A new twist on the old Juan’s battery Dilemma Vanessa Hunt, Timothy Sorey, Evguenia Balandova, and Bruce Palmquist

hen life hands you , make a battery! In this article, we de- scribe an activity that we call Juan’s Dilemma, an extension of the familiar lemon-battery activity (Goodisman 2001). Juan’s Dilemma integrates oxidation and reduction chemistry with circuit theory in a Wfun, real-world exercise. We designed this activity for a ninth-grade physical science class, and our students have found it to be intriguing and challenging.

52 The Science Teacher Juan’s Dilemma

F i g u r e 1 Lemon-battery diagram. Below is a digital multimeter (DMM) and a lemon battery with as a reference, and two rows of metal pieces, placed 1.5 cm apart.

0.956 V

Pb Zn Cu

Zn Pb Cu Zn Pb Cu

Pith Peel Side view of lemon Top view of lemon

The dilemma If we suspect that the class will find the scenario too im- We begin this activity by introducing Juan and his dilem- probable, we bring a few props (e.g., a toolbox, jewelry kit, ma. Juan’s team has just arrived at the National Radio- an RC car, a sign for Sloppy Sorey’s Lemonade Stand, and Controlled (RC) Car Championships when he realizes so on) and have students act it out. that he has left the battery for their battery-powered car at home. It is a two-hour drive to the local RC shop, and Lab activities the qualifying heat begins in one hour—so his team has After presenting students with this scenario, we assign to think fast. They must be able to light up the car’s light- two lab activities that can be completed in one 2-hour emitting diode (LED), a small electronic device, to avoid lab session or two 50-minute classes. Teacher explana- being disqualified. If they can find a way to create a make- tion is minimal; students measure produced by shift battery for this device, they might be able to make it combinations of three different metals—using a lemon to to the next round. In the meantime, Juan’s father can drive complete the —and order these voltages hier- to the nearest RC shop and pick up a new battery before archically. They then solve Juan’s dilemma by creating a the second heat begins. lemon-battery system to power the LED. These activities Juan knows that only 3 volts (V) are required to light up are designed to help students develop a conceptual under- the car’s LED, but he is not sure how much current he needs. standing of oxidation–reduction reactions and the electro- It is a hot day in July, so he and his team head to Sloppy chemical series for metals. Sorey’s Lemonade Stand to cool off and think. Tim, the We recommend that students have a basic knowledge goofy vendor, is squeezing lemons for fresh lemonade. of simple electrical circuits; the ability to connect batteries When the team tells him about their predicament, he jok- in series and parallel; and the use and operation of a DMM ingly offers them some of his lemons, saying he once saw a before completing the activity. They should also understand lemon-battery “trick” at a children’s party. that the LED is a polar device, and be able to connect it to Although skeptical, Juan’s team decides that the lemons a battery. All materials—such as lemons, LEDs, and small are worth a shot. They take the fruit and search through pieces of common metals—are inexpensive and easily ob- Juan’s toolbox for ideas, where they find screwdrivers, tained. Adequate DMMs are available for less than $20. wrenches, fishing line, and a digital multimeter (DMM). In terms of safety, chemical-splash goggles are required to Juan’s sister, Yarisel, contributes items from her jewelry- prevent juice and sharp metal edges from contacting making kit, including pieces of metal and wire, needles, the eyes. Students should wash their hands after handling pliers, and beads. The team has two hours to rescue its entry metals and lemon juices, reducing the risk of skin irritation for the RC championship! and exposure to heavy metals, such as lead.

October 2010 53 Figure 2 Experimentally produced voltages. Reference Zinc (Zn) (Cu) Lead (Pb) Pb = 0.512V Pb = -0.435V Zn = -0.492V Volts Cu = 0.956V Zn = -0.925V Cu = 0.432V Zn = 0.000V Cu = 0.000V Pb = 0.000V

