X840: Electrolysis of Water

Introduction

Electrical current is applied to water containing Na2SO4 as an electrolyte.  The water is converted into gaseous H2 and O2 at the electrodes.

ANODE HALF REACTION:                 2H2O —> 4H+ + O2 + 4e-

CATHODE HALF REACTION:             4H2O + 4e-—> 2H2 + 4OH-

OVERALL REACTION:                        2H2O —> 2H2 + O2

To Conduct Demonstration

  1. Attach one alligator clip from the battery pack to each electrode.
  2. Oxygen and H+ are formed at the anode (oxidation), and the indicator turns red.  Hydrogen and OH- are formed at the cathode (reduction), and the indicator turns blue. After a few minutes, a relative volume ratio of 2 H2:1 O2 should be visible.

Demo Time: ~2 – 5 minutes

NOTES:

  1.  It takes up to 5 minutes for the reaction to show well.
  2. Put a paper towel under to open end of the apparatus to catch the overflow.
  3. The solution can be used over again, but must be returned to the green color using 0.01M or 0.1 M NaOH or .01 M or 0.1M H2SO4.  It should only take a few drops. (If it takes a lot, remake the Na2SO4 solution).
  4.  It is not necessary to make up a universal indicator.  A commercially prepared preparation is fine.

Safety and Disposal

The electrolysis apparatus is somewhat fragile, and care should be taken in packing and transporting it.

REFERENCE:

C.L. Borgford and L.R. Summerlin, Chemical Activities, Teacher Edition, 1988.