In the Laboratory edited by
Second-Year and AP Chemistry
John Fischer Ashwaubenon High School Green Bay, WI 54303-5093
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Isolation of Copper from a 5-Cent Coin An Example of Electrorefining Steven G. Sogo Laguna Beach High School, 625 Park Ave., Laguna Beach, CA 92651;
[email protected] Although many laboratory investigations and demonstrations use United States pennies (1–5), relatively little chemistry has been done with the penny’s big brother, the “nickel”. A United States 5-cent coin is made of an alloy that is 25% nickel and 75% copper (6). In this experiment, students isolate copper from a 5-cent coin using electrolysis (an example of the process of electrorefining). The isolation of pure copper metal from a coin that, at first glance, appears to be devoid of copper is a transformation that is both entertaining and highly educational for students (Figure 1).
Electrolysis in a Chloride Solution Refining of metals through electrolysis can be performed either through a process of selective oxidation or selective reduction (7). In this experiment, pure copper is obtained through a process of selective reduction. The 5-cent coin is used as the anode and a graphite rod is used as the cathode. 6 M HCl is used as the electrolyte. HCl is chosen as the electrolyte for two reasons: the acidity promotes the solubility of the transition metal ions produced during electrolysis, and the chloride ions form coordination complexes that also enhance the solubility of the transition metal ions. A schematic diagram of the experimental setup is shown in Figure 2. When electrolyzed in typical aqueous solutions, the metals that make up the 5-cent coin are oxidized according to the half-reactions: Cu → Cu2+ + 2e− 2+
Ni → Ni + 2e
−
E ⬚ = ᎑0.34 V E ⬚ = +0.25 V
However, when a significant concentration of chloride ion is present in the electrolytic bath, the following oxidation halfreaction for copper is favored (8, 9): Cu + Cl− → CuCl + e−
Figure 1. Copper metal isolated from a 5-cent coin through a process of selective reduction on a graphite cathode.
connect to negative pole
During the experiment, the 5-cent coin is gradually dissolved, resulting in an intensely green-colored electrolyte bath. The green color of the bath results from aqueous Ni2+ ions (which may react with excess chloride ions to form complex ions such as NiCl42−). The copper(I) chloride complexes are nearly colorless, but may slowly (over a period of hours) be air oxidized to form green copper(II) chloride complexes (10). In a bath containing aqueous H+ ions, Cu+ complex ions, and Ni2+ ions, the important reduction potentials are:
5-cent coin
6 M HCl
CuCl + e− → Cu(s) + Cl− +
−
2H + 2e → H2(g) 2+
−
Ni + 2e → Ni(s)
Figure 2. Schematic diagram showing the experimental setup for electrolysis of the 5-cent coin.
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CuCl is almost insoluble in water, but in a bath of 6 M HCl, the CuCl will react with additional Cl− ions to form soluble complexes such as CuCl2− and CuCl32− (9). This solubility allows the copper ions to migrate through solution to eventually become reduced at the cathode. Experimental Observations
connect to positive pole
graphite post
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E ⬚ = ᎑0.14 V
E ⬚ = +0.14 V E ⬚ = +0.00 V E ⬚ = ᎑0.25 V
During the first few minutes of this electrolysis experiment, only hydrogen gas is formed at the cathode, but soon, as the
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concentration of aqueous copper ions increases, copper metal also begins to form at the cathode. Because of their unfavorable reduction potential, Ni2+ ions cannot compete with Cu+ and H+ ions for electrons. Therefore, the only solid material that forms at the cathode is copper. As the electrolysis reaction progresses, the concentration of unreduced nickel ions in the bath increases, resulting in an increasingly dark green color in the electrolyte bath. When using a 6-V battery (or power supply), the current in the cell typically reaches 1–1.5 A. With this level of current flow, easily measurable quantities of copper metal can be isolated in approximately 20–30 minutes. If the current is measured during the experiment, the quantity of solid copper formed can be compared to the number of coulombs transferred in the experiment, and a percent yield calculated. The calculation of percent yield of copper in this experiment is more interesting (and instructive) than in many other labs because of the competing reaction that results in the formation of H2 gas. Typical yields of copper metal are around 40%, which helps students learn that even when they do everything “right”, they may be unable to approach 100% yield owing to factors inherent in the system in which they are working. Summary This experiment is very interesting for students for a number of reasons. The intense colors and bubbles that are formed in the bath give the experiment great visual appeal. The isolation of pure copper from a “nickel” results in significant surprise and wonder. Many students find the remnants of the “nickel” at the end of the electrolysis to be a souvenir worth taking home. Students come away from the lab with a much stronger conceptual understanding of the significance of reduction potentials in a situation where there is a competition for electrons. I have run this lab at the high school level in both AP and honors classes. The laboratory instructions for this experiment are written in a way that encourages student discovery. The main objective stated in the instructions is to determine what “mystery metal” is alloyed with nickel to create a 5-cent coin. Presumably the experiment could also be adapted for an introductory college course. There is a wealth of chemistry involved in this experiment, and the depth and challenge it offers can be adjusted to fit many different levels of student ability and achievement.
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Hazards 6 M HCl can burn skin and eyes if not handled with caution. During electrolysis, the bath will warm up, resulting in increased formation of HCl fumes. Use of a cardboard lid on the reaction beaker during electrolysis minimizes the risk of these fumes. Use of individual student fume hoods can further reduce the risk. If the electrodes are reversed when connected to the power supply, highly toxic Cl2 gas will be produced. This gas can readily be detected by its strong odor, and corrections made. Acknowledgment The assistance of Erich Uffelman of the Chemistry Department at Washington and Lee University in the preparation of this manuscript is acknowledged. W
Supplemental Material
Instructions for the students, notes for the instructor, and an optional extension of the laboratory are available in this issue of JCE Online. Literature Cited 1. Mauldin, Robert F. J. Chem. Educ. 1997, 74, 952. 2. Szczepankiewicz, Steven H.; Bieron, Joseph F.; Kozik, Mariusz. J. Chem. Educ. 1995, 72, 386. 3. Phillips, J. P. J. Chem. Educ. 1983, 60, 740. 4. Lemlich, Robert. J. Chem. Educ. 1957, 34, 489. 5. Shakashiri, Bassam. Copper to Silver to Gold. In Chemical Demonstrations: A Handbook for Teachers of Chemistry; The University of Wisconsin Press: Madison, WI, 1983; Vol. 4, p 263. 6. Wu, Corinna. Science News 2000, 157, 216 7. Rosenquist, Terkel. Principles of Extractive Metallurgy, 2nd ed.; McGraw-Hill Company: New York, 1983; pp 424–252. 8. Jacobsen, C. A. Encyclopedia of Chemical Reactions; Reinhold Publishing: New York, 1949; Vol. 3, p 287. 9. Latimer, Wendell. The Oxidation State of the Elements and Their Potentials in Aqueous Solutions 2nd ed.; Prentice-Hall: New York, 1952; pp 183–187. 10. Sogo, Steven G. Laguna Beach, CA. Unpublished data obtained from experiments done using a sample of pure copper rather than a 5-cent coin as the anode, 2001.
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