A PENNY'S WORTH OF HYDROGEN

*PURPOSE

In this experiment, students generate hydrogen using inexpensive materials.

*DESCRIPTION

This experiment is suitable for a general or first-year college-prep course. By placing a prepared penny in hydrochloric acid, students are able to observe the evolution of hydrogen. If the mass of the penny is determined before and after reaction, students may determine the percent of zinc in the penny and/or the mass and volume of hydrogen produced from the penny.

*TIME REQUIRED

Fifteen to twenty minutes on two consecutive days.

*MATERIALS

Chemicals:

6 M HCl solution (500 mL concentrated HCl solution diluted to 1.00 L distilled or deionized water)
penny

Equipment:

250-mL breaker
triangular file
graduated cylinder
forceps
balance

*See Modifications/Substitutions

*HAZARDS

Concentrated solutions of HCl will burn skin and damage clothing. Fumes from acid can be caustic and/or irritating; do this experiment in a well-ventilated area (hood, if available). Goggles must be worn throughout this experiment.

*MODIFICATIONS/SUBSTITUTIONS

  1. HCl solution is available from a hardware store as muriatic acid, 28% HCl. This solution is approximately 8 M and may be substituted for 6 M HCl solution.
  2. A wide-mouth jar may be substituted for the beaker.
  3. If a triangular file is not available, the penny may be scratched by scraping it on a brick, cement block, or the sidewalk.

*PROCEDURE

  1. Using a triangular file, make several small notches on the edge of the penny. The zinc color should be evident.
  2. In a well-ventilated area or fume hood, place the penny in a 250-mL beaker and cover it with 25 mL of 6 M HCl solution. Use separate beakers if more than one penny is to be used.
  3. Observe the evolution of gas as the reaction proceeds.
  4. Allow the reaction to continue overnight.
  5. The next day, or when the evolution of gas has stopped, carefully pour acid into sink while cold water is running. Using forceps, carefully remove hollow penny from beaker without crushing it. Fill beaker with water and return penny to beaker to remove all traces of acid.
  6. Carefully remove the penny and allow it to air dry.
  7. If the experiment is to be done quantitatively, include the following steps:
    1. Mass penny before the reaction.
    2. Mass dry penny after the reaction
    3. Calculate the percent of zinc in the penny.
    4. Calculate the mass and volume of hydrogen at STP produced from the penny.

 

*DISPOSAL

Acid solution may be flushed down the drain with water. Pennies may be disposed of with solid waste.

*DISCUSSION

Two reactions occur:

  1. Any copper oxide on the surface of the penny reacts with HCl (aq) according to the following reaction:
    CuO (s) + 2 HCl (aq) ----> Cu2+(aq) + 2 Cl-(aq) + H2O (l)
  2. The zinc core of the penny reacts with HCl(aq) according to the following reaction:

    Zn (s) + 2 HCl (aq) ----> Zn2+(aq) + 2 Cl- (aq) + H2 (g)

    The copper coating over the zinc does not react with the acid. When all of the zinc has dissolved, the hollow penny may float if gas bubbles are trapped within the shell.

*REFERENCES

Herman, R.A., Chem 13 News, 146 (1984).

Submitted by Patricia Noel

Woodrow Wilson Leadership Program in Chemistry * lpt@www.woodrow.org
The Woodrow Wilson National Fellowship Foundation * webmaster@woodrow.org
CN 5281, Princeton NJ 08543-5281 * Tel:(609)452-7007 * Fax:(609)452-0066