Which metals make the best lemon battery? tions of any pattern that emerges. In the process, students In the first activity, students are guided to puncture a sin- discover that identical metals—copper and copper, for gle lemon with three dissimilar metals—zinc (Zn), lead example—do not undergo electron transfer reactions (Pb), and copper (Cu). A DMM is then used to confirm (i.e., oxidation and reduction), or produce a that small voltages are produced across different com- between them. binations of these metals (Figure 1, p. 53) (step-by-step When students record their experimentally produced lab instructions are available “On the web”). Students are voltages (Figure 2), they observe that zinc always produces guided to systematically measure the voltage produced by a negative voltage with the other metal, and copper always each combination, using each metal in turn as a common produces a positive voltage. From these observations, stu- ground or reference. The different combinations yield a dents deduce that zinc is losing electrons (i.e., oxidizing) hierarchy of voltages, which may be positive or negative and copper is gaining electrons (i.e., reducing) (Figure 3). (Figure 2). In ordering these metals hierarchically, students are led to We then explain that production of voltage implies the the experimental derivation of a basic electrochemical series capacity for current flow and the transfer from one metal to (Chang 2007). another, and that reactions in which electrons are transferred Formally developing the idea of the electrochemical are called oxidation–reduction reactions. A positive voltage on series and oxidation numbers at this grade level is optional, the DMM implies that the reference metal is being reduced so we sometimes choose not to do this if the majority of (i.e., gains electrons); a negative voltage implies that the the class lacks adequate chemistry knowledge. However, reference metal is being oxidized (i.e., loses electrons). This we believe that the conceptual understanding developed direct instruction provides valuable scaffolding for the activ- by the activity provides a valuable foundation for future ity and allows students to understand whether the reference science courses. metal is gaining or losing electrons for each of the voltages they measure. Lighting an LED with a lemon battery Students then produce three data tables that illustrate Once students have a working knowledge of the relative the relative tendency of each metal to oxidize, or “give effects that dissimilar metals play in electrochemical volt- away” electrons, and are asked to consider the implica- age production, they are ready to design and construct a solution to Juan’s dilemma (step-by-step lab instructions Figure 3 are available “On the web”). It Hierarchy of voltages. can be fun to make this a com- These voltages were produced with respect to the electrochemical series, using petition between student teams, lead as the reference metal. but students can also work in- dividually. One lemon battery is un- Lead (Pb) as a Voltage Gains Loses Metal Neither likely to yield a voltage greater reference (V) electrons electrons than 0.9–1.0V (Figure 4), so we Most positive voltage 0.432 Cu X reiterate that Juan’s dilemma requires lighting an LED 0.000 Pb X with a minimum of 3V and an Least positive voltage -0.492 Zn X adequate amount of current (we tell students that 10–15

54 The Science Teacher Juan’s Dilemma

Students choose the metals they wish to use for Figure 4 —using data from the first activity—and Lemon-battery voltages. then experiment with different configurations for The following voltages were produced by lemon-battery their lemon batteries. The goal is to produce sufficient combinations of zinc and copper (Zn–Cu). voltage and current to light the LED. In our experi- ence, three copper–zinc lemon batteries connected in series—with copper connected to the positive lead of Parallel Series Number the LED and zinc connected to the negative—works Voltage Current Voltage Current of lemons best (Figure 4). (V) (mA) (V) (mA) We conclude with a conceptual explanation of how 1 0.933 0.045 0.933 0.045 an electrochemical battery works, the role of dissimilar 2 0.800 0.800 1.736 0.040 metals, and how the lemon battery mimics a “regular” galvanic cell battery in terms of oxidation and reduc- 3 0.731 0.132 2.58 0.038 tion processes. The depth and detail of the explanation 4 0.770 0.143 3.27 0.038 varies according to the class’s science background, but we always cover the essential components of a func- tioning : two dissimilar metals (an anode and a cathode) as electrodes or terminals, and a salt milliamps is generally sufficient to produce a “glow,” bridge to chemically connect them. but they can also research this question—online or in The salt bridge provides a solution for the involved the library—as a preactivity homework assignment). We in the oxidation–reduction reaction between the dissimilar remind students how to use multiple batteries in series metals, so that electrons can be released and current can flow. and parallel configurations to meet these minimum power The salt-bridge connection enables the anode (a metal that requirements (Figure 5). oxidizes) to give up electrons to the cathode (a metal that

Figure 5 Parallel and series circuits. The diagrams below represent lemon batteries arranged in parallel (top) and series (bottom) circuits (Zn–Cu).

