In the Laboratory
Incomplete Combustion with Candle Flames: W A Guided-Inquiry Experiment in the First-Year Chemistry Lab Joseph MacNeil* Department of Chemistry, Chatham College, Pittsburgh, PA 15232; *
[email protected] Lisa Volaric Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599
First-year chemistry students enter the laboratory with a background rooted in expository experimentation. Our course objectives include transitioning these students to discovery-based learning and independent, critical thinking. In his famous Christmas Lecture series, Michael Faraday remarked, “There is no better, there is no more open door by which you can enter into the study of natural philosophy than by considering the physical phenomena of a candle”(1). This Journal has recently published a number of classroom demonstrations related to combustion (2–5). The most complete of these (2) makes two important points: incomplete combustion is typically a consequence of kinetics rather than thermodynamics and candle flames generally self-extinguish before consuming all the available oxygen. This latter result strikes many as counter-intuitive and has previously been debated (2–5). The current report extends these principles into the laboratory setting with students’ natural comfort with candle flames providing the jumping-off point to explore the concepts of incomplete combustion, thermodynamics, kinetics, and gas chromatography.
puff of ambient air. Students were left to explain for themselves how room air could extinguish a flame, yet blowing on the embers of a campfire could reignite it (2). In the Laboratory Small groups of students (2–4 students per group) were first asked to measure (by GC) the relative amounts of O2 and N2 in ambient air. Each student injected a 20-mL sample of room air onto a Perkin-Elmer XL Autosystem GC. Baseline resolution of the two components (Figure 1) was achieved with a two minute, isothermal (35 ⬚C) program on a 30-m PE-MOLSIEVE (N931-6361) megabore capillary column. Samples were introduced via split-mode direct injection (80 ⬚C) and were measured on a TCD detector (100 ⬚C). Detector response to O2 and N2 is effectively equivalent so separate calibration curves were not required (see Supplemental MaterialW for supporting data). Student results were consistent and reproducible when measured by relative peak area.
Outlining the Problem In a prelaboratory lecture, students were presented with a Hollywood-type scenario that described people trapped inside a cave watching their lone candle go out. Students immediately presumed that the flame failed because it ran out of oxygen, but were less confident when asked, “How, if the oxygen is all gone, could the people still be conscious?” Following a brief discussion of “bad science” in the movies, students were asked to form an initial hypothesis and, almost invariably, assumed that the scenario as presented was physically impossible.
N2
Background Information Students were provided with individual tea-candles and matches and were introduced to the concept of the “fire triangle”. Initially the fuel and oxidant were used to write and balance a sample combustion equation assuming wax was a single hydrocarbon, C25H52. Students readily understood that if the wax is not all consumed then oxygen must be the limiting reagent. Thermodynamic concepts were introduced to explain the self-generation of heat in a candle flame. The chemical profile of a flame, the concept of wicking, and the differences between diffusion flames and premixed flames were all briefly discussed. Finally, to promote a more broadminded approach to the problem, candle flames were extinguished with a snuffer, by blowing on it, and with a sudden
302
O2
0.0
0.5
1.0
1.5
2.0
Retention Time 兾 min Figure 1. GC trace for room air, with a molecular sieves capillary column and TCD detector.
