The Gold Rush-Turning Pennies into Silver and Gold (Jennifer Kim)

Author

Jennifer Kim

Principle(s) Illustrated

1. Oxidation-Reduction

2. Alloys

3. Chemical Reaction

Standards

• HS-PS1-1. Use the periodic table as a model to predict the relative properties of elements based on the patterns of electrons in the outermost energy level of atoms.

• HS-PS1-2 Construct & revise an explanation for the outcome of a simple chemical reaction based on the outermost electrons states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties

• HS-PS1-5 Apply scientific principles and evidence about the effects of changing the temperature or concentration of the reacting particles on the rate at which a reaction occurs.

Questioning Script

Prior knowledge & experience:

• The signs of a chemical reaction

• The single replacement reaction in terms of plating out zinc on the surface of the penny's copper exterior

• Seeing brass instruments, knobs, decorative pieces

Root question:

Explain how the pennies are able to turn from a copper coat, to a "silver coat," and then to a "gold" coat.

Target response:

Brass is a copper-zinc alloy. An alloy is a mixture of two or more metals dissolved in each other when molten ( or a metal and non-metal fuse together). The percentages of copper and zinc in brass vary depending on the type of brass. These differences allow the penny in this activity to turn several different colors. When a copper penny is added to the zinc solution, the zinc ions migrate to the copper where they are reduced to metallic zinc and deposited. The silver coating on the penny is the gamma-form of the brass alloy with zinc content greater than 45 %. This gives the penny its silver coloring. When the zinc-coated penny is heated, the penny becomes gold in color. The gold color is due to the zinc migrating through the coper to convert tot he alpha-form of brass allow which has a zinc content of less than 35%. This form of the brass alloy is a golden color.

Common Misconceptions:

Elements cannot change from one element to another through a chemical reaction. A chemical reaction is just a rearrangement of atoms to form new molecules. The elements stay the same.

Materials (for each demonstration)

Copper pennies, very clean and shiny, 2 Beakers, 100-mL, 2

Sodium chloride, NaCl, 2.5-3g Graduated cylinder, 50-mL

Vinegar, 15 mL Hot plate

Zinc chloride solution, ZnCl₂, 1 M, 25 mL Tongs

Zinc, granular, Zn, 10 g Towel or paper towel

Balance Distilled water

Preparation

1. Weigh out and place 2.5-3 g of sodium chloride and 15 mL vinegar in a clean 100-mL beaker.

2. Clean two pennies by placing them in the sodium chloride/vinegar solution until they are shiny.

3. Remove the pennies using tongs and rinse thoroughly with water. Dry thoroughly with a towel.

Note: Do not handle the clean pennies with your hands. The oils from your skin may interfere

with the zinc-plating reaction.

Procedure

1. In a clean 100-mL beaker, mix together 1.0 g of granular zinc and 25 mL of 1 M zinc chloride solution.

Note: Chemical splash goggles must be worn.

2. Place the beaker with the ZnCl₂ and zinc on a hot plate set to a medium heating setting.

3. Carefully and gently heat the mixture until the solution boils.

4. Using tongs, immerse two pennies in the mixture until they appear "silver."

5. Use tongs to remove the pennies. Caution: The pennies will be very hot. Carefully dip the pennies into a

beaker of distilled water. Shine the pennies with a towel. Set one treated penny aside to be used for

later comparisons.

6. Using tongs, place the other treated penny on the hot plate until the penny turns to a golden color.

heat-resistant glove or tongs, flip the penny every 30 seconds to avoid burning.

7. Use tongs to remove the penny from the hot plate and immediately dip the penny into a fresh beaker

of distilled water. The penny will be extremely hot and should be handled with tongs until it has cooled

for several minutes. Students will enjoy showing their friends their "silver" and "gold" pennies.

Disposal

1. Granular zinc can be disposed in the trash can while the zinc chloride solution can be poured down the sink

followed with a lot of water.

Photographs and Movies