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Offline Farrah Day

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How does electrolysis actually work?
« on: 26/07/2008 13:31:15 »
Ok, it might seem like a pretty dumb question, but I'm not looking for the high school answer, whereby nothing is really explained.

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Firstly let me say that I have a keen interest in on-demand oxyhydrogen production and have for some time been intrigued by the claims of achieving over-Faraday gas outputs.

Now, just before anyone jumps to any wrong conclusions about me, I should say that I'm as sceptical as most educated people are on this subject, and take nothing on face value. However, I'm open-minded enough to consider ideas and claims before dismissing without a thought. And, given the world fuel crisis it seems to me some things might just warrant a little further thought and investigation.

OK, let me pose the first question that I'm having trouble with, in the hope that someone can provide the answer I'm looking for.

Now, bear with me as this may initially seem like a ridiculous question to ask. Here goes:

Can anyone tell me exactly how adding a non-reactive electrolyte to water in order to increase ion content and hence increase current flow from any given applied voltage, actually increases production of hydrogen and oxygen?

This may seem like a simple question, but I fear it's actually a lot more complex than most people realise. In fact I'm really struggling to get a proper answer to this.

Any help much appreciated.


This was the moderators reply:

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Pure water is a non-conductor. Since the O amd H production is proportional to the current flow, anything that increases the current also increases the production.

The standard high school answer, and not exactly what I was expecting from advanced physics!

Can anyone elaborate a little on that answer?


 

Offline Bored chemist

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How does electrolysis actually work?
« Reply #1 on: 26/07/2008 18:06:32 »
Well, pure water isn't quite a non conductor, but it's a very poor one. The concentration of ions in pure water is about 2E-7 molar.
Since only the (chraged) ions can carry curent through the (neutral) water that means that, with pure water, you don't get a lot of cuurent and so you don't get a lot of gas.
Adding something like sulphuric acid to the water massivlely increases the number of ions in the solution because the acid dissociates into H+ and SO4- ions.
A solution of acid is, therefore, a much better conductor.

The over unity claims have never really stood up to any sort of investigation.
 

Offline Farrah Day

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How does electrolysis actually work?
« Reply #2 on: 26/07/2008 19:05:40 »
Thanks for the reply BC, but I think you're missing the point of my question.

I understand what you are saying... it's what everyone says, but I'm looking for a little more depth.

Exactly how and why does an ion current through water cause ionisation of the water?

Exactly what and where are the reactions taking place?

We all know that adding an electrolyte such as NaSO4 to water will add a lot of charge carriers in the form of -ve and +ve ions as the NaSO4 dissociates. This effectively allows an electric current to flow more easily through the liquid medium, OK, but how does this increase ionisation of water given that these ions actually play no part in the final reaction?
« Last Edit: 26/07/2008 19:07:15 by Farrah Day »
 

Offline Bored chemist

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How does electrolysis actually work?
« Reply #3 on: 26/07/2008 19:52:24 »
"Exactly how and why does an ion current through water cause ionisation of the water? "
It doesn't.
The ions form when the electrolyte dissolves in the water.
The reaction is H2SO4 --> 2H+  + SO4--

"but how does this increase ionisation of water given that these ions actually play no part in the final reaction?"
it doesn't
 

Offline Farrah Day

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How does electrolysis actually work?
« Reply #4 on: 26/07/2008 21:06:20 »
Come on BC, this is your chance to kill the boredom.

Your last post simply dances around the issue and explains nothing! It was a pointless post unless you can follow it up with a half useful answer.

I'd rather not play silly games - I take it you don't know!

 

Offline Bored chemist

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How does electrolysis actually work?
« Reply #5 on: 27/07/2008 13:44:34 »
You will be amazed how far that attitude will get you.
 

lyner

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How does electrolysis actually work?
« Reply #6 on: 27/07/2008 17:37:06 »
Come on BC, this is your chance to kill the boredom.

