In the Laboratory
Exploring Faraday’s Law of Electrolysis Using Zinc–Air Batteries with Current Regulative Diodes
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Masahiro Kamata* and Miei Paku Department of Science Education, Faculty of Education, Tokyo Gakugei University, Tokyo 184-8501, Japan; *
[email protected] Faraday’s law of electrolysis, stating that the number of moles of substance produced at an electrode during electrolysis is directly proportional to the number of moles of electrons transferred at that electrode, is one of the most important laws in the field of electrochemistry, and is presented in high school chemistry in Japan. In most cases, electrolysis of a CuSO4 solution is used to demonstrate it (1, 2); however, this kind of experiment needs not only chemicals but also several elaborate devices such as an analytical balance, a dc power supply, and a dc ammeter. In addition, it requires a long time for students to obtain quantitative data. From such a viewpoint, we developed new educational experiments using a zinc-air battery (PR2330) and a resistor to discharge it (3, 4). Although these experiments did not need any chemicals or a dc power supply, an ammeter (or a voltmeter) was needed to obtain accurate data because the discharging current was not necessarily constant if the resistor was directly connected to the battery to discharge it. Therefore, in this study, we applied current regulative diodes (CRDs) to our previous experiment and made it much simpler, which means the cost of the experiment is quite low and we can conduct the experiment in a classroom instead of a laboratory. In addition, the measurement can be accomplished within a short time and the data are quantitative enough for educational purposes. Zinc-Air Battery and CRD As shown in Figure 1, a zinc-air battery does not contain any cathode material, such as MnO2, inside the battery. Instead, it uses the oxygen in the air that comes into the battery through several holes on the cathode-side. The electrode reactions at the anode and the cathode are anode: Zn + 2OH − ZnO + H2O + 2e− (1) (2) cathode: 4OH − O2 + 2H2O + 4e−
Figure 1. Schematic of the zinc-air battery.
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and the total reaction is 2Zn + O2
2ZnO
Since four electrons are involved in the cathode reaction, eq 2, where one oxygen molecule reacts, the amount of oxygen molecules absorbed and reacting in the battery can be calculated as It amount of reacting oxygen = 4 F
(4)
where, I, t, and F are the discharging current, time of discharging, and Faraday constant, respectively. As one mole of oxygen gas occupies 22.4 L at 273.15 K and its volume is proportional to the absolute temperature, the volume of reacting oxygen can be calculated as
It volume of reacting oxygen = Vm 4 F = 22.4 L
= Vm°
T It T ° 4F
T It 273.15 K 4 F
(5)
where T, T °, Vm, and Vm° are the temperature, standard temperature, molar volume, and molar volume at standard temperature, respectively. A CRD is a diode that supplies constant current to an electric circuit, even when the applied voltage is not constant. In the case of the E-102 diode, the value of the constant current is slightly greater than 2 mA. On the other hand, the PR2330 zinc-air battery can be discharged stably with the current up to 20 mA, even when the room temperature is relatively low. Therefore, by connecting nineteen CRDs (E102) in parallel as shown in Figure 2, we created a CRD array that supplies 20 mA.
Figure 2. Array made of nineteen CRDs.
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(3)
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In the Laboratory
Figure 3. Basic characteristic of the CRD array: the array can supply constant current when the voltage is over 3 V.
Figure 5. Experimental apparatus during measurement.
Figure 4. Vertical cross section of the experimental apparatus.
Figure 6. Experimental data at 20 ⬚C showing the linearity of the volume of reacting oxygen with time.
The property of the array is presented in Figure 3. Here it can be seen that the array can supply constant current when the voltage is over 3 V. Since the terminal voltage of a zincair battery is 1.4 V, three zinc-air batteries have to be connected in series, or the combination of one zinc-air battery and one lithium battery (3 V), such as a CR2032, can be used.
line of 0.1, 0.2, 0.3 mL of the pipet. When the experiment was over, the battery was resealed to conserve its power for a while (a few months). A typical example of the results is presented in Figure 6, where the theoretical values derived from eq 5 are expressed in a solid line. Although the experiment was finished within several minutes, including preparation for measuring, the obtained data was coincident with the calculated data within a few percent.
Experimental Method The equipment developed in this study is simple, as shown in Figure 4. After enclosing the batteries in the plastic capsule, we connected the CRD array to the two terminals coming out of the capsule and put a small quantity of water into the end of the volumetric pipet. Then we fixed the apparatus in a horizontal position as shown in Figure 5 so that the water could move smoothly inside the pipet and the pressure inside the capsule was equal to the atmospheric pressure without being affected by gravity. Since the capsule was airtight, the pressure inside it decreased as the air-zinc batteries discharged (absorbed oxygen in the capsule), which made the water in the pipet move toward the battery. Thus we measured the time required for the water to pass the scale www.JCE.DivCHED.org
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Hazards This experiment does not use any hazardous chemicals and only a few small batteries that are commercially available; thus there are no significant hazards. Practical Study at Setagaya Senior High School We had 47 students in 12th grade conduct the experiment. They were divided into small groups of two or three students and conducted the experiment in their groups. Since they had already learned Faraday’s law of electrolysis in their chemistry classes, it was assumed that they could compare
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In the Laboratory
our new experiment with their old one using CuSO4 solutions. After the experiment, several questions were asked, using a questionnaire; for example, • Is this experiment easier to understand than the old one using aqueous solutions? • Is this experiment technically easier than the old one using aqueous solutions? • Is this experiment more interesting than the old one using aqueous solutions?
Most responses were positive, with the proportion of negative responses for each question being less than 20%. The students were also asked what they thought about our new experiment. Most of their answers were positive. The following are examples: • I am happy to find that measured values were in good agreement with the theoretical ones. • I am very impressed with the equipment.
• Zinc-air batteries are interesting. I want to try other experiments that make better use of zinc-air batteries. • Zinc-air batteries help us to see the reaction rate more clearly than in the electrolysis of CuSO4 solutions.
Conclusion The combination of zinc-air batteries and the CRD array has allowed us to make the experiment on Faraday’s law not only much simpler and less expensive but also very quantitative for high school students. According to the practical study conducted in senior high school chemistry classes, almost every student that conducted our experiment obtained accurate data and felt satisfied with it. WSupplemental
Material
Instructions and a worksheet for the students are available in this issue of JCE Online.
• I came to understand the experiment as I conducted it. • The amount of reacting oxygen was more than I had expected. • Although I am not good at electricity, this experiment was easy to understand. • I have learned a lot in one experiment. I like this experiment. • A CRD is very convenient, isn’t it? It makes Faraday’s calculation easier because the current is constant.
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Literature Cited 1. Gomes, M.; Teresa S. R.; Oliveira, M.; Manuela O.; Fonseca, M. Arminda; Oliveira, João A. B. P. J. Chem. Educ. 2004, 81, 116. 2. Thompson, C. C. J. Chem. Educ. 1973, 50, 435. 3. Kamata, M.; Kawahara, T. Chemistry and Education 2000, 48, 192. 4. Kamata, M. Chemistry and Education 2000, 48, 330.
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