ChemLab #34 - Chemical Plating of Copper with Zinc
Date Submitted: Score:   
Lab Group Members:
Introduction:   In this experiment, you will plate zinc onto copper metal by chemical means, which is just the opposite of what is done to make today's pennies. You know that the Zn+2 ion combines with two OH− ions to form zinc hydroxide Zn(OH)2. However, in the presence of sodium hydroxide (NaOH), zinc will form a complex ion known as the zincate ion, Zn(OH)4−2. It is this ion which allows zinc to plate on copper.

Before 1982, pennies were 95% copper and 5% zinc. In 1982, the penny was changed to 97.6% zinc, coated with a thin electroplating of copper.

      CAUTION: This lab uses a strong sodium hydroxide solution which WILL burn your skin.
 

Materials:   ring stand, two ring clamps, wire gauze, bunsen burner, 25 ml graduated cylinder, two 100 ml beakers, 250 ml beaker, tongs, weighing dish, mossy zinc, 3 Molar NaOH solution, two pennies
Pre Lab:
To prepare 15 ml of 3M NaOH: Dissolve 1.8 g sodium hydroxide in enough distilled water to make a total volume of 15 ml.

To prepare 200 ml of 3M NaOH: Dissolve 24.0 g sodium hydroxide in enough distilled water to make a total volume of 200 ml.

Experimental Design:
  1. Assemble the ring stand with ring clamp and wire gauze. Place the 250 ml beaker on the wire gauze and surround the beaker with a second ring clamp so that the beaker is prevented from tipping over.
  2. Obtain two pennies.
  3. Weigh out a 0.5 g sample of zinc in the weighing dish. Pour the zinc into the beaker.
  4. Obtain a 15 ml sample of the sodium hydroxide solution and carefully pour into the beaker.
  5. Heat the beaker gently with the Bunsen burner until the solution just begins to bubble. Then adjust the burner flame to keep the solution below boiling. CAUTION: Hot sodium hydroxide solution is very caustic.
  6. Using tongs, carefully add two pennies to the hot sodium hydroxide solution. Do not drop the coins into the solution so that you cause a splash.
  7. Observe the changes in the appearance of the coins. Let these react until the coins have a uniform "silver" color. Carefully turn them over once so both sides are coated completely.
  8. Fill the two 100-ml beakers with distilled water.
  9. With the tongs, remove the two pennies from the solution. Place them both in one of the beakers of distilled water. Remove heat from the beaker of sodium hydroxide.
  10. Using the tongs, remove the coins from the beaker of water. Rinse and dry with a paper towel.
  11. Holding the coin edge to edge and vertically with the tongs, gently heat the coin in the outer cone of the burner flame. Continue to heat for about 3 seconds after the appearance changes. Do not overheat it. The zinc wafer in the new pennies will melt easily, ruining your project.
  12. Immediately immerse the coin in the second beaker of distilled water.
  13. Repeat steps 11 and 12 with the other coin.
  14. Remove the coins from the water and dry them
 
Conclusion:     When you heated the coins the zinc combined with the copper, forming a bronze coating. Because the bronze is high in zinc, your coins look "gold".