Your last post simply dances around the issue and explains nothing! It was a pointless post unless you can follow it up with a half useful answer.

I'd rather not play silly games - I take it you don't know!


Well, what is your alternative (and, no doubt, brilliant) alternative explanation?
Your superior knowledge is clearly wasted in such a slough of ignorance as this forum.

BC - shame on you for holding with the established scientific models!
 

Offline Farrah Day

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How does electrolysis actually work?
« Reply #7 on: 27/07/2008 20:44:16 »
I didn't come on here to get into a pie throwing contest, I'm simply asking a legitimate question to which no one as yet given me an answer.

Sophie, if I had an explanation, however brilliant or not I wouldn't be asking the question. I simply thought there may be someone that already knew the answer. Is that asking too much?

From past posts you seem pretty level-headed, so I can't quite understand your post given that BC has actually explained nothing to me in his!

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"Exactly how and why does an ion current through water cause ionisation of the water? "
It doesn't.
The ions form when the electrolyte dissolves in the water.
The reaction is H2SO4 --> 2H+  + SO4--

"but how does this increase ionisation of water given that these ions actually play no part in the final reaction?"
it doesn't

Is the above really supposed to be an explanation?! I would have settled for an intelligent reply - I thought he was just taking the P*** - hence my somewhat frustrated response.

You seem to be offended to my response to BC's post, but I can get a bucket full of those kind of responses from the numerous 'muppet laden' water fuel/watercar forums. This being a science forum I expected at least an intelligent answer.. that's all.

And you say this as though something has actually been explained... Where? What?
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BC - shame on you for holding with the established scientific models!

This is a shame I had high hopes you might be able to help me on this.

I don't think either of you are getting the gist of my question. If neither of you can help then OK, but let's not waste time posting for postings sake!

Try this:

1) If oxygen and hydrogen are evolved is it from the ionisation of H2O?
2) If yes, how exactly is this reaction increased by the passing of an electric current?
3) What reaction takes place at the electrodes when an electrolyte is used?

I think you will find the answer is not quite as straight-forward as high school science would have us believe!
 

lyner

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How does electrolysis actually work?
« Reply #8 on: 28/07/2008 00:11:35 »
F D
Fair enough and I now appreciate that yours was a more sensible post than I first took it for.  Sorry.
Reading it through again, though, I think you could have made it clearer that you were coming from a more informed position. A bit more technical detail could have helped to establish your cred; like where, precisely you found the explanation lacking.
BC, myself and others have more than our fair share of heckles from the totally uninformed when  we're trying to give the standard view of scientific topics.

I could suggest that the reason that extra production of H and O is due to action at the electrodes themselves where the local field could be high enough to produce the extra  ionisation.

btw, well de-ionised water is a pretty poor conductor; it can be piped quite safely round the cooling circuits of high power radio valve anodes with 20kV EHT supplies.  'Near as dammit' an insulator if it's pure enough.
 

Offline Farrah Day

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How does electrolysis actually work?
« Reply #9 on: 28/07/2008 23:27:52 »
Ok, let's put recent posts down to a misunderstanding, wipe the slate clean and start afresh.

I appreciate that there is often input on these forums from uniformed posters that totally lack any education or background in science. And, that such posters can indeed test the patience of saints with their nonsense, but I'm really not one of them.

My initial question may have seemed like something from a junior school lesson, which no doubt did nothing for my credibility, but that said, I did my best to emphasise that I was looking for more than the standard high school answer.

Bear in mind that the Advanced Physics forum (who also initially thought I was just asking a dumb question), were unable to come up with answers once they realised the question was not so dumb afterall.

The problem I think lies in the fact that electrolysis is often over simplified:

Pass an electric current through water via two (inert) electrodes and hydrogen is given off at the cathode, oxygen at the anode.

