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Pretty Pennies: Lesson in Copper Oxidation and Balancing Chemical Equations
Materials List:
Approximately 20-30 tarnished pennies (dark brown or green)
water
1 small non-metallic bowl
1 plastic spoon or fork
1 teaspoon salt
1/4 cup white vinegar
1 Paper clip
Paper towels
Clock
Plastic gloves
pH paper (optional)
Procedure:
Step 1: Examine the pennies. (Do not use any pennies from a coin collection because this experiment actually removes some of the copper from the penny.) The pennies are probably very dull, a dark brown, perhaps with some green substance on them. Pennies used to be mostly copper, but since the early 1980’s, pennies are mainly zinc, with a coating of copper so that they retained the same appearance. Copper pennies get dull over time because the copper (Cu) in the pennies react with the oxygen (O2) in the air to form copper oxides (cupric oxide, CuO) and carbonates (discussed in the last step.)
Step 2: Balance the chemical reaction. The reaction of the copper in the pennies and oxygen to form copper oxide is shown below as Equation 1 in chemical equation format, with the empty brackets [ ] present to show a number needs to be placed within for the equation to be balanced. The left hand side of the equation should hold the chemicals present before the chemical reaction, while the right hand side of the equation should show the chemicals present after the chemical reaction. So we are showing that copper (Cu) in the presence of oxygen (O2) creates copper oxide (CuO). What we don’t know at this time is the proportions or amounts of each ingredient.
[ ] Cu + [ ] O2 = [ ] CuO Equation 1
Chemically balance the equation. You balance the equation by ensuring the number of different atoms are the same on both sides of the equal sign. It can be seen from equation 1, that there is one copper atom on the left side and one copper atom on the right side as part of the copper oxide. However, there are two oxygen atoms on the left side (since it takes two oxygen atoms to make one molecule of diatomic oxygen) while there is only one oxygen atom on the right side as part of copper oxide. How can we balance the number of oxygen atoms on both sides? How about assuming we form 2 molecules of copper oxide?
[ ] Cu + [ ] O2 = [2] CuO Equation 2
Is this equation balanced now? Clearly, the number of oxygen atoms is balanced (two on each side), but the number of copper atoms is no longer balanced. To balance the equation, let’s try starting with 2 copper atoms on the left side.
[2] Cu + [ ] O2 = [2] CuO Equation 3
Is the equation balanced now? Yes, equation 3 is balanced. Therefore our balanced equation is
2 Cu + O2 = 2 CuO Equation 4. Balanced Equation for Copper Oxide
That’s all there is to balancing a chemical equation. You want to make sure you have equal number of element atoms on both sides of the equation.
Step 3: In the small non-metallic bowl, place the 1 teaspoon salt into the ¼ cup white vinegar, stir with the plastic spoon until dissolved. If you have pH paper, test the solution and determine the pH. You should find the solution is acidic (has a pH less than 7.) The table salt is sodium chloride (NaCl) while the vinegar is dilute acetic acid (CH3COOH + H2O). The salt dissolves in the water in the vinegar and forms a weak solution of hydrochloric acid (H+ + Cl-). The superscript + or – signs indicate the charge on the particle or ion, thus the hydrogen ion (H+) has a positive charge of one. The chlorine ion (Cl-) has a negative charge of one. Take one of the dull pennies and place half of it into the solution for a few seconds. What happened? Place all the pennies into the bowl with the salt-vinegar solution. Record your observations of what happens. The pennies are now very shiny, the copper oxide on the pennies has been removed. The unbalanced chemical equation for this reaction is shown below in Equation 5. Balance this equation.
