, the real scientific learning begun as they researched the chemicals to determine why they behaved the way they did. In our class, the concepts that this analysis touched upon was reaction types, ionic vs. covalent chemical bonding, energy creation from broken chemical bonds, and of course the nature of chemical reactions. By performing these experiments, they understood the ideas behind ionization and rearrangement of the atoms in the compound which included different reaction types. It is true that the upfront vocabulary behind the experiments was minimized but the #1 most common complaint I received as a teacher is that science education has too much complicated vocabulary. In this experiment, the students organically came by the various vocabulary with research which is similar to real life lab environments.
The only rules I have regarding the experimental analysis is that the students are done writing when they are no longer able to write original thoughts that they understand. This means that the students cannot repeat themselves and they also cannot copy down words or concepts that they can’t explain in their own words. Whenever students aren’t given some guidelines, there is the danger that they will begin copying down passages from textbooks that they don’t understand which is not productive by any means.
The more advanced students delved into the electron configurations of ionic vs. covalent bonds while some examined the physical chemistry of reactions and the students scientific mastery simply discussed the basics of endothermic and exothermic reactions and the links to the chemicals they worked with. As the students were writing their conclusions, I offered feedback and helped guide them away from quagmires. All students should however, discuss and suggest the reasons for the differences in energy and namely, why the addition of a chlorine ion to sodium and lithium lowers the heat generated in solution. Finally, when we performed this lab, I asked the students to write their results on the whiteboard so that they can compare their results to that of other students and to potentially discuss it in their lab notebooks.
In conclusion, this unit provided a view on the nature of energy and chemical reactions. From here, we moved into the optimization of chemical ratios in producing a more energetic reaction which I will discuss in my next blog. However, additional jumping off points can be an analysis of the individual reactions, food calorimetry, or even enzymes and energy in organic materials.