H2O + Electricity = hydrogen and oxygen
H2O + Electricity = H and O, or more accurately H2 + O2, or more accurately 2H20 = 2H2 + O2

But to get to the stages above we have to have some of the water ionising into OH- and H+. The H+ ion is attracted to the cathode where it picks up an electron to become a hydrogen atom. This hydrogen atom quickly bonds with another hydrogen atom to become more stable H2, which is evolved as gas.

At the anode, the OH- ion gives up it's electron which destabilises the OH bond. The O quickly bonds with another O to be evolved as O2. But, what happens to 'Billy no-mates' the neutral hydrogen atom left at the anode?  This seems to be often simply ignored.

Well, from what I understand, the hydrogen atom can't exist by itself for very long and so quickly bonds with the nearest H2O molecule to become H3O or hydronium (not often is hydronium ever mentioned in high school - if at all).

Now, given that pure water is a phenomenal insulator with a stand-off voltage in the order of many kilovolts per mm, how can it ionise in the first place?

OK, so pure water is extremely hard to come by as being a universal solvent many things, including gases in the atmosphere dissolve in it.  But, assuming that it was contaminant free, it also self-ionises. Not by much, as ions associate back into water just as quickly as water dissociates into ions, (figures given by by BC somewhere above), but nevertheless something causes this to happen without any externally added energy.

As far as I'm aware, this self-ionisation is put down to fluctuations in the electric fields surrounding molecules, due to molecule interactions - collisions.

Now, if it is fluctuations in electric fields that cause ionisation, what part does electric current play in all this? Are we simply dragging ions through the water creating water ionisation from ion current collisions?

If we add an electrolyte to water then apply a voltage to the electrodes submerged in it, we get +ve and -ve ions travelling through the liquid medium - a two-way ion current flows. They do all the work getting to the electrodes, then take no part in the reaction. Instead water ionises at the electrodes! Why? What is happening here? Is water ionising just at the electrodes or throughout the liquid medium? Does the very small amount of water self-ionisation remain constant throughout the liqiud medium or does the electrolyte act as a catalyst to encourage water to greatly ionise? And what becomes of the electrolyte ions themselves? If they don't lose their charges would they not simply act to clog up the electrodes so blocking them from interaction with water? This obviously does not happen - so what is happening?

Something is bugging me about all this, but I can't quite put my finger on it yet!

Hope I've explained myself better.
 

Offline chris

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How does electrolysis actually work?
« Reply #10 on: 29/07/2008 00:09:42 »
This is a good question and one wonders whether the movement of ions through the solution in sympathy with the current makes the dissociation of water molecules (into H3O+ and OH- ions) more likely, perhaps by provoking those random fluctations a bit more? A moving charge will try to pull electrons off things as it passes - alpha particles do this very nicely to things as they pass through. Possibly, when accelerated by the current the ions have sufficient energy to ionise a few more water molecules? These then migrate to the other electrode where they have the most favourable electrode potentials for evolution as gases.

My two penneth

Chris
 

lyner

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How does electrolysis actually work?
« Reply #11 on: 29/07/2008 10:24:50 »
FD

It is much too complicated for School and, as usual, they just teach the result rather than the 'how'.
I read through your above and can answer this bit, I think:
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as water dissociates into ions, (figures given by by BC somewhere above), but nevertheless something causes this to happen without any externally added energy.

The 'added' energy for individual ionisation can be there by virtue of the distribution of energies about the average energy (the temperature). It would be natural to expect self ionisation to increase as temperature to increase.

As for the effect of other ions on the level of water ionisation, could the electron from a self ionised water molecule be delayed in its recombination with that ion because of the presence of the other negative ions. A delay would result in an increased concentration. Could that be a big enough effect?
It could be a similar action to that of a catalyst, as you say. Perhaps the impurity +ions grab the free electrons and change the balance of things enough to reduce the probability of recombination.

