Place your pre-1982 copper penny in an watch glass and heat with a mixture that first turns it silver, then suddenly turns it “gold” when the penny is then heated on a hot plate.
Caution: Wear safety goggles and gloves and do the reaction in the fume hood with the sash down. Note step 10: special disposal.
Place approximately 2 g of zinc in an watch glass.
Add enough NaOH solution to cover the zinc and fill the dish about one-third.
Place the dish on a hot plate and heat until the solution is near boiling.
Prepare a copper penny (pre-1982) by cleaning it thoroughly with a light abrasive (steel wool pads work well).
Using crucible tongs or tweezers, place the cleaned penny in the mixture in the dish.
Leave the penny in the dish for 3-4 min. You will be able to tell when the silver coating is complete.
Remove the penny, rinse it, and blot dry with paper towels. (Do not rub!) Remove particles of zinc.
Using crucible tongs or tweezers, place the coated penny on the hot plate. The gold color appears immediately.
When the gold color forms, remove the coin, rinse it, and dry it with paper towels.
Special disposal procedures: Do not discard the waste zinc in the trash container. When zinc dries, it forms a powder that may spontaneously ignite. Rinse the NaOH-zinc mixture several times with water. Then add the solid to a beaker that contains 200 mL of 1 M
. When all of the solid dissolves, flush the zinc sulfate solution down the drain.
The golden penny experiment.
Your TA will set this up for you by putting 8 g of 30 mesh zinc in the bottom of a 400-mL beaker. It is best to weigh-out the zinc in a watch glass and pour the zinc into a tilted 400-mL beaker so as to keep the zinc on one side of the beaker. Use a spatula and tilt and tap the beaker on the bench top in order to get all the granular zinc to cover about half the bottom of the beaker (one-half not covered: see Figure 5). Carefully pour 200 mL of 1 NaOH down the side of the beaker, being careful not to disturb the distribution of zinc. Use a stirring rod or spatula to clear any remaining granules so that half of the beaker bottom is completely free of zinc granules. Place the beaker on a hot plate in the fume hood and turn the hot plate to medium heat. The solution should be heated to about 80-90
; if it is heated to boiling the distribution of zinc granules will be disturbed. Continually monitor and check the temperature to keep it in this range.
20-25min.
While waiting for the solution to heat, buff six copper pennies with steel wool until they are shiny. Wash them with deionized water and dry. Solder 10-cm lengths of 20-gauge copper wire to two of the pennies, overlapping the wire and penny about 2 to 3 mm from the edge. Solder the free end of one of the copper wires to a 5 x 100 mm strip of zinc metal, as shown in Figure 5. Clean any rosin off the soldered joints with steel wool, and rinse with water.
Figure 2
When soldering, place the penny on a ceramic fiber square, with the end of a copper wire overlapping the penny 2-3 mm. Ask a partner to apply pressure to the wire to hold it in place while you are soldering the joint -use a rubber stopper as before to do this.
Four arrangements of copper pennies in the golden penny experiment.
