Getting a Charge Out of It Electrochemical cells (sometimes called batteries) often seem magical to students. Understanding the steps necessary to make electric current flow can be seen by removing components from battery cells and discovering what doesn t work. Next Generation Science Standards (NGSS) Science Practices Practice 2: Develop and use models Standards and Disciplinary Core Ideas HS-PS2-6. Communicate scientific and technical information explaining the importance of molecular-level structure to the functioning of designed materials. PS1.B: Chemical Reactions. Substances react chemically in characteristic ways. In a chemical process, the atoms that make up the original substances are regrouped into different substances, and these new substances have different properties from those of the reactants. 1
ETS1.C: Optimizing the Design Solution. Although one design may not perform the best across all tests, identifying the characteristics of the design that performed the best in each test can provide useful information for the redesign process that is, some of the characteristics may be incorporated into the new design. Engagement Engagement brings students into a topic by connecting the topic to previous material and understandings while offering context. These activities may not be needed for students already very familiar with the concepts in the Exploration. Some relevant starting ideas: Oxidation and reduction are the loss and gain of electrons. (OIL RIG is a useful mnemonic; Oxidation Is Loss, Reduction Is Gain.) Reactions in which electrons are lost by some substances and gained by others are called redox reactions, and they are always accompanied by energy changes. Exothermic changes are those that release energy. The motion of charges can be used to generate useful energy. Activity 1: Copper and Zinc Battery A Wet Cell A copper and zinc electrochemical cell (the Daniell cell) is often the most basic cell that students are introduced to. In the usual arrangement, the zinc molecules in one half of the cell each give up two electrons (i.e., are oxidized ) and become zinc ions, which then enter the cell solution; at the same time, the copper ions in the cell solution gain the two electrons produced by the zinc molecule oxidation (i.e., are reduced ) and form copper molecules that fall out of the cell solution. The movement of these electrons from the zinc molecules to the copper ions can be used to provide energy for many devices. More specifically, the electric current produced by such redox reactions can be captured by physically separating the oxidation and reduction reactions in two halves of a voltaic cell. When the two halves of the voltaic cell are joined together by a salt bridge, electrons from one half of the cell (the anode ) spontaneously flow to the other side of the cell (the cathode ), producing a 2
current that can be used to do work. The salt bridge is necessary to even out the unbalanced electrical charge that develops between the two sides; however, the salt bridge must not allow the zinc and copper ions to move between the two sides only the electrons. Materials (per group): 5 g copper sulfate, 5 g zinc nitrate, copper strip (a copper plumbing fitting or a pre-1982 American penny will work), zinc strip (a galvanized nail or other galvanized metal will work), saturated sodium chloride solution, sturdy paper towel, 150 ml of distilled water, three glass beakers or jars, non-soap steel wool (optional), multi-meter, alligator clips, small red LED, safety goggles and a chart of standard reduction potentials. Warning: Copper sulfate and zinc nitrate are poisonous. Don t drink. Avoid touching. Handle with care. You can save the materials to use in future labs. Dispose in a responsible (and legal) way for your community. 1. Wear safety goggles. Wear appropriate garments for working in a chemistry lab. 2. Pour 50 ml of water into a beaker. Add salt and dissolve completely. Add additional salt until it accumulates on the bottom. While more salt can be dissolved by heating the water, this concentration of this solution is adequate for the task. 3. Pour 50 ml of water into two more beakers. Add 5 g of copper sulfate to one and 5 g zinc nitrate to the other. Stir to dissolve completely. 4. If the copper or the zinc strips look corroded or heavily oxidized, use steel wool to clean them. 5. Place a copper strip in the beaker with the copper sulfate so that part of the strip is submerged and part is out of the solution. Place a zinc strip in the other beaker so that it too has part of the zinc strip submerged and part of the strip out of the solution. 6. Soak the towel in the salt water solution. Squeeze the water out of the towel so that it is wet but not dripping. This will be used as the salt bridge. 3
