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Electrolysis of NaCl solution

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Jango:
In electrolysis of concentrated NaCl solution (using graphite electrodes and, for example: 12V), H2 is formed at the cathode, while Cl2 is formed at the anode. I believe no oxygen is formed at the anode.

The water (H+ + OH-), has its H+ taken away, so the OH- ion is left in solution. The Na+ ion left in solution goes together with the OH- ion to form NaOH.

So the half equations are:

(at cathode) 2H+ + 2e- -----> H2(g)

(at anode) 2Cl- -----> Cl2(g) + 2e-

(in solution) Na+ + OH- -----> NaOH(aq)

The full equation goes like this:

2NaCl(aq) + 2H2O(l) -----> H2(g) + Cl2(g) + 2NaOH(aq)

Therefore, no oxygen is made in this way, as it is all used in the process of making the NaOH. Using a different voltage and electrodes, you may do.

If you used iron electrodes, hydrogen would form at the cathode as usual, but most of the chlorine at the anode would react with the iron to from a yellow/green solution of iron(III) chloride because chlorine is so reactive.

billnotgatez:
Jango -

There are indication that many think that oxygen is formed as well when in an aqueous solution of sodium chloride even though many of us learned otherwise

Grundalizer:

--- Quote ---There are indication that many think that oxygen is formed as well when in an aqueous solution of sodium chloride even though many of us learned otherwise
--- End quote ---

What does that mean?

Zerm:
I know for a fact that chlorine can be produced by electrolysis of a brine solution and I would think evolution of chlorine would be more thermodynamically favorable than oxygen but there are a lot of variables and I certainly can't say that I know its not produced because it likely is to an extent.  I therefore propose an experiment! 

Setup three inverted test tubes in three beakers of water.  In one tube, use the afore mentioned catalytic decomposition of HCl with MnO to trap pure Cl2.  In the second tube, use catalytic decomposition of H2O2 with MnO to trap pure O2.  In the third tube, trap the gasses given off at the anode of a NaCl solution under electrolysis.  Compare the gasses by color first.  If color isn't a clear indicator, carefully insert an iron nail or wire into each tube and observe the reactions.  Yellow ferrous chloride should form rapidly in a pure Cl2 environment. 

I just might do this over the weekend.  If I do, I'll let you know how it turns out.

billnotgatez:
Grundalizer -

http://electrochem.cwru.edu/encycl/art-b01-brine.htm
http://en.wikipedia.org/wiki/Chloralkali_process
http://scienceaid.co.uk/chemistry/applied/electrolysis.html

I have seen somewhere that oxygen is also produced in some quantity, but I can not find it now. So here are some discussions I found while Googling "electrolysis of brine". Maybe I just remembered incorrectly.

Thanks for getting me thinking.

In any case if any oxygen were created in combination with Chlorine, how would we easily determine the purity.


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