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Author Topic: How does electrolysis actually work?  (Read 13888 times)

lyner

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How does electrolysis actually work?
« Reply #25 on: 30/07/2008 23:12:35 »
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Also, if enough negative ions pile up round the positive electrode it stops looking like a positive charge.
That doesn't make sense to me. The electrodes are connected to a low impedance source - a constant voltage supply. Or are you saying there is a screening effect around the electrode?  OH yes - I looked at the link.

But, for a current to flow, there must be a net flow of ions; if they are diffusing back, then how can there be a net flow?

The problem is that those links seem to be dealing with ions in solution and what happens to them. They don't seem to be dealing with the problems that FD introduced.

There has to be a net flow of cations and anions and I think they must be water bits - the logic seems to demand it.

So what produces all these extra available water ions?
 

Offline Farrah Day

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How does electrolysis actually work?
« Reply #26 on: 30/07/2008 23:26:48 »
I sense a bit of progress here... or at least another angle of approach.

Guys, I don't always seem to be able to make it crystal clear exactly what I'm getting at or what I'm struggling with, but from your last post Sophie, it would appear that you now see where I was coming from in relation to the finite number of electrolyte ions. It's nice to be on the same page as everyone else every now and then, :)!

Thanks for that link BC. This bit particularly interests me:

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If a water-soluble electrolyte is added, the conductivity of the water rises considerably. The electrolyte disassociates into cations and anions; the anions rush towards the anode and neutralize the buildup of positively charged H+ there; similarly, the cations rush towards the cathode and neutralize the buildup of negatively charged OH− there. This allows the continued flow of electricity.[1]

By now, you all probably wish I'd settle for some answer - any answer - and stop digging. Trouble is I often find an answer rather annoyingly poses yet another question!

From the quote above something is bothering me ... nothing new there then,  ;).

So let me try to get this straight. Assuming that what BC says is correct (and I'm inclined to think he is), and the water molecule itself reduces and oxidises at the electrodes, this would leave OH- ions near the cathode and H+ ions near the anode. Now, if the electrolyte cations rush toward the cathode and neutralise the H+ and electrolyte anions rush toward the anode to neutralise the OH-... Wait, how would a sodium ion neutralise a OH- ion? Are we talking about the formation of a new compound now, sodium hydroxide? And the sulphate ions... hydrogen sulphate? Why don't the OH- ions at the cathode simply now migrate towards the anode? Wouldn't this allow the continued flow of electricity?  Also I thought that the H+ could only exist by itself for a few femtosceonds and so would form H3O+ with the nearest water molecule?

I'm just a little worried, and I may be wrong but isn't Wikipedia a voluntary information site, so may not be 100% correct. This paragragh uses the term 'disassociates' where I always thought the term was 'dissociates'. Might be something or nothing!

Sophie, I bought an aquarium ph meter to do just as you suggest, but the probe won't fit between my closely spaced electrodes without shorting them. So, I tested a couple of ss plates 1cm apart and indeed the water does seem more acidic and alkaline closer to the electrodes, which would tend to support BC's oxidation and reduction at the electrodes.

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btw, you say the sodium - water reaction doesn't occur. That isn't surprising because a Na+ ion has less attraction for a free electron (screening of the inner shells in Schoolboy terms) than an H+ ion. In fact, doesn't that explain why H2 is produced, preferentially?

I think there is something important in this paragragh that my inferior chemistry knowledge is not allowing me to grasp. However, it might explain why H2 is produced preferentially, but does it explain how it encourages water to reduce in the first place?

It's way past my bedtime and the monitor is now becoming a blur, so, before my brain explodes, I'm going to print off some of the info from the links provided by BC and give it time to sink in.

Thanks again guys.

« Last Edit: 30/07/2008 23:29:55 by Farrah Day »
 

Offline Bored chemist

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How does electrolysis actually work?
« Reply #27 on: 31/07/2008 19:22:34 »
"and the water molecule itself reduces and oxidises at the electrodes, this would leave OH- ions near the cathode and H+ ions near the anode."
Hydroxide ionas are produced at the cathode and are actively repeled into the rest of the solution. Similarly H+ iona are generated near the + charged anode and are sent on their way.
Since water is a mixture of H2O, H+ and OH- in equilibrium these ionic species react when they reach one another.
 

