Lesson 3: Biobutanol vs. Bioethanol
The final lesson in the series involves students once again calculating total bond energies and making comparisons. I will have them do an activity similar to the activity where they calculated the carbon dioxide emissions from the combustion of gasoline. I will give them structural diagrams of both ethanol and butanol, and have them calculate the relative amounts of energy contained within the molecules (See Appendix 3). After they have all agreed on calculated energy content numbers, we will combust known volumes of both alcohols and determine the energy output by using my set of classroom calorimeters.
A calorimeter measures the heat energy of a combustible product by measuring the temperature rise in a vessel of water. The students then can relate the relative temperature differences and get a fairly good approximation of the difference in chemical energy released by the combustion process. They will be able to relate the energy content of the ethanol to the energy content of butanol. This lab will once again reinforce good lab practices. After they have derived their experimental data, I will reveal the actual numbers and we will see how close they came to the actual energy content numbers. We may then discuss the sources of error, and how we could design a more rigorous experiment. This type of after-lab reasoning is very good for students to learn. Too often they think that the lab is over once they have a number and have filled in some worksheet. I try to emphasize to them that science is ever-evolving, no experiment is perfect, and there are always things that could be done to improve techniques and data collection methods.
The students will also get some articles that outline the production of biobutanol using strains of bacteria rather than enzymes. We will have a discussion of the relative merits of the different biofuels, their production, and their carbon footprint, and students will ultimately be required to write a position paper, and defend a certain fuel using the data and research that have gathered. I typically assign these types of position papers as a way to get the students to really think about all of the implications of a topic.
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