MadSci Network: Chemistry

Re: Electrolysis of Water - No Oxygen produced.

Date: Fri Jan 14 13:03:19 2000
Posted By: Dr. Michael Gallagher, Senior Research Chemist
Area of science: Chemistry
ID: 946856031.Ch


When I read your question, I also remembered seeing a similar question in the archives. After a bit of poking around, I found two questions that are remarkably similar to your question.

Answer 1 - ID: 889125512.Ch
Answer 2 - ID: 893300205.Ch

I believe that the answer to your particular question is a mixture of both of these archived answers.

So, here we go:
If you were able to electrolyze pure water you would have only two reactions occurring:

At the Anode (Oxidation) the reaction would be:

	RXN 1		2H2O  ==>  O2 + 4H+ ions + 4 electrons
While at the Cathode (Reduction) the reaction would be
	RXN 2		2H2O + 2 electrons  ==>  H2 + 2OH- ions
After balancing, the net reaction would become:
	RXN 3		2 H2O + energy ==> 2 H2 + O2
However, pure water is a poor electrical conductor. To facilitate the reactions, we add some sort of an electrolyte. In your case, table salt NaCl.

Now we have the possibility of other reactions occurring.

	RXN 4, Anode 		2 Cl- ions  ==> Cl2 gas + 2 electrons
	RXN 5, Cathode	Na+ ion + 1 electron ==> Na metal
Of these, RXN 5 will not occur since the hydrogen in the water is easier to reduce via RXN 2 than is the sodium ion to sodium metal shown in RXN 5.

Oxide and Chloride ions are fairly close to each other in ease of oxidation. RXNs 1 and 4. (-1.23 volt O, -1.36 volt Cl) If you are using a 6 volt battery, you have plenty of voltage to drive either or both of the reactions.

Depending on the NaCl concentration, you can generate predominately Oxygen or Chlorine gas at your anode. In general, the greater the Chloride concentration, the greater the generation of Chlorine gas. One problem is that Chlorine gas immediately reacts with water to form Hydrochloric acid and mixed Chlorine oxides, predominately Hypochlorous acid, all of which are water soluble, THUS NOT GENERATING ANY BUBBLES AT THE ANODE !!!

The corrosion of the Copper wire is another sign that some other reaction is occurring at the anode, rather than producing Oxygen gas. In your case, the Chlorine, Hydrochloric acid, and Hypochlorous acid are all attacking the Copper, oxidizing it to Copper +1 or +2. If the corrosion was black, that is probably Copper Oxide. If the anode or the NaCl solution turns kind of a blue green, then Copper Chloride is being formed.

Try varying your concentration of NaCl to the very minimum amount that will conduct electricity. You should get some Oxygen production. To demonstrate the 2:1 ratio of Hydrogen to Oxygen, you would want to change your electrolyte to something that will not oxidize at the anode. Sulfuric acid will work very well. Be sure to add the acid to water so that enough mass is there to prevent superheating and spattering. Sodium sulfate will also work well.

Have fun experimenting!!

Dr. Michael M. Gallagher
Senior Research Chemist
J.R. Simplot Co.

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