Offline Farrah Day

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How does electrolysis actually work?
« Reply #28 on: 31/07/2008 21:16:35 »
Yes BC, could be right.

Effectively then, the majority of the water ions left at the wrong electrodes would associate back into water and so neutralise where they meet, as they head in opposing directions.

But then what is this electrolyte ion neutralising action mentioned all about?

Sophie, it's probably a good thing they don't go into greater detail on this subject at high school, as there would be little time remaining for anything else!

 

Offline Farrah Day

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How does electrolysis actually work?
« Reply #29 on: 02/08/2008 12:36:46 »
Although to-date, I'm still not happy that I have any better understanding of 'the action of the electrolyte in electrolysis', I wanted to throw something else into the mix whilst it's fresh in my mind.

From recent posts and the links provided, I think I may have learned something today.

I had always assumed that platinum was used as the electrodes in an electrolyser simply because is is relatively inert.  But so is ss in the same environment, as well as being far cheaper - so let's use ss.

Then I read about platinum catalysts and realise that there is another reason why platinum electrodes are used. Now if I understand this correctly, at the platinum cathode no over-potential is required to reduce the water into H+ and OH- (this s the catalyst part), whereas with all other materials, including ss, we have to provide an external voltage in order to initiate electrolysis.  Please put me straight if I'm misinterpreting here.

Though the same can not be said for the oxidation reaction at the anode, the overall efficiency of a platinum electrolyser would be far greater than the ss equivalent. Interesting.

Now I daresay that many of you will not be familiar with any of the WFC (water fuel cell) experiments and the so-called conditioning of stainless steel electrodes. By all accounts, this conditioning of the electrodes enhances gas output, but no one ever seemed to understand why. Many WFC experimenters use dc pulsing, and it was thought that the conditioning caused minerals from within the tap water to build up on the cathode.  It was then suggested that the cell more readily acted as a capacitor, with this layer acting as the dielectric.  It does seem that with conditioning, gas output does indeed improve from any given power.

I was always troubled by this, as surely to be a dielectric would mean that the mineral build-up would need to be a fairly good insulator. Therefore, at the cathode the hydrogen ion would not be able to pick up an electron and hence no gas should be evolved. This simply was not the case. Furthermore, dc works just fine, with no apparent loss of efficiency due to this mineral layer!?

The problem I find with this kind of thing is that there is so little reliable information available. This opens the door for the WFC crackpots and mindless fanatics to invent all sorts of crazy science fiction to explain things. Most of it really is complete and utter nonsense, without any scientific foundation. Furthermore I found that many of the fanatics themselves appeared to be bordering on lunacy. One guy even stated that, 'the electricity atom is called the lithium atom!!!' What was more worrying was that I seemed to be the only person to be totally dumfounded by that statement. Staying around those guys for too long and you begin to question your own sanity!

Anyway, the mineral build-up.

Well, I decided that the mineral build-up was akin to scale in pipes and kettle elements etc., found in hard water areas. I could get my electrodes to achieve this coating from our tap water after many hours.

Gambling that the greatest part of this mineral build-up would be from limestone, and hence calcium carbonate, I doped-up my tap water with it. Sure enough I had a much faster and greater mineral build-up on my electrodes (cathodes).  By taking the electrodes out of the water and drying, the mineral build-up, which is not that apparent when submerged, quickly shows itself as a white powdery coating on the cathodes.

As it dries it hardens, and then I repeat the process. If I initially run too much current through the electrolyser, then the mineral build-up is too fast and layers too thickly - it then easily flakes off.  This I assume is a similar action to what Sophie mentions above somewhere, concerning electroplating. So controlling this action to achieve very thin layers, drying out and repeating, is the way to proceed.

By this stage you probably are wondering what I'm getting at - nearly there.

Now, I know that this mineral layer does not insulate the cathode as might be expected, so could it be acting itself as some kind of catalyst, similar to platinum?

I'm not sure what is actually occuring here, but I too find that my cells are more efficient once this mineral build-up is present on the cathode.  I'm not sure whether or not there is a level of thickness over which there are no gains in efficiency or indeed if there is an ideal thickness, but in the near future I intend to do some controlled experiments.

Again I find this all very interesting. 
 

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How does electrolysis actually work?
« Reply #29 on: 02/08/2008 12:36:46 »

 

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