As discussed in the enzyme experiment, chemical reactions occur when separate atoms or molecules interact and combine their molecular building blocks to create new and different compounds. The addition of enzymes to a reaction will speed the reaction up, but even without the enzyme, the reaction will still occur. It will just take a little more time.
Displacement or Replacement Reactions are those that when molecules from one substance or compound change place with molecules from another substance or compound, forming a third, different substance or compound. Oxidation is a good example of a replacement reaction. When iron interacts with water and oxygen, the oxygen from the air and the water combines with the iron to form iron oxide (rust). Other molecules present in the air and the metal will combine with the hydrogen to produce an acid. This acid seeps into the microscopic spaces in the iron and opens them up, like tunnels, to allow more oxygen to penetrate into the metal, allowing the metal to oxidize even further. Left alone for a long period of time, the iron can be completely oxidized, until there is nothing but rust left.
In this experiment, we will create a displacement reaction on the surface of a penny.
The Experiment
Supplies: Paper towel, four or five pennies, vinegar, and a non-metal saucer or bowl. A plate with a raised edge or a shallow baking dish works well. Dark or “dirty-looking” pennies may work better than bright shiny ones. Try to use an assortment of pennies, if possible.
What to do: Fold the paper towel in half and fold again into a square. Place the paper towel in the dish. Pour enough vinegar onto the paper towel to thoroughly wet it, but without creating a pool of vinegar. Place the pennies on the paper towel and fold the towel loosely over onto the pennies. Wait 24 hours. Observe what happens. What happened to the pennies? What happened to the paper towel?
What is happening: Modern pennies have a zinc core and a copper alloy exterior, while older pennies are entirely copper alloy. Alloy is just a fancy word meaning a mixture of metals, and “copper alloy” means that the main metal in the mixture is copper. The dark, dirty-looking material on the dingy pennies is actually copper oxide (copper’s version of “rust”). The vinegar penetrated the copper oxide and disolved it slightly (decomposition reaction), freeing the copper to interact with the oxygen in the air. The copper, vinegar, and air combined to create copper acetate, which is green. The copper acetate is a substance completely separate from the penny, the paper towel, and the vinegar. This explains why it was able to settle onto both the paper towel and the penny.
Left on its own, copper will eventually change in appearance, from its shiny orange-red color to the green that appeared on the pennies. When construction began on the Statue of Liberty, back in 1875, Lady Liberty was not the dull, pale green she is today. She was shiny and orange-red, like a new penny. Exposure to the air, combined with acid rain (rain containing pollutants that lower the pH of the water, causing it to be slightly acidic), has caused a patina (surface discoloration) of copper acetate to form on the statue’s surface. This infographic demonstrates the color change over time. Notice how, during the first several years, the color of the statue was closer to that of an old penny?
The Statue of Liberty is almost 150 years old. She has aged quite nicely!
Links
For a variation in this experiment, head to The Exploratorium. Their experiment, involving pennies, vinegar, and salt, actually creates free-floating copper in the vinegar!