Electrolysis of WaterAn Example of Chemical Decomposition

by

Joseph A. Castellano, Ph.D.RESEED Silicon Valley

A Decomposition Reaction

A simple experiment is used to show how low voltage electricity decomposes the water molecule into hydrogen and oxygen atoms.

Create the ElectrodesCut the eraser off two number 2 pencils and cut the pencils in half to give two pieces about 10 cm long. Sharpen both ends of each pencil.

The graphite in each pencil will become the “electrodes.”

Using two small binder clips (2 cm wide) and one large paper clip (5 cm long), connect the pencils together:

Form the Electrode Assembly

Prepare the Electrolyte

Mass 20 grams of pure sodium chloride * or sodium sulfate and transfer it to a 500 ml Erlenmeyer flask.

Add 200 ml of water to the flask and stir to dissolve all of the crystalline solid to produce a clear 10% solution. The salt solution acts as an electrolyte.

Pour the 10% solution into a 250 ml beaker.

* Make sure the salt does not contain calcium silicate,which tends to make the solution cloudy.

Complete the Experimental Set-up Place the “electrode” assembly on top of the beaker containing the electrolyte solution; make sure the tips of the pencils are well immersed in the liquid. Attach an alligator clip with a red wire to the tip of the pencil on the right and connect the red wire to the positive (+) terminal side of a 9-volt battery. The right side pencil with then be the anode.

Attach an alligator clip with a black wire to the tip of the pencil on the left and connect the black wire to the negative (-) terminal side of a 9-volt battery. The left side pencil with then be the cathode.

The Water Electrolysis Set-Up

Cathode(-)

Anode(+)

Electrolyte(10% Na2SO4 or NaCl solution)

9 voltBattery

The Water Electrolysis Experiment

Notice more gas bubbles form at the cathode (left) than at the anode (right).

Cathode & Anode Reactions The reaction at the cathode is REDUCTION – a gain of electrons:

2 H2O + 2 e- 2 H2 + 2 OH-

The reaction at the anode is OXIDATION – a loss of electrons:

2 H2O - 2 e- O2 + 4 H+

Combining the two reactions, we obtain: 4 H2O 2 H2 + O2 + 2 H2O or 2 H2O 2 H2 + O2

H HO

HHO

H

H

O

O

H

H

2 H2O 2 H2O2

Two molecules of water decompose into one molecule of diatomic oxygen and two molecules of diatomic hydrogen.

Electrolysis of Water Summary

This presentation was produced as a public service to help middle school and high school science teachers develop experiments and demonstrations that can be used in their classrooms.

More science experiments and demonstrations are available on the RESEED Silicon Valley web site: www.reseed-sv.org.

RESEED (Retirees Enhancing Science Education through Experiments and Demonstrations) is a program aimed at stimulating greater interest in science by middle school students.

Music: “K Twist,” Kenny Burrell, Blue Note, 1999