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JAMES 0. SCHRECK Univenity of Northern Colorado Freeby, CO 80639
An Easy-TO-DOPlating Experiment Mary S. Herrmann University of Cincinnati, Raymond Walters College, Cincinnati, OH 45236
Plating is a process in which a layer of metal is bonded to the surface of another substance, which i s usually another metal. Metallic coatings are applied to many commercial products in order to prevent their corrosion,' a s with zinc-plated or galvanized steel, or to enhance their beauty, as seen in silver- or gold-plated jewelry. Plating, both commercial and educational, is typically done by immersing the object to be plated in a salt solution of the plating metal. For example, in copper plating, the salt solution is frequently a copper sulfate solution. Usually additional components are added for various purposes.2 An electric current is then applied, while the object to be plated serves a s the cathode. This type of plating is called electroplating or electrodeposition. This experiment demonstrates the concept of plating without the use of electricity. A nail is plated with copper metal from a penny. The experiment is quick, safe, and very convenient because the materials involved are readily available and inexpensive. Students a t all educational levels can benefit from this encounter with two familiar metals and a n industrially important process. Experimental Materials
Nails (unealvanized iron) Pennies !&ed before 1982) Vinegar Magnesium sulfate (epsam salts) Nylon wool or steel wool 5% Copper sulfate solution in 1%sulfuric acid3 Container (250-mLbeaker or clear plastic cup) Tongs (if available) Each student should have a penny, 2 nails, and a container. Thoroughly clean the penny and the nails by rubbing them with the fine steel wool. Slow Plating Put 25 mL (about 1oz) of vinegar into a container. Add 1 g (about 112 teaspoon) of magnesium sulfate to the container, and stir to dissolve. Place the penny and the nail in the container in such a way that they will remain on o p posite sides of the container. Allow the container to sit for two days. Plating is more uniform if the nail is turned over after the first day. After two days, pour off the liquid. Rinse the nail with water and dry it. (Do not let the container sit longer than two days because rusting will occur.) Fast Plating Caution:Wear goggles and handle the solutions with care.
Put the second (uncoated) nail into a container, and add 25 mI. of copper sulfhte solution. Observe the nail cl~~icly.
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Journal of Chemical Education
Copper plating of the nail should occur very quickly. After about 10 min, remove the nail with tongs and rinse it. If tongs are not available, the solution can be poured off and the nail rinsed several times with water while still in the container. Compare this coating with that obtained by slow plating. Results and Discussion When a penny is placed in vinegar, the copper metal is slightly solubilized as it is converted to the copper(I1) ion as shown below. Cu + ~ u & + 2 e (1)
Vinegar is a 5%acetic acid solution. Its presence speeds up the Laction because the acid attacks thk copperketaL4 The conversion to copper ion is slow. I t is rate-determiningfor the entire process.Once the copper ion is formed, it r e a d s quickly to form copper metal a t the surface of the nail according to the following reaction. CU" + Fe + Cu + Fez+ (2) This is a s ~ o n t a n w ureaction. s I t is enereeticallv favorable because copper ion has a greater affinity fur electn)na than iron metal does. Comer is oxidlzed and then reduced in the slow plating (eqs 1 .Ad 2). It is reduced in the fast plating (eq 2). The two plating procedures yield very different appearances. The slow plating gives a smooth, shiny, copper coating on the nail. The slow movement of copper ions from the penny into solution ensures that the application of copper will be smooth. This is in contrast to the rapid plating that occurs when the nail is added to copper sulfate solution. The concentration of copper ions is much higher in this case, and we get a thicker and rougher copper coating. Additional Information 1. CopperUI) ions give a characteristic blue color in solution, which can be seen with the copper sulfate solution. This blue color is not observed with the slow-plating process, however, because the concentration of copper(I1) ions is very low. 2. In the slow-plating process the magnesium sulfate is used to increase the ionic strength of the solution, thus allowing the reactions to proceed a t a faster rate.
'Gillurn, D. E.; Herrmann, M. S. J. Chern Educ. 1990, 67,62. 'Bruno, T. J. J. Chern Educ. 196663,883. 3 ~ prepare, o dissolve 5 g of CuSO, in 100 mL of 1% sulfuricacid, which was prepared by adding 1 mLof concentrated sulfuricacid to water. 4The Merck Index, loth ed.: Windholz, M., Ed.; Merck: Rahway, NJ, 1983: p 358. endri ricks on, E . J. Chern. Educ. 1988, 65,986.
3. For the slow-plating process in which pennies are the source of copper metal, pennies dated before 1982 are recommended because they are a better source of copper. They are predominantly copper (95%),whereas newer pennies are predominantly zinc (97.6%)with just a thin copper
coating (0.0002 inA5Any Cu(I1) that is formed in the slowplating process will react with exposed zinc, thus replating the penny, rather than plating the iron nail. (Pennies dated after 1982 can be used, but the results will be slightly inferior.)
Volume 69 Number 1 January 1992
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