It would be interesting to find out whether anyone has studied the potential gradient across the gap. I think there must be a lot of action at the electrodes. Otherwise, the separation would be critical - double the separation and the field would halve, if it were uniform. With that idea, you have to assume that there is much higher effective conductivity (either more ions or less electrical energy{voltage drop} involved in making them move per unit distance) in the gap and low conductivity {more voltage gradient} at the electrode surface. Most of the work would be done at the surface. Sounds reasonable, actually.
That last para was very lumpy but is says what I mean.
 

Offline Farrah Day

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How does electrolysis actually work?
« Reply #12 on: 29/07/2008 16:13:42 »
Thanks for the input guys.

I think it is quite amazing given the coming fuel crisis that there seems to be so little - if any - research into the ionisation of water. I have scoured the internet and found next to nothing on the subject.

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As for the effect of other ions on the level of water ionisation, could the electron from a self ionised water molecule be delayed in its recombination with that ion because of the presence of the other negative ions. A delay would result in an increased concentration. Could that be a big enough effect?
It could be a similar action to that of a catalyst, as you say. Perhaps the impurity +ions grab the free electrons and change the balance of things enough to reduce the probability of recombination.

Possibly.

Initially I had assumed that with the H+ and OH- ions being attracted to, and reacting with, the electrodes that the water, now effectively with an ion shortfall, simply redressed the balance by further ionising. However, if this was the case gas evolved would surely be very limited by this very low level of ionisation.

I am actually finding myself having to re-evaluate everything I thought I knew about water. I now have to conclude that this seemingly simple 3 atom molecule is far more mysterious and intriguing than I ever gave it credit for.

To emphasise this, we have a small molecule, that scientists predicted would, by right, be a gas at SATP - it's not! It is an extremely good coolant in that it can absorb a lot of energy before it changes state, and when it moves from liquid state to solid, it expands instead of, like most things, contracting. To boot, it's a universal solvent.

Of course we can also throw into the mix the often ignored fact that water molecules are not just H20, but cluster into larger molecules of H3O2, H5O2, H7O3, H9O4, H13O6. Which might go someway into explaining the fact that it is liquid at SATP, but what role do these molecules play in other reactions and interactions.

Agreed that the level of self-ionisation should increase as we add heat. Which would also suggest that electrolyte ion movement through the water would be responsible for causing water to ionise - analogous to driving a bus through a pedestrian only town centre on a busy market day.

But like Chris says, perhaps it is not down to collisions, but merely the effect of charged ions disrupting electric fields as they speed past, like the wake behind a powerboat! Or the moons effect on the seas and oceans. 

However, if collisions are not the cause, then why does the water heat up so quickly under heavy current? Surely this would be due to collisions... wouldn't it?

Whatever the mechanism, I daresay this could illustrate just why electrolysis is so inefficient. Only ionisation near to the electrodes has any chance of reacting to evolve gas, most ions will simply, very quickly associate back into water and the energy that caused ionisation will have been effectively wasted.

Placing the electodes as close together as possible dramatically increases gas production as there is less resistance between them. I find that even de-ionised water will electrolyse at relatively low voltages if you can get the electrodes close enough. The only problem being that, if the electrodes are too close, the gas evolved gets trapped and so acts to slow the process down.

Now, if we assume that electric field fluctuations due to collisions are what cause water to ionise, can this be done by another method?

Can we induce ionisation by using pulsing electric fields rather than using the energy sapping brute force of electrolyte ions?



« Last Edit: 29/07/2008 16:17:46 by Farrah Day »
 

Offline rosy

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How does electrolysis actually work?
« Reply #13 on: 29/07/2008 17:28:15 »
I'm really sorry if I'm repeating stuff that's already been discussed above, or that everyone who's got this far down the discussion knows, I've only skim-read some of the posts.. but basically in its equilibrium state water's known to be partially ionised [H+]x[OH-] = 10-14 (very roughly... actually of course it's H3O= etc.

However, this is really a very rapid thermodynamic equilibrium, so which specific molecules are in molecules or ions varies much faster than the electron transfer at the electrodes. So it isn't really that ions move through the solution as such, more that the ionisation tends to move about (does that make sense?) So yeah, as the ions are "used up" they're immediately replaced.

