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TRACKING ELECTRONS

* PURPOSE

The objectives of this experiment are to construct a simple electrochemical cell and to trace the path of the electrons involved in the reaction.

* DESCRIPTION

This experiment is appropriate for a general or first-year college-prep course. Electrochemical processes have a practical importance in everyday life; students take for granted the chemical reactions occurring inside the batteries that power our flashlights, toys, radios, and calculators. In this experiment two iron nails are connected to the terminals of 9.0-volt battery. The nails are inserted in a petri dish of clear gelatin to which color indicators have been added. The flow of electrons in the reaction can be traced by the color changes that occur.

* TIME REQUIRED

Two lab periods (one for preparation of gel).

* MATERIALS

Chemicals
phenolphthalein solution (1 gram in 99 grams of ethanol)*
0.10 M K3Fe(CN)6 (3.30 grams K3Fe(CN)6 dissolved in distilled or deionized water and diluted to 100 ml)
plain gelatin
#4 ungalvanized iron nails
Equipment
sandpaper
9-volt battery
leads with alligator clips attached
600-mL beaker*
petri dish
10-mL graduated cylinder
*See Modifications/Substitutions

* HAZARDS

Care should be taken when handling the K3Fe(CN)6 solution; cyanides are poisonous. Goggles must be worn throughout this experiment.

* MODIFICATIONS/SUBSTITUTIONS

  1. Phenolphthalein solution may be prepared by dissolving Ex-Lax or Feenamint from a drugstore in ethanol and filtering. Ethanol is available from a drugstore as rubbing alcohol but label should be checked to be sure of the contents.
  2. Any suitable glass container may be used in place of the 600-mL beaker.
  3. A shallow plastic dish or jar lid may be used in place of the petri dish.

* PROCEDURE

  1. Make gelatin according to package directions but add 10 drops of phenolphthalein and 10.0 mL of 0.1 M K3Fe(CN)6.
  2. Attach leads from the battery to two clean iron nails using alligator clips for the connections. Place the nails as far apart as possible head down in a petri dish to which the gelatin solution has been added.
  3. Let the dish stand until gel is formed or overnight. Students should observe initial color around each nail.
  4. Draw a diagram showing the flow of electrons from one electrode through the battery to the other electrode.

* DISPOSAL

The gelatin and nails should be discarded with the solid waste.

* DISCUSSION

Students should be familiar with the color change of phenolphthalein, the formation of complexes, and oxidation-reduction reactions prior to doing this experiment.

At the anode where blue color is produced the following reactions are occurring:

Fe(s) ---> Fe2+(aq) + 2 e-
3 Fe2+(aq) + 2 Fe(CN)6(aq) [yellow] ---> Fe3[Fe(CN)6]2(s) [ "Turnbull's blue"]
At the cathode where a pink color is produced the following reaction is occurring:
2 H20(l) + 2 e- ---> 2 H2(g) + 2 OH-(aq) [pink with phenolphthalein]

* TIPS

  1. Ungalvanized common nails with the largest possible head should be used.
  2. For more advanced students a variety of electrodes could be used. One example is to use copper electrodes in (NH3)aq gel.
  3. Aluminum nails will not work.

* REFERENCES

This experiment is adapted from work done at 1985 I.C.E. Program at the University of California, Berkeley.

Merrill, P., Parry R.W., Tellefsen, R.L., and Bassow, H., Chemistry Experimental Foundations - Laboratory Manual, Prentice-Hall, Inc., Englewood Cliffs, NJ, 1982, p. 81.
- Describes an experiment which uses this chemical system in agar gel to demonstrate corrosion of iron.

Submitted by Lillie Tucker-Aiken, John Davik, John Garner, Martha Muir, and Harry Palin

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