臺灣化學教育

Educational Experiment to Verify
Faraday’s Law of Electrolysis
using Zinc–Air Batteries

Takahiro SHIBATA and Masahiro KAMATA*

Department of Science Education, Faculty of Education, Tokyo Gakugei University, Japan
*[email protected]

n  Introduction

We developed a new educational experiment to verify Faraday’s law of electrolysis using button-type zinc-air PR44 batteries and a resistor to discharge them. Although the output current from a PR44 battery is small, by connecting four PR44 batteries in series, a clear result was obtained just as in an experiment using larger batteries such as the PR2330 battery. In addition, the apparatus was not only inexpensive but also easy to build and handle for high school students.

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 learned by studying senior high school chemistry students in Japan. In most cases, electrolysis of a CuSO₄ solution with a copper plate is used to verify it ^{[1, 2]}; However, this kind of experiment needs not only a certain quantity of chemicals but also several elaborate devices such as an analytical balance, a DC power supply, and a DC ammeter. In addition, it takes a long time for students to obtain quantitative data.

From such a viewpoint, we previously reported a new educational experiment using a zinc-air PR2330 battery with a resistor or current regulative diodes to discharge it ^[3]. These experiments did not need any chemicals, and therefore, no waste treatment was required. In addition, the quantitative measurement can be accomplished within a short time. However, the production of the PR2330 battery has ended because they were produced originally for pocket pagers which are not used today. An alternative experiment cannot be created by just changing the type of battery because the output current of the PR2330 battery was exceptionally large and there is no zinc-air battery commercially available which is equivalent to the PR2330 battery. Therefore, we developed a new experiment using an apparatus with a different structure which uses four smaller PR44 batteries instead of the PR2330 battery. The apparatus is not only inexpensive but also easy to build and handle. In addition, the obtained data is educationally quantitative enough, just like the experiment using the PR2330 battery.

n  Experimental

Apparatus

The structure of the PR44 battery, as shown in Fig. 1, is the same as the PR2330 battery which we used in our early studies.

Fig. 1. Zinc-air PR44 battery (Left: the plastic tab was removed, right: the air holes was covered with the plastic tab

The anode material is powder of zinc metal and it is stored inside the PR44 battery, but no cathode material, such as MnO₂, is stored inside it. Instead, it uses the oxygen gas in the air that comes into the battery through several holes on the cathode side. As potassium hydroxide is used as the electrolyte, the electrode reactions at the anode and the cathode are expressed as the equations [1] and [2] respectively, and the total reaction is expressed as the equation [3].

Anode (oxidation): Zn + 2OH⁻ → ZnO + H₂O + 2e⁻ [1]

Cathode (reduction): O₂ + 2H₂O + 4e⁻ → 4OH⁻     [2]

Total reaction: 2Zn + O₂ → 2ZnO                              [3]

The PR44 battery is produced by several companies. The battery was used in this study was the product of VARTA Microbattery GmbH in Germany. A plastic tab is put on the cathode side of the battery to seal the air holes, and it must be removed before using the battery. Immediately after the tab is removed, the battery absorbs a certain amount of oxygen even if it is not electrically connected to a circuit. Therefore, it takes at least one hour before carrying out the experiment, the tabs of four PR44 batteries should be removed without being connected to anything so that the cathode is in equilibrium with oxygen in the air.

The PR44 battery with the diameter 11.6 mm is much smaller than the PR2330 battery with that 23 mm, thus the electric current coming from the PR44 battery is much smaller than that from the PR2330 battery. In order to compensate for this difference, four PR44 batteries are connected in series in this study. Each PR44 battery is squeezed into a short flexible PVC tube (12 mm OD, 10 mm ID, and 14 mm Length) as shown in Fig. 2(a). After four tubed PR44 batteries are prepared, they are connected using paperclips and PVC tube spacers (12 mm OD, 10 mm ID, and 5 mm Length) as shown in Fig. 2(b). The spacers are needed to avoid connecting three batteries with the same one paperclip which would create a short circuit.

Fig. 2. Four PR44 batteries connected in series

Then, as shown in Fig. 3, the four connected batteries are put in a test tube (18 mm ID, and 90 mm Length) and sealed with a silicone rubber plug through which two electric leads and a scaled part cut out from a 1 mL of plastic pipette, as shown in Fig.4, are fixed. The two leads are connected to both ends of four PR44 batteries respectively using the same paperclips. The pipette is used to find out the volume of oxygen gas reacted in the batteries during discharge.

Fig. 3. Experimental apparatus

Fig. 4. How to cut out a scaled part from a pipette

How to use

To verify Faraday’s law, a 500-W resister is connected between the ends of the two leads coming out of the plug as shown in Fig. 3. After putting a small quantity of ethanol into the pipette, the apparatus is placed horizontally so that the ethanol can move smoothly inside the pipette without being affected by gravity when carrying out the experiment. The pressure inside the test tube decreases as the air-zinc PR44 batteries discharge (oxygen reacted in the test tube), which makes the ethanol in the pipette move toward the battery. Thus, the time required can be measured for the ethanol to pass the 0.05 mL scale lines of the pipette. In this way, the relation between the volume of reacted oxygen gas and the time of discharge can be obtained.

On the other hand, Faraday’s law can be expressed as the equation [4];

Amount of reacted oxygen in one battery = It / 4F    [4]

where, I, t, and F are the discharging current (A), time of discharging (sec), and Faraday constant (96500 C/mol, 96500 s A/mol), respectively. The number ‘4’ represents that one mole of oxygen gas needs to be reacted with four moles of electrons (see the equation [2]). Assuming that one mole of oxygen gas occupies 24.8 L at room temperature, the volume of reacted oxygen gas in the four batteries, V in mL, can be expressed as the equation [5];

    [5]

Although the nominal voltage of a zinc-air PR44 battery is 1.4 V, the terminal voltage of four PR44 batteries in series is 5.0 V when they are connected to a 500-W resistor. Therefore, the current coming from a PR44 battery can be calculated as I = 5.0 V / 500 W = 0.010 A, and replace a formula in the equation [5] with the calculated result and the specified values as shown in the equation [6], and then the equation [6] can be simplified as the equation [7];

V mL = 4 × [0.010 A × t s / (4 × 96500 s A/mol)] × 24.8 L/mol× 1000 mL/L    [6]

V mL = 0.0026 × t mL    [7]

The equation [7] can be compared with the experimental results.

n  Results and Discussion

A typical example of the result is presented in Fig. 5, where the relation between the time required for the ethanol to pass the 0.05 mL scale lines and the volume of oxygen reacted is plotted using the blue spots and compared with the theoretical line, derived from the equation [6], drawn using the orange color. Although the experiment was finished in several minutes, including preparation for measuring, the obtained data was coincident with the calculated data within a few percent. It is noted here that a Current Regulative Diode (CRD) can be used instead of a resistor to obtain more quantitative data.

Fig. 5. Typical example of experimental result

By the trend line of the blue spots in Fig. 5, the result indicates that the volumes of oxygen gas reacted is directly proportional to the time required at cathode electrode. Therefore, the moles of oxygen gas reacted is directly proportional to the number of moles of electrons transferred at that electrode. This trend line is very close to the theoretical line (the orange line). Therefore, the result verifies Faraday’s law.

n  Reference

1.        Kamata, M.; Kawahara, T. Chemistry and Education (in Japanese) 2000, 48, 192.

2.        Kamata, M. Chemistry and Education (in Japanese) 2000, 48, 330.

3.        Masahiro Kamata and Miei Paku, Journal of Chemical Education 2007, 84, 674.