The degree of ionisation of the water won't be affected by addition of acid or, indeed, salts such as magnesium sulphate (which is what we use to do the same job when we're demonstrating electrolysis to kids, it being less corrosive etc).

Water's a liquid under standard conditions because it has lots of intermolecular hydrogen bonds... compared to (say) ammonia, which is similar but has one more "H" it can form twice as many hydrogen bonds, which are pretty strong forces in the scheme of intermolecular attractions (even if not on any other scale). It's not particularly mysterious... oxygen is held together only by van der Waals forces and so is a fairly low boiling gas.

The change in pH (which is linked to the degree of water ionisation) on adding heat doesn't need to be attributed to the movement of the species directly... it's a direct consequence of the thermodynamics... ΔG = ΔΗ - T ΔS as T increases the term in ΔS becomes more important.

Urghh... I have no idea of your background, so I don't know what level of chemistry background I should be assuming here and what I should be explaining in detail. I suspect this post is just going to be confusing... but I'll post it anyway incase you can make something of it. Maybe I'll try again later when I'm not supposed to be working.


(Apologies for the hand-wave-y qualitativeness of the above, I don't have the figures at my fingertips and don't really have time to go and look for them)
 

Offline Farrah Day

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How does electrolysis actually work?
« Reply #14 on: 29/07/2008 21:56:08 »
Rosy, I can guarantee you that your chemistry knowledge far exceeds mine.

What you say about the equilibrium state of water interests me in that when ions of water react to form gas, then water will redress the balance. This backs up what I understood to happen.

Am I right then in thinking, you are saying that the random self-ionisation of water is so fast that the ions would associate back into water too quickly to travel unless they were close enough to the electrodes to react almost instantly. And, so we can fairly dismiss the movement of OH- and H+ species through the liquid medium. Somewhere in the back of my mind just a few femtoseconds rings a bell.

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The degree of ionisation of the water won't be affected by addition of acid or, indeed, salts such as magnesium sulphate (which is what we use to do the same job when we're demonstrating electrolysis to kids, it being less corrosive etc).

Given what you say above, how then does the electrolyte influence water to react at the electrodes and evolve gas?

Personally I tend to use sodium sulphate for electrolyte as being a mum with kids around I did not want to use caustic soda or anything so corrosive.

Apologies too if I tend to repeat myself from one post to another.

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The change in pH (which is linked to the degree of water ionisation) on adding heat doesn't need to be attributed to the movement of the species directly... it's a direct consequence of the thermodynamics... ΔG = ΔΗ - T ΔS as T increases the term in ΔS becomes more important.

Can you elaborate on the above paragragh.

Are you saying that it is heat, or more specifically the heat generated by the resistance of electrolyte ions travelling through water that encourages ionisation?  I'm probably misinterpreting this as otherwise instead of increasing the current we could alternatively heat up the water!

It's just nice to have an outlet; somewhere to bounce thoughts and ideas around to see what comes back. Sometimes the seemingly most insignificant post or comment can throw new light on the subject providing an all new perspective from which to view things.
 

lyner

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How does electrolysis actually work?
« Reply #15 on: 29/07/2008 22:02:37 »
Rosy. Interesting.
Your mechanism for conduction reads a bit like conduction in a doped semiconductor. A very small doping level dramatically increases the conductivity from very low (intrinsic)  - either by electron or by hole conduction, depending on whether p or n type doping is added.

You say that separation distance is very relevant, is it linear once the separation is above a certain value?

My Chemistry is crude but my Physics is better. I usually wait for someone else to 'dump' on dodgy ideas in Chemistry. I go along with you until this happens!

FD
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However, if collisions are not the cause, then why does the water heat up so quickly under heavy current? Surely this would be due to collisions... wouldn't it?
When a wire heats up with a current it's electrons interacting with positive ion cores - losing KE, if you like - but no movement of atoms (natch). So, why can't it be similar for conduction in water? If you measure V and I that would give you a value of Resistance - the power dissipation would be VI.

