Electrolysis of Water - Teacher's Page

Electrolysis of Water and Other Electrolytes

Your students will electrolyze solutions of both 1 M NaCl and 1 M NaHCO₃. Use of NaCl and baking soda from the supermarket is prefectly acceptible.

In the case of both salts, it is the negative pole that is supplying electrons. There are two possible elements that could be reduced - H⁺ and/or Na⁺ to H₂ and Na°, respectively. If H₂ is produced it bubbles away, and if Na° is produced, it reacts with water to form H₂ and NaOH. So at the negative electrode, it is always H₂ that is seen bubbling up.

At the positive pole the solutions act differently. In the case of NaCl, the possible oxidations are O^-2 and/or Cl^- to O₂ and/or Cl₂. The O₂, with its low 8ppm solubility in water, essentially all bubbles away; but Cl₂ has a much higher solubility in water and would not fizz out to add volume to the bubble trapped in the inverted tube - at least not the long time it takes to saturate the water churning around the electrode.*

But in the case of the NaHCO₃, the case is more limited: the carbon in carbonate cannot be oxidized any more than it already is. As you remember, carbonate in water is merely a form of dissolved carbon dioxide, which is at the pinnacle of the oxidation chain of carbon (from most reduced methane at the bottom through alcohol to aldehyde and elemental carbon, to carboxylic acids and finally to CO₂).

BEFORE you introduce all this to the students, make sure that you yourself have run both solutions so that:

you are sure that the batteries are good
you are acquainted with how fast the devices work. (Higher concentrations of the salts make them go faster.)
you have answered all the questions on the students' page.

You can give the class a foretaste of good things to come by mentioning that there is some way of measuring the number of electrons that the battery supplied, and that will take you to an exercise on electroplating - since you can weigh the amount of copper or silver deposited and calculate just how many moles of electrons were moved. (Hint: think Faraday and Coulomb!) And, certainly, you could try to measure the volume of H₂ produced above to determine how many moles were made and from that determine the number of electrons that flowed. But volume measurements usually don't have the precision of weighing measurements.

CLEAN UP!

Proper clean up is required because salts solutions tend to be corrosive to metals over long periods of time such as between when you have used this in class, and then will use it again for the next semester or next year. Thus, for both short and long-term:

Remove and separate the pencil lead electrodes and the rubber florists caps
Rinse the electrodes and the florists caps with fresh water and set aside to dry.
Rinse the inside of the device with lots of flow down the barrel so that water flows out the two electrode portals.
Long-term storage: Stand the device upside-down in some container along with the caps and the electrodes. The insides will dry during storage.

*Use of thick NaCl brines is commercially the most important electrolytic process in the chemical industry because the evolved chlorine and hydrogen are high-energy molecules that can later be used to drive other chemical reactions. A classic example is Dow Chemical Company and its being in the State of Michigan in the USA. Much of Michigan rests upon ancient salt beds, which when mined provide the brine.