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the cell can help physics teachers to answer their stu- dents’ questions. References 1. E.L. Littauer and J.F. Cooper, “Metal air batteries,” in Handbook of Batteries and Fuel Cells, edited by D. Lin- den (McGraw Hill, New York, 1984), pp. 30-1–30-21. 2. D. Rathjen and P. Doherty, Square Wheels and Other Easy-to-Build Hands-On Science Activities (Explorato- rium, San Francisco, 2002), p.85. 3. Figure courtesy of Mark Hespenheide. 4. M. Tamez and J.H. Yu, “Aluminum–air battery,” J. Chem. Educ. 84, 1936A (Dec. 2007), or http://www. exo.net/~pauld/activities/AlAirBattery/alairbattery. html. 5. The Exploratorium Science Snackbook (Exploratorium, San Francisco, 1991), p. 57-1, or online at http://www. exploratorium.edu/snacks/hand_battery.html. 6. Oxidation is the loss of electrons at the anode, and re- duction is the gain of electrons at the cathode. 7. J.P. Hoare, “Oxygen,” in Standard Potentials in Aqueous Solution, IUPAC, edited by A.J. Bard, R. Parsons, and J. Jordan (Marcel Dekker, Inc., New York, 1985), pp. 54–66. 8. S.-M Park, “Boron, aluminum and scandium,” in Stan- dard Potentials in Aqueous Solution, IUPAC, edited by A.J. Bard, R. Parsons, and J. Jordan (Marcel Dekker, Inc., New York, 1985), pp. 566-580. 9. P. N. Ross, “Hydrogen” in Standard Reduction Poten- tials, IUPAC, edited by A.J. Bard, R. Parsons, and J. Jordan (Marcel Dekker, Inc., New York), pp. 39-48. 10. Al 3H2O Al(OH)3 32H2 E0 1.89V. PACS codes: 01.50.My, 41.00.00 Stephanie V. Chasteen has a PhD in condensed mat- ter physics is currently a science education postdoctoral fellow at CU in Boulder. She was previously a postdoc at the Teacher Institute at the Exploratorium in San Francisco. Publications, activities and blog at http://www. exo.net/~drsteph. She is also the daughter of N. Dennis Chasteeen. N. Dennis Chasteen is a professor emeritus of physical chemistry whose primary work has been in the area of iron transport and storage. Paul Doherty has a PhD in physics from MIT and is staff scientist at the Exploratorium. He has written dozens of articles and books on teaching physics. He has received the NSTA’s Faraday award for excellence as a science communicator. Activities at http://www.exo.net/~pauld. tate– ions from the dissociation of the weak acid make the solution conductive. A squirt of vinegar to your saltwater cell will make the LED glow brighter. But don’t be fooled—stirring the solution does the same thing. Upon settling, the performance of the vinegar/ salt cell is generally comparable to that of the saltwater cell alone. However, you’ll probably find that vinegar/salt cells maintain their current for a longer time than saltwa- ter cells. If you leave a vinegar/salt cell overnight, the surface of the copper will not be coated with a reddish oxide but stays shiny and clean. The acetate in vinegar tends to dissolve the cuprous oxide coating as it forms by forming a complex with the Cu(I). This allows bet- ter contact with the solution and thus better electron transfer over time. Additional details about the role of vinegar in the cell can be found at http://www.exo.net/pauld/ saltwater. Adding bleach You will find you’ll get a much more stable and powerful cell if you add a teaspoon of bleach to the saltwater (or to plain water) with current and voltage around 10 mA and 1 V, respectively, with only the multimeter in the circuit. So a bleach-powered cell produces about 20 times as much power! Why is that? When bleach is added, the battery is no longer an air battery; instead of oxygen from the air, sodium hy- pochorite (NaOCl), the major constitutent in bleach, and hypochorous acid (HOCl), a minor constituent, are reduced. The full equations for this cell can be found online. This cell potential (3.93 V) is quite a bit higher than the 3.12 V for the saltwater battery [Eq. (3)]. So this reaction proceeds more rapidly, gen- erating more electrons per unit time and thus greater current. We also observed a higher voltage. Another indication that the reaction proceeds more rapidly for the bleach battery is the striking abundance of white fluffy particulate as the cell is left over time—this is Al(OH)3(S). Additional details about the role of bleach in the cell can be found at http://www.exo.net/pauld/ saltwater. In conclusion, the saltwater aluminum/air battery is a rich activity combining elements of chemistry and physics. Understanding some of the chemistry behind THE PHYSICS TEACHER Vol. 46, December 2008 547PDF Image | Salty Science of the Aluminum-Air Battery
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