Question: What would the recombination time be for an electron moving through the liquid?

 

Offline Farrah Day

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How does electrolysis actually work?
« Reply #16 on: 30/07/2008 00:23:04 »
Hi Sophie

When researching the possibility of pulsing electrodes, I mistakenly assumed that electrons in metals moved a lot faster than the ions would in a liquid. My mistake was to think that because electron flow appears to be near instantaneous that they moved very quickly.

From further research I found that an electron travels relatively slowly, around 3 inches an hour was indicated, though I can't recall the voltage. However as electrons hop from one atom (ion) to the next, and there are so many of them, it seems fast. In reality is it not unlike pushing a line of touching ballbearings - push one end an the other end moves simaltaneously.

Ions on the other hand would seem to have to travel with their charge the complete distance to and from electrodes, no passing the baton here! So does this mean that effectively an ion generates a lot more energy than a single electron? Not to mention the fact that it is gigantic compared to the electron!

I always thought that ions are always the charge carriers in liquid mediums and that electrons by themselves do not travel through liquids at all - is this not correct?

Agreed VI will give the power dissipation, but - as I'm sure you know - unlike in metals, the resistance of an electrolytic solution is non-linear. And from my experiments the resulting power curve varies greatly with electrolyte concentration. This non-linear curve I would assume relates to the greater mobility or potential to mobilise of the molecules in a liquid.

Is there not also another difference between a metal conducting and a liquid... or am I just now getting too tired to think straight?

Example: Do not better conducting metals have a greater number of loosely bonded electrons and are less resistive, dissipating less power for any given voltage, whereas when we add more charge carrying ions to a liquid to increase conductivity the resulting increase in current causes more heat to be generated.

 

lyner

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How does electrolysis actually work?
« Reply #17 on: 30/07/2008 07:58:24 »
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So does this mean that effectively an ion generates a lot more energy than a single electron?

The work for each charge is QV when moved through a PD of V, irrespective of the mass of the particle - and of how many charges might 'take up the baton' on the way.
Some of this work / energy will show up as heat and the rest, I presume, is involved in what goes on at the electrodes, as the gases or solid deposits are formed.

The speed of flow of charges depends on current and charge density. In the case of a metal, the density of conduction electrons is huge, so the drift rate is low. A Coulomb of flowing charge in a metal takes up some incredibly small volume. Without the protons in there to keep it all together, it would explode with a real bang!

As for the non linearity; this may be due to some 'avalanch' effect, as in an arc in a gas, where the moving charges cause further ionisation. This doesn't happen in metals as the electrons are all very available.
 

Offline Farrah Day

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« Reply #18 on: 30/07/2008 10:22:18 »
That's interesting Sophie as I would have assumed that it took much less energy to move a tiny low mass electron than it would to move a relatively enormous ion - the bus and the motorbike analogy. (Hold on. There is a minimum voltage required to initiate current through a liquid that is not true of a metal - why is this?)

Does not the speed of the flow of charges relate directly to the pd, irrespective of current or charge density?

Fascinating stuff.

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The work for each charge is QV when moved through a PD of V, irrespective of the mass of the particle - and of how many charges might 'take up the baton' on the way.
Some of this work / energy will show up as heat and the rest, I presume, is involved in what goes on at the electrodes, as the gases or solid deposits are formed.

As far as I am aware, and if I've got this right, ionisation is an endothermic process, so should itself not add heat to the liquid medium. Meaning that the action of the ion flow is primarily resposible for generating heat.

That said, this of course is not necessarily so, as I am still yet to learn of the actual reaction at the electrodes once the non-reacting electrolyte ions reach them.

I'm struggling to picture what exactly can be happening here and why it remains a continuous process. Why doesn't there come a time when all the electrolyte ions are pulled to their oppositely charged electrodes and hence current flow ceases?  I'm obviously missing a very important piece of the puzzle here! But what is it?

