LAB: Chemical Plating of a Penny

CHEMISTRY LAB
CHEMICAL PLATING OF A PENNY

INTRODUCTION:

In this experiment, we will observe the plating of one metal with another by chemical means. We will plate zinc on the copper of a penny, which is just the opposite of what we normally would expect to happen.

We know from our study of ions that the Zn⁺⁺ ion combines with two OH^- ions to form zinc hydroxide Zn(OH)₂. However, in the presence of sodium hydroxide NaOH, zinc will combine to form the complex ion known as a zincate ion.

[Zn(OH)₄]^{- -}

[Zn(OH)₃ * H₂O]^-

it is this zincate ion which allows zinc to plate on copper.

The old 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. WEAR SAFETY GOGGLES throughout this lab. If any of the solution gets on your skin, wash immediately and thoroughly with water.

MATERIALS NEEDED:

SAFETY GOGGLES
Ring stand
Two ring clamps
Wire gauze
Bunsen burner
25-mL graduated cylinder
Two 100-mL beakers
250-mL beaker
Long tongs
Weighing dish
Mossy zinc
*3 Molar NaOH solution
Two pennies

LAB PREP:

* 15 mL of 3M sodium hydroxide is needed. To prepare:

15 mL:

1.80 g

200 mL:

24.00 g

PROCEDURE:

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 (see the drawing above).

2. Obtain two pennies.

3. Weigh out a 0.5g sample of zinc in the weighing dish. Pour the zinc into the beaker.

4. Obtain a 15mL 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

When you heated the coins, the zinc combined with the copper forming a brass coating. Because the brass coating is high in zinc, your coins look "gold".

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