What's in a Penny? - Experiment 25

The procedure will allow the students to use chemical reactions to observe the composition of an alloy.

Materials Substitutions
Pennies minted after 1982
12M (concentrated) hydrochloric acid muriatic acid
2 150 mL beakers 2 small jars
2 400 mL beakers 2 large jars
6 M NaOH solution
elemental zinc, granulated
metal shears
hot plate
evaporating dish
tongs

Procedure
Percentages of Copper and Zinc in a Penny

  1. Obtain a penny minted after 1982 and record the mint date. Use metal shears to cut the edges of the coin in several places.
  2. Weigh the penny and record the mass.
  3. Under the hood, place the penny into a 150-mL beaker and add approximately 20 mL of concentrated hydrochloric acid.
  4. When the coin stops producing gas bubbles, decant the acid into another 150-mL beaker. Record the reaction time.
  5. Wash the penny in distilled water. Then rinse with acetone. When the penny is dry, weigh and record the mass of the copper shell.
  6. Calculate the percentages of copper and zinc in the penny.

Preparation of Brass Alloy

  1. Place an evaporating dish under a hood, with approximately 5 g. of zinc and approximately 50 mL of 6M sodium hydroxide. While the volumes are not critical, assure that the zinc is covered with the NaOH solution. With the hot plate, heat the mixture to boiling. Carefully (with tongs), place the copper shell into the mixture.
  2. Leave the coin in the solution until it turns a silver color (about 45 seconds).
  3. Remove the coin (with tongs) and dip it into a beaker of water to remove any remaining NaOH solution. Dry the coin.
  4. With the tongs, place the coin on the surface of the hot plate (Be careful! It will be VERY HOT). Turn the coin to heat evenly. A gold color will appear. Do not overheat! The gold color will disappear if the coin is overheated.
  5. Dip the coin into a beaker of water and dry.

Data
Mint date of penny ______________________
Mass of penny ______________________
Mass of copper ______________________
Percentage of copper ______________________
Mass of zinc ______________________
Percentage of zinc ______________________

Questions

  1. List two observations that give evidence of a chemical reaction occurring between the zinc and the hydrochloric acid.
  2. What type of reaction is represented in question 1 (gas producing, precipitation, oxidation-reduction, etc.)? Why?
  3. Would the reaction of the penny with hydrochloric acid have occurred if the penny had not been cut?

Teacher’s Notes
Pennies that have been made after 1982 are a composite of zinc and copper. The copper is plated on top of the zinc. What if we could reverse this composite by placing the zinc on top of the copper? The zinc can be removed from the penny by cutting the coin and creating a reaction between the zinc and the concentrated hydrochloric acid:

Zn(s) + HCl(aq) -> Zn2+(aq) + Cl (aq) + H2(g)

Copper does not react with hydrochloric acid. After removing the zinc, reweigh the penny and obtain the mass of copper that is present. The remaining copper can be plated with zinc and the brass alloy produced. This process entails first creating a reaction between zinc and 6M sodium hydroxide:

Zn(s) + 2 OH(aq) -> ZnO22-(aq) + H2 (g)

The zinc will adhere to the copper. Upon heating, a brass alloy forms.

Answers to Questions

  1. Two observations: gas evolution; consumption of zinc inside penny
  2. Type of reaction represented in question 1: gas producing and oxidation-reduction.
  3. The reaction with HCl occurs only if the HCl contacts the zinc.

Safety Precautions

  1. Proper eye protection should be used at all times.
  2. Handle hydrochloric acid and concentrated sodium hydroxide with care! Gloves should be worn when working with these chemicals
  3. Hydrogen gas, produced in Part A, is very reactive! Do not have open flames or sparks near gas production or storage area. Pressure will build up quickly inside the flask or jar so the container should never be tightly sealed. Explosions could occur from increased pressure.

Disposal
All aqueous solutions may be flushed down the sink with copious amounts of water. Use care when disposing of concentrated acid since it may spatter when poured into the sink.

*This experiment is based upon similar ones from Hubert Alyea described in “Tested Demonstrations”.