I find it's far easier to understand the reactions of electrolytes that take an active part in the reactions than those that do not. Electroplating for example quickly makes sense, with every ion and electron seemingly playing it's part. It's when these annoying non-reacting electrolyte ions seem to do all the work and then effectively 'pull-out' at the finishing line that I'm really struggling to get my head around.

Has anyone got a definitive answer - or otherwise - to this question that I might be able to make some sense of?

« Last Edit: 30/07/2008 12:06:14 by Farrah Day »
 

lyner

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How does electrolysis actually work?
« Reply #19 on: 30/07/2008 15:17:12 »
The E = QV  merely refers to the work done moving a charge - basic principle. In a vacuum, accelerating an electron through 1V will get an electron moving much faster (Root 1800 times faster) because they will have the same KE at the end.
In a 'medium', they will go much much slower because they keep interacting with the stuff they pass. Their KE, at the end will be very low because they lost it all on the way - heating up the medium.

Yes, Ionisation is endothermic (the neutral state is more stable than the charged state) but, once you have got an electron away from the atom, it gets KE from the electric field and then crashes into another atom, transferring some KE - heating the bulk.

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I'm struggling to picture what exactly can be happening here and why it remains a continuous process. Why doesn't there come a time when all the electrolyte ions are pulled to their oppositely charged electrodes and hence current flow ceases?  I'm obviously missing a very important piece of the puzzle here! But what is it?
There are always fresh electrons coming out of the Cathode. I suggest that there is probably a gradient of Anions and Cations - concentrated around their respective electrodes but, just beside the elecrodes, it is the 'water' ions which end up losing and gaining the electrons, preferentially. There is a large reservoir of these - until the jar is nearly empty.

When you say that ordinary electrolysis is easier to understand, I suggest that there will be lots of similar oddities if you look deeply enough. Electroplating with Silver, for instance, uses Cyanide ions to 'slow' the Silver ions down so they lay better on the Cathode. I tried to get Silver from Used Photographic Hypo. It was very unsuccessful because, if you tried to increase the current, it just got hot and produced black guff. At low currents it produced a meagre whitish powder and was really disappoihnting.

The threshold voltage for an electrolytic cell could just be a lot higher than for a metal because of the binding energy of the conduction electrons. My suggestion of an avalanche effect would fit in with this. There is a similar 'knee' in a semiconductor diode where the applied voltage must be great enough to get charges to move across the junction.
 

Offline Farrah Day

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« Reply #20 on: 30/07/2008 16:58:57 »
Sophie, yes, I've read that various electroplating is enhanced by careful control of the current, and hence reactions at the electrodes.

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There are always fresh electrons coming out of the Cathode. I suggest that there is probably a gradient of Anions and Cations - concentrated around their respective electrodes but, just beside the elecrodes, it is the 'water' ions which end up losing and gaining the electrons, preferentially. There is a large reservoir of these - until the jar is nearly empty.

Yes, I believe there is a gradient of ions near to the electrodes, but is it not also the case that too many charged ions at the electrodes tend to repell some of the same species (from memory, are they known as diffusion layers?). I understand that with an applied pd across the electrodes there will always be a supply of holes and electrons available to skip ship, but I'm still not getting the full picture.

I know that the electrolyte level in our electrolyser will remain unchanged and only the water would ever need topping up as the electrolyte does not react - I see the end result, I just don't see how we got there!

There might be an unlimited supply of holes and electrons available at the electrodes, but given that the electrolyte will only provide a limited number of ions, and that these ions do not lose or gain charges at the electrodes... what happens to them?  Ultimately current only  flows because ions of water take on and drop off charges. From here I find myself back at my initial question... exactly what role are the electrolyte ions playing?