7. Drape the salt bridge between the two cups. 8. The students are going to attach the multi-meter to the metal strips with the alligator clips. Have the students predict whether or not the multi-meter will show a voltage about 0.1 V. 9. Have students explain why the multi-meter shows a voltage. 10. Have the students attach LED to the plates. If it doesn t light up, switch the legs. Keep track of which leg should be attached to which plate. 11. Have students remove the salt bridge and set it aside. 12. Students should predict what will happen to the voltage measured by the multi-meter. They should provide support for their prediction. After completing the task, they should explain why the voltage is much lower. 13. Students should replace the salt bridge and remove the copper plate from the solution. Again they should predict what will happen to the voltage on the multi-meter. 14. The students should then set the multi-meter so that it measures current. They should slowly insert the copper plate into the copper sulfate. Again they should first predict what the results will be and compare the actual results to their predictions. 15. Repeat with the zinc plate. 16. What happens to the brightness of the LED as the plates are submerged? 17. Have the students write an explanation of how the electrochemical cell generates electricity and how is it related to the amount of copper and zinc submerged. 18. Students can then calculate the voltage of a standard Daniell cell using the table of Standard Reduction Potentials. The values on the table of standard reduction potentials are measurements of potentials using the standard hydrogen electrode (SHE) as a reference electrode under standard conditions so their calculation will not be an accurate predictor of their cell voltage: Zn 2+ + 2 e Ag (s) E = -0.76 V Cu 2+ + 2 e Cu (s) E = 0.34 V E CELL = E RED (cathode) - E RED (anode) = 0.34 (-0.76) = 1.1 V Students should explain, using what they observed during their investigation, why their cell voltage is not 1.1 V. If they are unfamiliar with the table, an explanation of standard conditions will be useful (1 M concentrations of solutions is a particularly important condition!). Exploration 4
Exploration is where students use prior knowledge to investigate ideas through activities to facilitate conceptual change. Activity 2: Aluminum Air (Oxygen) Battery (a dry cell) The copper and zinc battery is fairly robust, but it requires liquids to work well. An aluminum and air battery is easier to use, but requires some help to arranging its components. In an aluminum air battery, four aluminum atoms combine with 12 hydroxide ions in water, creating aluminum hydroxide and liberating 12 electrons. Those electrons then combine with three oxygen molecules in the air and six water molecules making 12 hydroxide ions. Like in the copper and zinc battery, a careful arrangement of wires allows energy to be derived from the movement of the 12 electrons. anode: Al(s) + 3OH (aq) Al(OH) 3 (s) + 3e cathode: O 2 (g) + 2H 2 O(l) + 4e 4OH (aq) overall: 4Al(s) + 3O 2 (g) + 6H 2 O(l) 4Al(OH) 3 (s) Al 3+ (aq) + 3e Al(s) O 2 (g) +4 H + + 4e 4OH (aq) E = -1.66 V E = 0.40 V E CELL = E RED (cathode) - E RED (anode) = 0.40 (-1.66) = 2.06 V Unlike the copper and zinc battery where all the elements and ions are easily available, the oxygen is in limited supply. The partial pressure of oxygen in air is 21%, and its solubility in water is low. Getting the electrons to react with the oxygen is the limiting factor for energy production in this battery. To get around this problem, activated charcoal is often used. Carbon, in the form of charcoal and graphite, is conductive enough that electrons can flow through it. Because activated charcoal is full of tiny holes and crevices, it has a very high surface area which means that it is in contact with much more air (and therefore oxygen) than a flat surface. The carbon itself isn t a participant in the reaction. In this activity, students will first make an aluminum air battery using graphite pencil leads and then repeat with activated carbon, showing the importance of the positioning of the materials and their structure. 5
Materials: Aluminum foil, graphite pencil leads or drawing graphite, activated charcoal, paper towels, sodium chloride salt, water, two alligator clips, small DC lightbulb like a single Christmas light, multimeter 1. Add salt to water until no more will dissolve. 2. Fold a sheet of paper towel into fourths and dampen the towels with the saturated solution. It should be wet but not dripping. 3. Cut a piece of aluminum foil so that it is approximately 15 cm x 15 cm. 4. Place paper towel on the aluminum foil. Place the graphite rods on top. Add some of the salt water solution to the graphite rods to ensure a good electrical connection. 5. The graphite needs to be on the paper towel and not touching the aluminum foil directly. 6. Have students predict the amount of voltage, if any, that will exist between the graphite and the aluminum foil. You may have to explain the chemistry of the reaction. Have students measure with the multi-meter. Switch the multi-meter so that it measures current and measure the current in several locations. 