[ ] CH3COOH + [ ] H2O + [ ] NaCl + [ ] CuO
= [ ] CH3COO- + [ ] H+ + [ ] OH- + [ ] Na+ + [ ] Cl- + [ ] Cu+2
Equation 5
After balancing the equation, the equation should look like Equation 6:
3(CH3COOH) + H2O + NaCl + CuO = 3(CH3COO-) + 3H+ + 2OH- + Na+ + Cl- + Cu+2
Equation 6. Balanced Equation for Removal of Copper Oxide
Step 4: Take a paper towel and label in a corner “rinsed” and label a second paper towel “non-rinsed”. Remove the pennies, saving the liquid in the bowl. Place half of the pennies on the paper towel labeled “non-rinsed”. Rinse the other half of pennies in water and place on the “rinsed”-labeled paper towel to dry. Note, and record the time.
Step 5: Untwist the paper clip and place part of the paper clip in the remaining salt-vinegar solution, leaving a portion of the clip out of the solution. Check after about 10 minutes. Record your observations of what is happening. Take the paper clip out of the solution and look at it, and record your observations. Replace the paper clip into the solution as it was and wait until one hour has passed. Reexamine the paper clip and record your observations. You can also leave the paper clip in the solution overnight if you don’t see much change. Just as the acid solution removed copper ions (Cu+2) from the copper oxide surface of the tarnished penny, placing a paper clip in the solution will remove iron (Fe+2) ions from the paper clip. Since positive ions have left the paper clip, the paper clip becomes slightly negatively charged. It will attract any other positively charged ions nearby…such as the copper ions floating loose in the solution. Thus the paper clip becomes plated with copper.
Step 6: After about an hour after removing the pennies from the solution, look at your rinsed and non-rinsed pennies. Record your observations. Use gloves to handle the blue-green pennies. The acid residue reacts with the oxygen in the air and copper on the pennies to form blue-green copper carbonate (CuCO3). Copper carbonate can be an irritant so the gloves are worn for safety. The unbalanced chemical formation of copper carbonate is shown in Equation 7. Balance the equation.
[ ] CH3COO- + [ ] O2 + [ ] Cu = [ ] CuCO3 + [ ] H 2 Equation 7
After balancing this equation, it should look like Equation 8:
2 CH3COO- + 4 O2 + 4 Cu = 4 CuCO3 + 3 H 2
Equation 8. Balanced equation for Copper Carbonate
Copper carbonate is used often for pigment and occurs naturally as malachite and azurite.
Step 7: Optional Study. The shining of the pennies occurs due to an acidic solution. The salt speeds up the reaction dramatically. But, you can use any acidic solution: cola soft drinks, ketchup, lemonade, etc. Try different solutions to see how long it takes to shine a penny. Test each initial solution with pH paper to see if there is a correlation between pH and reaction time. Record all your data and have fun.
Step 8: For questions or comments on this Do-It-Yourself Experiment, please e-mail us at seekalaska@aol.com.
Sources:
1. “Copper Magic”, The Complete Idiot’s Guide to Science Fair Projects, by Nancy K. O’Leary and Susan Shelly, Alpha Books, Indianapolis, IN, 2003.
2. A Green Penny??, http://www.haverford.edu/educ/knight-booklet/greenpenny.htm, 3/13/2006
3. Chemistry Fun with Pennies, http://chemistry.about.com/cs/demonstrations/a/aa022204a.htm, 3/13/2006.
4. Clean Pennies, http://www.uen.org/Lessonplan/preview.cgi?LPid=11258, 3/13/2006.
5. Copper Caper, http://www.exploratorium.edu/science_explorer/copper_caper.html, 3/13/2006.
6. Tarnished Pennies, http://www.newton.dep.anl.gov/askasci/chem00/chem00456.htm, 3/13/2006.
7. Copper Acetate, http://yarchive.net/chem/verdigris.html, 3/13/2006.
8. Copper (II) Carbonate, http://en.wikipedia.org/wiki/Copper(II)_carbonate, 3/13/2006.
9. ChemShorts for Kids – 2006, http://membership.acs.org/C/Chicago/ChmShort/CS06.html, 3/13/2006.