[Farrah Day]

Hi, first let me apologise if this post initially seems a little dumb, but please bear with me.

I'm really struggling to understand the reactions taking place at the electrodes during electrolysis. Don't get me wrong here, I've read and researched, but I am unable to find a satisfactory answer. I do get answers, yes, but none seem to precisely answer the nitty-gritty of my posed questions. I'm not well educated in chemistry, so my lack of understanding may well come down to my own ignorance. I'm rather hoping someone here can put me out of my misery.

Electrolysis of pure water seems straight forward enough. If we have 2 platinum electrodes immersed in pure water, and drop 2 volts across the electrodes we get the H+ ion reacting at the cathode and -OH ion reacting at the anode, with hydrogen and oxygen evolved respectively. With pure water being a very poor electrolyte, relying solely on ions produced by self-ionisation, the amount of gases evolved is very low. This is all straight forward and makes complete sense to me.

So now a compound is added to the pure water that makes for a good electrolyte, in this case Na2SO4, which provides the ions 2Na+ and SO4-2.

Now, here's the thing, many people just tell me that because we now have an abundance of charged species in the solution, much more current can flow. They often say this as if this statement explains everything. However, when I push for greater detail and ask tougher questions, if they answer at all, they usually can provide me with no satisfactory answer.

I am told that, in the above case, the electrolyte acts only as a catalyst and takes no part in the reactions at the electrodes. So what exactly do these ionic species do? They may be charge carriers, but what good are they if they do not react at the electrodes?

This is where I'm struggling, this is what I just can't get my head around.

Could it be that the Na+ ions are in fact reacting at the cathode, taking an electron and becoming sodium, which then instantly reacts with the water molecule to produce hydrogen and sodium hydroxide (H2 + 2Na+ +2OH-) ? But then what precisely is occurring with the SO4-2 at the anode?

If on the other hand, we still simply have H+ and OH- ions reacting at the electrodes, what is the point of the electrolyte? And furthermore, if they themselves are not taking part in reactions at the electrodes, why don't the sodium and sulphate ions act to polarise the electrodes and so serve to inhibit further electrolysis?

I mean, if the anode is surrounded by countless sulphate ions, effectively forming a negative barrier - ions that are not going to take part in any reaction - why doesn't everything simply grind to a halt.

I'm hoping that I'm missing something fundamental here because the more I consider this, the more baffled I am becoming.  Thanks in advance for any useful input.

Farrah

NB. If I've posted in the wrong place, please feel free to move this post to a more appropriate place.

[Farrah Day]

Whilst I initially thought that maybe my question was so stupidly simple that no one was interested in providing an answer, I'm now rather inclined to think the opposite may be the case.

Further research leads me to believe that there is much conjecture and much over-simplification based simply on the resulting products.

The electrolysis of an electrolytic solution is in fact so very little understood that no one as of yet can explain with any degree of confidence actually what occurs or how. It seems that the reactions taking place at atomic level may in fact be extremely complicated and so not yet categorically agreed upon in the scientific community. I guess this is why I have had so much difficulty in finding any reliable answers.

This quote from an 'A' level website would seem to bear this out:

If there are more than one kind of negative ion present, how do you decide which gets discharged? You are only likely to come across the problem here for aqueous solutions.

As a general rule, if there is a halide ion present, you will tend to get the halogen. Otherwise you get oxygen coming from the water.

The only halogen you have to worry about for this statement is chlorine. Concentrated solutions of a chloride produce chlorine, but more dilute ones give oxygen as well.

There are two ways of thinking about this. For neutral solutions such as sodium chloride solution, CIE prefer you to think of this in terms of a reaction involving the water itself:

2H2O - 4e-   ------------------ 4H+ + O2

But some teachers will explain that the oxygen comes from hydroxide ions from the water:

4OH- - 4e-   ------------------- 2H2O + O2

--------------------------------------------------------------------------------
Note:  The truth is probably more complicated than either of these, and varies with the pH of the solution. Use the version CIE prefers.

Note: 'There are two ways of thinking about this.'

And then I find this. A quote from the full link below:

Revising theories

Koper’s research group is now the first to unravel the mechanism behind oxygen formation during electrolysis with gold electrodes. Their results appeared in the latest edition of the British scientific journal Chemical Science. With the help of spectroscopic techniques, Koper’s research group analysed oxygen formation in the experiment carried out by his Haarlem predecessors in 1789. Electrolysis turns out to work in a fundamentally different way than was previously assumed. Koper: ‘Existing ideas and theories will have to be revised.’

http://www.news.leiden.edu/news-2013/marckoper.html

Very interesting read.

[Borek]

Actually there are thick books on electrochemistry and electrode reactions - while there are still some fine details that require more research, bulk of the problem is well known for at least half a century. Trick is, it is complicated, so often even explanations on the undergraduate level tend to be simplified.

During water electrolysis electrolyte like Na₂SO₄ is inert. Ions travel through the solution speeding up charge transfer, but they don't react neither at the anode nor at the cathode, it is water that reacts. I don't know exact mechanism of the water reduction nor oxidation. What I do know is that the neutralization and dissociation of water is so fast, it doesn't matter whether water has to dissociate first and it is H⁺ & OH^- that reacts, or if these are water molecules that react directly. Many electrochemical processes consist of separate chemical (C) and electrochemical (E) steps, so we have processes like CE, EC, ECE and so on - they can be distinguished only if the difference between their speeds is large enough, otherwise we see all of them at the same time (unless you design some very fast experiment, like ultra high scan rate cyclic voltammetry). I do remember reading papers on the the water electrolysis mechanism, but it was so many years ago I don't remember any details.

Unfortunately the only books I can suggest are those publish in my neck of the woods, so they don't have to be easily accessible for you. Try to find Electrochemistry by Dvorak, Koryta and Bohackowa, or Fundamentals of Electrochemical Analysis by Galus. Perhaps you may have more luck finding Modern Electrochemistry by Bockris and Reddy, or Electrochemical Methods by Bard and Faulkner. Note these are rather dated - not that the basics covered there changed.

[Farrah Day]

Thanks for the reply Borek.

Since my initial post I now realise that what is happening - how, where and when - is a lot more complex than many people realise and certainly the reactions are more complicated than we are led to believe at most levels of science.

If the sodium and sulphate ions do not themselves react, then it would be nice to know the mechanism by which they in some way either induce the water molecule to ionise, hence more H+ (H3O+) and OH- within the solution or cause the water molecules to react directly at the electrodes due to their presence. No doubt you are right in that there will be a mixture of chemical and electrochemical steps to the process.

Granted, most explanations serve their purpose and seem to work... up to a point - that point usually being when I ask one of my 'annoying' questions  . It's just that I have read so many conflicting explanations for what an electrolyte actually does and what reactions are taking place that I have little faith in any one explanation and I'm more inclined to doubt them all. 

Anyway, I'll try to follow up on the info you've provided, and come back if I find some illumination.

Otherwise I might just have to revise the whole electrolysis business myself! 

Edit: I thought the discovery by a certain John Kanzius in 2009, whereby combustible gases evolved and burned steadily when a test tube of saltwater was irradiated with RF might give some insight into certain reactions (especially given that no charge exchange medium was present). However, I'm still yet to see any scientific explanation or even suggestions as to what mechanism is in play there. Fascinating stuff though.