7. Connect the bulb and look at the brightness. 8. Have students remove the graphite from the top layer and replace with activated charcoal. Have students predict whether the voltage and the current will be different. Use the light as a visual indicator of the change in current. 9. Have students gently press down on the pile. Does the increased pressure change the voltage and current? Why? 10. Students can use the table of standard reductions to compare the reactions between the two cells and explore how their set-ups differ from standard conditions. They can use this knowledge to explain the voltage differences between their cells and the predictions made using the standard reductions potentials. 6
Explanation This is where students put together their ideas to create a mental model of how a system works. 1. Have students explain the differences in current and voltage in the amount that the metal plates are submerged in Activity 1. Typical answers include that a greater mass of metal allows more reactions to occur by liberating more electrons which allows more current to flow. 2. Have students explain why the graphite has such a small current in Activity 2 compared to the activated charcoal. Students should note the smaller surface area of the graphite. 3. Why does the activated charcoal work better if it is pushed together? Typical answers include better contact which reduced internal resistance. 4. Why do multiple layers increase the voltage? Elaboration Students extend their understanding to new systems. 1. Students should watch the video. 2. The teacher should pause the video at key locations to emphasize the ideas with the students and help them become active viewers. 3. After watching the video, the teacher should lead a discussion where the students determine and speculate a. Why does the scientist/engineer want to make a better battery? b. What is a better battery? (What are the limitations of the batteries available today?) c. How does precision placement of the materials make a better battery? How is this related to the Activities? d. Why is this technique superior to the chemical only methods described here? e. The virus coat (capsid) is composed of protein (polymers of amino acids). What properties must the capsid have in order for the gold and cobalt oxide ions to stick to the virus coat? It may be helpful to see the molecular structures of the 20 amino acids that make up proteins. 7
Evaluation Teachers and students determine if they understand the material. In this case, we will apply their understanding of making models to a novel situation. A battery can be made by sanding the coating off one side of pennies made after 1983 and placing saltwater-saturated paper in between the sanded penny layers. Post-1983 pennies are a sandwich of zinc and copper. Students can make predictions about what will increase voltage and current Why is sanding the layers smooth important? Would they work as well if they were rough? 1. Why do the layers need to be separated by the salt water soaked paper? The pennies are already copper and zinc touching. Why does the paper need to be soaked in salt water? Why not pure (deionized) water? 2. How does the number of layers affect the voltage? Why? 3. If we bought copper and zinc plates at the store so that their area was larger than the pennies, would the current and/or voltage be higher? Why? Patents Patents are a way for inventors to create property rights in their inventions. Patents provide inventors with the right to exclude others from making, using, offering for sale, or selling the invention in the United States or importing the invention into the United States. Please see What Are Patents, Trademarks, Service Marks, and Copyrights? In exchange for this right, inventors must disclose to the public how to make and use the inventions in their patent applications. This information often can help other inventors make improvements, as well as spur on new inventions. Reading a patent introduces students to technical language and gives them familiarity with the way inventors describe their work. The language can sometimes be difficult for students; however, it can also be instructive to see what claims the inventor has made, and to learn more about how the device or process works. Drawings can also be helpful in understanding some of 8
the key design elements of an invention. Please look at The Anatomy of a U.S. Utility Patent and then at the following two patents related to using viruses to make batteries. US 8283325 Class 514/21.8 (Drug, Bio-Affecting and Body Treating Compositions) AU 1675 US 8685323 Class 422/68.1 (Chemical Apparatus and Process Disinfecting, Deodorizing, Preserving, or Sterilizing) AU 1798 The Aluminum Air Battery is based on work developed by the Teacher Institute at the Exploratorium in San Francisco, California. 9