(+) 1V, 5mA (+) 1V, 5mA (+) 1V, 5mA Circuit total: 1V, 15mA (-) (-) (-)

(+) 1V, 5mA

(-) Circuit total: 3V, 5mA (+) 1V, 5mA

(-) (+) 1V, 5mA

(-)

October 2010 55 Juan’s Dilemma

reduces), and these electrons flow Figure 6 provides a list of problems we have encountered through the external circuit, creating with these activities and our proposed solutions. a current. This current can power an electronic device that is connected to Assessment Keywords: Battery the two metal terminals. We assess understanding on an “acceptable” or “unac- at www.scilinks.org In a lemon battery, the pith and cit- Enter code: TST101002 ceptable” basis for the first lab activity—students should ric acid act as the salt bridge. Once the be able to state whether each metal is gaining or losing connection is made between the two metals, the oxidation– electrons (i.e., reducing or oxidizing) and identify the elec- reduction reaction continues until trochemical series from the voltage hierarchies produced. For the second lab activity, students produce series and u the anode is completely oxidized; parallel configurations of lemon batteries with enough u the salt bridge is depleted of ions as the citric acid is used voltage and current to light the LED. For this activity, up; or we assess students’ success as “excellent,” “acceptable,” or u the cathode is depleted of ions. “unacceptable.” Rubrics for both activities are available online (see “On the web”). Figure 6 Conclusion Observed problems and solutions. Oxidation and reduction reactions are an important com- 1. Initial voltage readings are imprecise. Select a low ponent of students’ chemistry understanding, yet high enough digital multimeter (DMM) voltage range to school students frequently fail to grasp this abstract pro- read millivolts to three significant figures. cess (Schmidt, Marohm, and Harrison 2007). The hands- 2. The lemon battery fails to provide consistent on experiment presented in this article actively guides voltages. Rolling the lemon firmly between your students through an interesting problem that delivers a palms releases the acid-containing juice within the conceptual understanding of the electron transfer process, lemon. Clean metal electrodes of any corrosion while integrating chemistry with the study of electric cir- with sandpaper and ensure that they are inserted cuits. To increase the inquiry level of the activity, students deep enough into the lemon to pierce the pith, can develop their own methods to determine two metals and are not touching each other either above or that produce the most effective batteries, or design data- below the surface of the lemon. collection sheets. n 3. Do not substitute wires for metal plates. Flat metal plates provide greater surface area for metal Vanessa Hunt ([email protected]) is a professor of biological scienc- release. es and science education, Timothy Sorey ([email protected]) is a 4. Students make mistakes in connecting multiple professor of chemistry and science education, Evguenia Balandova lemons in series and parallel. Encourage students ([email protected]) is a health science and chemistry minor to determine and follow the current path of their graduate, and Bruce Palmquist ([email protected]) is a profes- circuit carefully (Figure 4, p. 55). sor of physics and science education, all at Central Washington 5. Students cannot light the light-emitting diode University in Ellensburg. (LED) and become frustrated and convinced that their LED is faulty. Two AA batteries set up in se- On the web ries in the classroom allows for testing of the LED. Juan’s Dilemma grading rubric: www.nsta.org/highschool/ To help students recognize that the LED is operat- connections.aspx ing though its light is dim, lower the light level in Lighting an LED With a Lemon Battery lab instructions: www. the classroom. nsta.org/highschool/connections.aspx 6. If students wish to make current measurements, Which Metal Makes the Best Lemon Battery? lab instructions: review use of the DMM as an ammeter. www.nsta.org/highschool/connections.aspx 7. Challenge early finishers to light two LEDs in series with lemon power. This extension activity requires References students to configure a new lemon-battery circuit Chang, R. 2007. Chemistry. 9th ed. New York: McGraw-Hill. that doubles the voltage supplied by the original. Goodisman, J. 2001. Observations on lemon cells. Journal of 8. Use fresh fruit for each lab. To reduce cost, fruits Chemical Education 78 (4): 516–518. may be halved or other citrus fruits (e.g., oranges, Schmidt, H.J., A. Marohm, and A.G. Harrison. 2007. Factors grapefruit, or limes) may be substituted. that prevent learning in electrochemistry. Journal of Research in Science Education 44 (2): 258–283.

56 The Science Teacher