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Experiments with a Penny- Home Lab Activity #6
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Introduction
In this experiment, we will investigate the properties of copper and zinc. Both are transition metals that can exist in different oxidation states.
A U.S. penny will provide our source of metals. In 1982, the U.S. mint changed the composition of the penny from 95% Cu, 5% Zn to a zinc plated coin containing 2.5% Cu. These two metals have some interesting chemistry that will allow us to differentiate between the two.
In a mixture of vinegar and hydrogen peroxide, Cu metal forms a black oxide, CuO. The acetic acid in vinegar then reacts with CuO to form blue-green coordination compound in which the Cu(II) ion forms six-secondary bonds to water molecules (Cu(H2O)6)2+. When household ammonia is added to this solution, the NH3 molecules replace water molecules forming a deep blue coordination compound, (Cu(NH3)4(H2O)2)2+.
Zn forms a white oxide (ZnO) which is amphoteric. In a basic solution it forms a white precipitate of Zn(OH)2; however, as the solution is made more basic, the precipitate dissolves forming the colorless zincate ion, (Zn(OH)42-). Zn(II) also forms a complex with NH3, but it is colorless.
The featured molecule is tetraamminecopper(II)sulfate. It is a deep blue-violet crystalline salt of the copper-ammonia complex.
Experimental Details: Experiments with a Penny
You will need the following household materials: 3% hydrogen peroxide, vinegar, post 1982 U.S. pennies, 3 small juice glasses (or disposable medicine cups), pure Cu (e.g. a pre-1982 U.S. penny or Cu wire), steel wool, U.S. nickels, household ammonia, sodium hydroxide, NaOH (optional; NaOH can be found in some solid drain cleaning products. NaOH is very caustic; handle with care.)
Thoroughly clean all metal surfaces with steel wool.
Add a penny to each cup. Then add 1 teaspoon (tsp) of vinegar to one cup, 1 tsp of hydrogen peroxide to the second cup, and 1 tsp of vinegar + 1 tsp of hydrogen peroxide to the third cup.
After 15 minutes, note any changes in the cups. Record your observations below.
Remove the solution from the third cup and divide into two portions. To one portion add household ammonia until a color change is noted. Add NaOH to the other portion until a precipitate forms. Keep adding NaOH until the precipitate dissolves.
Repeat the experiments with pure copper and a U.S. nickel coin. Record your observations.
DATA TABLE| Metal | Vinegar | H2O2 | H2O2 + vinegar |
|---|---|---|---|
| Cu Penny | |||
| Pure Cu | |||
| Nickel Coin |
Which of the following statements can be concluded from your experiments on the nickel coin?
| The nickel coin contains both Cu and Zn. | There is not enough information to show that the nickel coin contains the metal Ni. | The nickel coin contains the metal Cu. |
CuO dissolves in acetic acid, because of its basic properties – this is a characteristic of metal oxides. Non-metal oxides, e.g. CO2, exhibit acidic properties. Our featured video, Basic Oxide, demonstrates this difference