We know in the final reaction at the electrodes that it is the water that takes precedent over the electrolyte ions... but why?  Why wouldn't the electrolyte ions simply congregate around the electrodes restricting other reactions?  How do these non-reacting ions remain dispersed throughout the water - which they obviously must do? Why do they not simply all migrate to the respective electrode and congregate there, getting in the way?  Do you see what I'm getting at?

I keep looking over previous posts in case the answer has already been given and I'm just not seeing it... but if it has, I can't find it.

I use sodium sulphate as my electrolyte, so I suppose that the sodium ion is only an electron away from becoming very reactive sodium. If the sodium ion was to collect an electron at the cathode I assume that this would react instantly with the water molecules. I don't think this reaction occurs, so there must be some order to the reactions that do take place - an order of priority?  Some reactions must take precedence over others? Why is this?

As far as work goes, the picture I had in my head was of electrolyte ions moving through the water, like tadpoles in a pond, which of course is nonsense. The reality I guess is analogous to a large container of white marbles with a few red ones interspersed. There will always be some interaction on the red marbles from the many neighbouring white marbles. From this analogy it it easy to see how trying to move a red marble is going to require considerable energy.

It's probably me. I think perhaps sometimes I generate too many questions at the same time and muddy the waters.

I currently find myself both intrigued and frustrated by electrolysis!
 

Offline Bored chemist

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How does electrolysis actually work?
« Reply #21 on: 30/07/2008 18:52:06 »
The second and third equations here
http://en.wikipedia.org/wiki/Electrolysis_of_water
are, perhaps, the answer you were looking for. The water itself (not an ionised species) is oxidised at the anode  to give oxygen (and H+ ions) and reduced at the cathode to give hydrogen (along with OH- ions).
The adding an electrolyte makes it easy for the current to get through the bulk of the liquid.
 

lyner

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How does electrolysis actually work?
« Reply #22 on: 30/07/2008 18:52:45 »
I have a suggestion. If the separation of your electrodes is not too small then you should be able to put the probe of a DVM at various positions across the gap. If what we are saying is on the right lines then you should find a S shaped variation from + to - electrodes, rather than linear variation. This would happen because the 'work' is being done right next to the electrodes and that would be where the PD drops fastest.

Another possibility would be to sample the liquid in the cell at each electrode. I would expect to find a surplus of + electrolyte ions near the cathode and vice versa. That would be messy Chemistry business and I don't do analysis, myself, but there are people who can!

I am getting the picture of some sort of 'space charge', such as you get round a thermionic valve cathode. The voltage on the Cathode, left to itself, becomes positive because of the electrons which have left. They return because of this charge but a certain proportion of them are kept in flight by the fact that the cathode is heated.
Your analogy with evaporating water has some relevance, too. It is the atmospheric pressure that balances the vapor pressure of the water and limits the rate of evaporation. It is the molecules with the highest KE which tend to leave the surface and this is why you get cooling in a breeze; the average energy gets less as the faster ones select themselves and are then blown away by an air current.
I hope the experiment(s) are feasible. I don't have easy access to a lab, particularly out of term time.

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?
 

lyner

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How does electrolysis actually work?
« Reply #23 on: 30/07/2008 18:55:59 »
BC
Bearing in mind that the electrolyte ions are finite in number, it isn't clear to me that they carry the current - they would all end up at one end, wouldn't they? There has to be a migration of the water 'bits'.
 That seems to be FD's opinion too.
 

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How does electrolysis actually work?
« Reply #24 on: 30/07/2008 21:17:36 »
Diffusion is enough to send at least some of them back. Also, if enough negative ions pile up round the positive electrode it stops looking like a positive charge.
A detailed analysis is complicated by a number of factors. Some of them pop up here.
http://en.wikipedia.org/wiki/Double_layer_%28interfacial%29
and here.
http://www.cartage.org.lb/en/themes/Sciences/Chemistry/Electrochemis/Electrochemical/ElectricalDouble/ElectricalDouble.htm
 

The Naked Scientists Forum

How does electrolysis actually work?
« Reply #24 on: 30/07/2008 21:17:36 »

 

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