Journal of Chemical Education • Vol. 80 No. 3 March 2003 • JChemEd.chem.wisc.edu
In the Laboratory
When the students were comfortable with the use of the GC and the software, they were provided with a reaction chamber (Figure 2) consisting of an inverted powder-addition funnel capped with a 24兾40 red rubber septum (Sigma– Aldrich #Z12,465-6) and asked to determine the quantities of O2 and N2 remaining in the sealed space after the candle flame had burned to extinction, approximately 1–2 seconds. No explicit procedure was provided at this stage, but the instructor demonstrated how the septum could be punctured by the syringe needle. As each student group determined how to run the experiment and completed its first trial, they were surprised to see that the oxygen levels had diminished only marginally. At this point the instructor discussed various aspects of the experiment that could have allowed O2 from the room air to contaminate their sample, and students were permitted to complete several more trials until they could achieve reproducible results. On a first post-combustion measurement, the O2 composition of the enclosed space drops from 21% to approximately 18%. As previously reported (2), the rapid changes in temperature and pressure inside the reaction chamber can lead to air first escaping, then being replaced with ambient atmosphere as the temperature and pressure drop again. This effect can be at least partially overcome by firmly pressing the funnel to the benchtop or by lining the funnel edge with vacuum grease. These modifications, as well as effective sampling technique, can reduce the final O2:N2 ratio to 15:75 (±1). This experiment may be modified to work in a more tightly sealed environment; full details of these modifications are included in the Supplemental Materials.W Because the water produced in the combustion reaction condenses, the pressure within the system drops by 5 ± 1%. In the absence of a tightly sealed reactor, the ambient air that leaks in causes the students to overestimate the real O2 concentration by approximately 0.4%. Before completing the experiment, each student group was asked to estimate the rate of oxygen consumption in the flame. This required students to determine the volume of an irregularly shaped object, a task that was solved in a number of different fashions. There was significant variation in these results, primarily because the combustion times were short.
present an interesting analysis problem. Students initially hypothesized that a person could not remain conscious in an environment that would not support a candle flame based on the assumption that O2 levels would be minimal or nonexistent. To complete their lab reports, students had to do further research on human respiration parameters. No special attention was paid to the physiological consequences of elevated CO2 and CO levels that would be present but this may be of interest to many classes. This experiment is used in our curriculum to introduce first-year students to modern chemical instrumentation and to ease them into discovery-based laboratory work. Modifications to this laboratory could include investigation of the temperature and pressure changes. CO2 is strongly retained by molecular sieves columns, but could be quantified with alternative GC columns. The relationship between ignition temperature and O2 consumption could be investigated by replacing the tea-candle with other ignition sources, such as an alcohol burner, if larger reaction chambers were used. Student Response Students were given a prelab quiz to evaluate their general knowledge on the subject. Following the completion of the lab, the students were asked to retake the quiz. Their performance improved significantly, increasing from 33% to 79%. The most dramatic improvement was in response to the question “What are the approximate concentrations of oxygen and nitrogen in dry air?” Scores increased from 11% to 97% on this question, despite the fact that composition of dry air had previously been discussed in the classroom sections. The postlab quiz also asked some subjective questions. Students reported enjoying the lab and the opportunity to work on a real-life problem, but were most enthusiastic about the opportunity to operate the GC by themselves. Additional comments were very positive; representative examples include, “I liked this lab because we were able to figure things out for ourselves” and “(This format) makes the students think more”. Acknowledgments The gas chromatograph used in this experiment was purchased though an educational grant funded by the John M.
Hazards Although this experiment presents no unusual hazards, care must always be taken when working with open flames in the laboratory. Goggles must be worn at all times. Conclusions and Modifications Students are readily engaged in the experimentation and data interpretation process because it does not produce the result they expected. Because students are comfortable with candles they are not intimidated by the lack of a set procedure and feel more confident selecting their own experimental protocols. The rapid analysis allows students to see the results of their experiment within two minutes and they are able to try variations on their initial design, as well as test the reproducibility of their best method. The results also
red rubber septum tea candle glass powderaddition funnel
Figure 2. Schematic design of the combustion reaction sample chamber.
JChemEd.chem.wisc.edu • Vol. 80 No. 3 March 2003 • Journal of Chemical Education
303
In the Laboratory
Hopwood Charitable Trust. The authors wish to thank their students for their participation and enthusiasm in the development of this experiment. Supplemental Material Prelab and postlab quizzes, raw quiz data, analyzed quiz results, teaching notes, and student handouts are all available in this issue of JCE Online. W
304
Literature Cited 1. Faraday, Michael. The Chemical History of a Candle; Cherokee Pub. Co.: Atlanta, Georgia, 1978. 2. Vitz, E. J. Chem. Educ. 2000, 77, 1011–1013. 3. Fang, C. H. J. Chem. Educ. 1999, 76, 898–899. 4. Parsons, L. J. Chem. Educ. 1999, 76, 898. 5. Fang, C. H. J. Chem. Educ. 1998, 75, 58–59.
Journal of Chemical Education • Vol. 80 No. 3 March 2003 • JChemEd.chem.wisc.edu
