[wireframewolf]

Hey all. I'm trying to teach myself chemistry via Linus Pauling's general chemistry book, but sometimes I get confused!

I'm a little confused as to why a solution of NaCL in water only produces Hydrogen and oxygen around the electrodes, rather than also having Metallic sodium and chlorine gas in the mix, as would be produced by electrolysis through molten NaCL.

Take the cathode reaction for example. I know that 2(H₂0) receives two electrons from the cathode, splitting it into H₂ and 2(HO^-). I know that Na+ ions heading towards the anode balance out the charge of the OH- ions, so that the area around the cathode doesn't because saturated with a negative charge, thereby allowing the continued flow of current.

But what is stopping Na⁺ from taking electrons as the hydrogen is? And similarly, what is keeping Cl^- from losing them to the anode?

[Borek]

Imagine for a moment metallic sodium actually appeared on the electrode in the presence of water - what will happen?

But in reality it is like trying to sell ice creams for $10 when someone standing next to you offers them for $1. You will not sell anything, as everyone will buy from your neighbor. In this particular case there are several possible reactions that can take place at different potentials. The one that is the "cheapest" occurs first, in this case this means hydrogen/oxygen are being produced till you run out of water (this is simplified, but correct enough at the early stages).

[billnotgatez]

Am I missing something? is it not Hydrogen and Chlorine at the electrodes, since we are using brine?
Well, I suppose it would be Hydrogen and Oxygen after the all the Chlorine is liberated.

[Borek]

It depends on several factors - pH, concentration of chlorides and electrode material (think overpotential). Basically if you look at standard half electrode potentials oxygen should evolve first.

[billnotgatez]

http://www.energyquest.ca.gov/projects/split_h2o.html

I guess we are both correct

[Borek]

http://www.energyquest.ca.gov/projects/split_h2o.html

My bet is that they mistake smell of chlorine for chlorine - that is, traces of chlorine are present, but evolving gas is mostly oxygen. But I can be wrong. I remember checking numbers in the past and being surprised by what I have seen (I thought like you do that it is chlorine that should evolve, but potentials shown it should be other way around), but I am too lazy at the moment to check them again.

[billnotgatez]

i always learned from HS and beyond was that Chlorine went first because it was in the right most column and is not inert.
Now you are suggesting the order would be (depending what was in solution) Florine oxygen chlorine. I assume this would take the handbook of chemistry and physics to figure out. For instance where would boron in this list (assuming the temperature could be high enough for it to be a gas).

[Borek]

i always learned from HS and beyond was that Chlorine went first because it was in the right most column and is not inert.

You can't compare them directly - that could be done if both will undergo identical electrode reaction, that's not the case.

2Cl^- -> Cl₂ + 2e^- (E₀ = 1.359V)

could be directly compared with (non existing!)

2O^- -> O₂ + 2e^-

but the real reaction is something different, like

2H₂O ->  O₂ + 4H⁺ + 4e^- (E₀ = 1.23V)

As you see oxygen evolves at lower potential, so it should appear first, even if the difference is not large.

That's only first approximation, as potential of water oxidation depends on the pH (four H⁺ being produced), so the real potential in neutral solution will differ. However, standard potential is given for a solution containing 1 mol/L H⁺, as concentration of H⁺ (which is a product) is much lower in neutral solution, we should expect much lower potential at which oxygen will evolve (this is in a way similar to Le Chatelier's principle). If my calculations are correct (I am not claiming they are, I just hope ), formal potential for the oxygen evolution should be given by

E = 1.23 + RT/4F ln(p_O2[H⁺]⁴) = 1.22 - 0.059 pH

(assuming p_O2 of 0.21 atm, hence 1.22 instead of 1.23)

For neutral solution that gives 0.81V, way below potential at each chlorine evolves.

To be precise we should also do similar calculations for potential of Cl^-/Cl₂ reaction, but it won't change the situation. Potential doesn't differ much from 1.359V - it changes by 0.059V for each tenfold dilution of chlorides and we are using most likely solutions with concentrations between 0.01M and 3M (which is close to saturation).

As I wrote before, this is not entire truth yet, as depending on electrode material and overpotentials gas evolution may occur at even different potentials then those calculated.

Then, I can be completely wrong, that won't be for the first time  

[wireframewolf]

So basically the reason that h20 splits first is because... it takes a smaller potential to split than NaCL? Sorry if I ask simpleton questions, but it is the only way I'll learn.

[Borek]

That's not a completely bad way of stating the reason Whatever needs lower potential reacts first.

Note it is not NaCl, but both ions separately.

[wireframewolf]

Right, sorry. Because in an aqueous solution, Na and Cl balance out eachother's charge's in a general sense, but are separated because of the polar water molecules. However, I assume that the potential needed to split h20 somehow encompasses the whole molecule, since it is a covalent bond. Is that correct?

[Borek]

Could be - but to be honest, I have no idea what the actual mechanism of the electrode reaction is. Could be it is something like

4OH^- -> 2H₂O + O₂ + 4e^-

in which case water has to dissociate first.

[wireframewolf]

The actual reactions are as follows:

At the Cathode, 2(H₂O) + 2e^- -> H₂ + 2(OH^-)

At the Anode, 2(H₂O) -> O₂ + 4H⁺ + 4e^-

Pretty straight forward as far as I can tell.

[Borek]

These are just overall reactions, they doesn't tell anything about actual mechanism. Charge transfer is stepwise, it doesn't occur in a one step.

[Borek]

http://www.chem1.com/acad/webtext/elchem/ec8.html#CHLOR

Thermodynamically - oxygen, kinetically - chlorine.