In the Laboratory
A Simple Penny Analysis Nicholas C. Thomas* and Stephen Faulk Department of Chemistry, Auburn University Montgomery, Montgomery, AL 36124-4023; *
[email protected] Using pennies to illustrate chemical principles has previously been reported in this Journal. This includes the popular “gold” penny demonstration (1, 2) and various brief references to X-ray fluorescence (3) and chemical (4) analyses. Since 1982, the U.S. Mint has been producing pennies made from a zinc core thinly plated with copper. By removing small sections of the penny’s copper rim, the exposed zinc core can be dissolved with hydrochloric acid, leaving the copper shell intact (4). We have now used a simple procedure to measure the zinc content of pennies by reacting the coins with acid and measuring the volume of hydrogen generated. Using basic gas law and stoichiometry calculations, students can easily calculate the percent of zinc in the penny. Recovering the “souvenir” hollow penny at the conclusion of the experiment is rewarding for most high school- and college-level students. Materials Graduated cylinder, 1 L Beaker, 2 L Parafilm Rubber tubing, 3 foot length Glass tubing, 5 in. piece bent into a V shape by heating Glass tubing, 3 in. piece inserted into a one-holed rubber stopper Erlenmeyer flask, 125 mL 6 M Hydrochloric acid, 50 mL One shiny, new penny Analytical balance Stand with clamp
Experimental Procedure Fill a 1 L graduated cylinder to the top with water and cover tightly with a piece of Parafilm so that no air bubbles are trapped in the cylinder when inverted. Lower the inverted cylinder into a 2 L beaker containing about 1 L of water and remove the Parafilm. Attach a 3 foot length of rubber tubing to one end of a V shaped piece of glass tubing (made from a 5 in. piece of straight glass tubing that has been bent into a V by heating). Place the V shaped glass tube under the water in the beaker, so that the open end of the tube is inserted into the graduated cylinder, which is then clamped to a stand. The other end of the rubber tubing is attached to a 3 in. piece of glass tubing inserted into a one-holed rubber stopper to fit a 125 mL Erlenmeyer flask. Clean a new penny (less than 1 year old) with soap, water, and a test-tube brush. Holding the penny with tongs, rinse off the soap by holding the penny under a running cold water tap, and dry the penny with a paper towel. Weigh the penny on an analytical balance.
Using a triangular file, lightly file a 2–3 mm section of the penny’s rim to reveal the zinc core. Repeat the filing at three other places on the penny’s rim. Reweigh the penny to determine the quantity of copper removed due to filing. (It will be < 0.05% of the penny’s mass). Place the penny in the Erlenmeyer flask, add ~50 mL of 6 M HCl, and seal the flask with the stopper to collect the hydrogen. When the gas generation has stopped, remove the V shaped glass tubing from the cylinder and hold the cylinder so that the levels of water in the cylinder and beaker are the same. Record the volume of hydrogen generated. After removing the penny shell from the acid, wash it carefully with water, and oven dry (~30 min at 50 °C) to remove all moisture from inside the copper shell. The shell is then weighed at room temperature. Hazards Wear safety goggles and rubber gloves when handling hydrochloric acid. The used acid should be flushed down the drain with plenty of water. Hydrochloric acid is very hazardous in case of skin contact (corrosive, irritant, permeator), of eye contact (irritant, corrosive), and of ingestion. Hydrogen is flammable and should not be used in areas near open flames. Results and Discussion Since the reaction between the acid and penny core is slow, the reaction can be left overnight or to the next lab period to go to completion. Approximately 900 mL of hydrogen is produced, the quantity varying slightly depending on the penny’s mass (~2.5 g). The reaction can be speeded up considerably by filing away more of the penny’s copper rim, but this tends to make the final penny shell collapse, and so is less appealing to students at the end of the activity. Using the volume of hydrogen gas produced, the balanced chemical equation between zinc and hydrochloric acid, the general gas equation pV = nRT, and the mass of the penny, the percent by mass of zinc in the penny can be calculated to within about 1% of the true value. For example, in one experiment we found a penny of mass 2.4867 g produced 915 mL of hydrogen. Using p = 0.980 atm, R = 0.08206 (L atm)∙(mol K), and T = 298 K, yielded 0.0367 mol of hydrogen and 0.0367 mol or 2.40 g zinc. This gives a zinc content of 96.5%. According to the U.S. mint web site, the zinc content of copper clad pennies is 97.5% (4). The zinc content of the penny can be also determined gravimetrically from the mass of the dry copper shell. In the above example, using a mass of the copper shell as 0.0658 g (including the copper removed by filing) and a mass of the penny before filing of 2.4867 g, this gave a copper content of 2.65%. By subtraction, this yields a zinc content of 97.3%, which is slightly closer to the quoted (true) mint analysis. Students can compare the accuracy of the two methods.
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 85 No. 6 June 2008 • Journal of Chemical Education
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In the Laboratory
For schools with limited equipment supplies, an inverted and clamped half-gallon plastic milk jug filled with water could be used in place of the graduated cylinder, and any large container could replace the 2 L beaker. The volume of water displaced should be marked on the side of the jug, which can be filled with a measured quantity of water to determine the volume of hydrogen generated.
3. White, J. M. J. Chem. Educ. 1983, 60, 142. 4. United States Mint. http://www.usmint.gov/about_the_mint/ fun_facts/index.cfm?flash=yes&action=fun_facts2 (accessed Feb 2008).
Literature Cited
Abstract and keywords
1. Dominic, S. J. Chem. Educ. 1995, 72, 389. 2. Szczepankiewicz, S. H.; Bieron, J. F.; Kozik, M. J. Chem. Educ. 1995, 72, 386.
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Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2008/Jun/abs817.html Full text (PDF) with links to cited URLs and JCE articles Supplement
Student handouts and instructor notes
Journal of Chemical Education • Vol. 85 No. 6 June 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
