A PENNY'S WORTH OF HYDROGEN

PURPOSE

In this experiment, students generate hydrogen using inexpensive materials.

DESCRIPTION

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 the mass 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:

50-mL breaker

triangular file

graduated cylinder

pliers

triple-beam balance

HAZARDS

Concentrated solutions of HCl will burn skin and damage clothing. Fumes from acid can be caustic and/or irritating. Goggles must be worn throughout this experiment.

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, place the penny in a 50-mL beaker and cover it with 14 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 pliers, carefully remove the hollow penny from the beaker without crushing it. Fill the 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. Calculate the following:
  1. Mass penny before the reaction.
  1. Mass dry penny after the reaction
  1. Calculate the percent of zinc in the penny.
  1. Calculate the mass of hydrogen 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)

Explain the above equation in detail. Describe what chemicals are involved, how they moved from the beginning to the end, and what was created.

  1. 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.

Explain the above equation in detail. Describe what chemicals are involved, how they moved from the beginning to the end, and what was created.