Extensions

1. Electrochemical gadgets. A number of suppliers provide gadgets that can be used to illustrate electrochemical principles. For example, toy and novelty stores often have clocks that can be powered by a copper/zinc voltaic cell. A "two potato" clock that uses potatoes as electrolytes and salt bridges is often available. A company that sells interesting gadgets like this is The Home Scientist/Jerryco, 601 Linden Place, Evanston, IL 60202. Another is Grand Junction, Ltd., Science and Surplus Center, 100 South Lynn Shores, Virginia Beach, VA 23452. See also information in the Demonstrations section.

2. Using school-built batteries. A small transistor radio or "walkman" can be powered by several voltaic cells connected in series. Any number of "homemade" cells can be used as long as their potentials add up to the proper electric potential. Fairly large electrodes should be used to pass sufficient current.

3. Electroplating unusual objects. Electroplating with copper is a fairly easy process. If the object to be plated is not conductive, first coat it with a film of conductive material. There are commercial sprays available for this purpose. Hook up an electrolysis bath using the object to be plated as the cathode and a piece of copper sheet as the anode. The electrolyte solution should be about 1 M copper(II) sulfate. NOTE: Copper does not plate well on a zinc object.

Projects

Many chemistry teachers like to encourage students to complete research projects, either for their own edification or to enter into science fair and paper presentation research competitions. Below are a few suggestions for such projects.

1. Gel electrophoresis. (This idea is by Sheila Kolb and Robert Roe, Jr., from the R. Arlotto et al. 1984 Dreyfus Institute Electrochemistry module.) Electrophoresis is a method for separating mixtures that contain charges on molecules, or molecules with + and ­ charges like proteins, DNA, etc. The ions are attracted toward electrodes and forced to move through a medium such as gel. The combination of size, charge, and tendency to interact with the gel determine how fast various species travel and consequently how well and how quickly they can be separated from each other. Electrophoresis is not normally encountered in high school because of perceived expense and danger. Your local biology department may have an inexpensive unit. (Commercial firms use high electric potential.)

However, your students can make electrophoresis separators with 9-V transistor batteries, a short piece of clear plastic tubing such as Tygon, some grocery store gelatin, and a small syringe and needle. The gelatin is used to fill a length of tubing. The pH of the gelatin can be controlled by gelling it with various buffer solutions made from acetate, phosphate, and ammonium salts instead of water. Dissolve about 10 g of gelatin in about 200 mL of water or buffer solution (a 250-mL beaker nearly full) in a hot water bath (a 400-mL beaker with enough water to form a "jacket" around the 250-mL beaker). Fill the tubing with gelatin and stopper at both ends until the gelatin cools and sets. Inject a small sample of the mixture to be separated through the tubing into the middle of the gelatin using the needle and syringe. A mixture of bromcresol green and methyl red is a good trial mixture for practice. Attach wires to the batteries and stick the ends of the wires in the ends of the gelatin.

Here are some questions that can be answered with this system.

1. What mixtures of ions can be separated? Can other mixtures such as amino acids or DNA fragments be separated like this?

2. What is the influence of pH on the separation?

3. What is the influence of concentration of ions in the water used for making the gelatin?

4. Does it matter how close together the electrodes are? Does the shape of the electrodes matter? For example, if you attached the wires to little flat electrodes that contacted the gelatin across the diameter of the tube, would the results be different?

2. Making a Polymer that Conducts Hydronium Ions at Room Temperature. Such a polymer was identified in Links and Connectionsas important for the economic development of fuel cells. Worthy (1985) reported news about such a polymer made from polyvinyl alcohol and phosphoric acid. One student might enjoy altering important variables such as reactant concentrations and preparation temperature to prepare such materials. The polymer is apparently made by mixing the two ingredients and evaporating the water. Details on testing the proton conductivity were not given in the article and might prove to be a challenge to a high school student. Before making a polymer, student(s) should look up the article and locate the research article by the original scientists by referring to Science Citation Index.

3. Effects of Magnetic Fields on Electrochemical Reactions. Ions moving in electrochemical cells are just moving charged particles subject to all the laws of physics including the one describing the force a magnetic field places on them. The rates and perhaps other features of electrochemical reactions involving ions should therefore be affected by placing them in magnetic fields. Test-tube reactors placed between strong magnets such as those available from many science supply companies are good systems for testing these effects. Students can also build Helmholtz coils and, with a variable electric potential source, can even subject the electrochemical cells to magnetic fields of different strengths. If you are unfamiliar with Helmholtz coils, consult a high school or college physics teacher. One easy system to measure is electrolysis of water. How does the presence of the magnetic field affect the rate at which gas collects in a tube?

Use Figure 11 as a guide in construction. Care is needed to not overheat any of the elements and to not cause a short by using too much solder in connections. Students interested in electronics may be induced to construct the polarity testers for you. If your school has an electronics course or a vocational education department, they might also help you build one. Once you have built a polarity tester or two, they take about 15-20 min to build. They are quite durable and should last many years with proper care. Inexpensivemultimetersofhigh impedancecanalsobe purchasedatmanyelectronics stores (such as Radio Shack). These are good alternatives if you do not have the time to make your own polarity testers.

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