[PFScience]

Hi

I've been looking back and revising through some old notes on electrochemistry and I've realised a lot of the things I thought were right might not be regarding my fundamental understanding of electricity, electrolytes and the moving of current. This in turn has caused me to question over things in my mind.... hence I'm having a major wobble!

Am I thinking right...or am I going bonkers!

- Taking a simple electrolytic cell with an aqueous ionic compound as the electrolyte and 2 inert electrodes:

1) The anode battery end (oxidation of electrons) leads to the cathode electrode (reduction of chemical species)
    The cathode battery end (reduction of electrons) leads to the anode electrode (oxidation of chemical species)

2) Cations go to cathode electrode and anions go to anode electrode with the associated formation of chemical species. The deposited electron at the anode travels back into the battery cathode.

3) Due to their still being a potential difference between the battery cathode and anode, and assuming a sufficient concentration of electrolytes remain, the process starts over again.
 
So essentially, the electrons themselves do not travel through the solution. It is the electrolytes that are doing the work. My basic misunderstanding I think has being hung up on electricity as the movement of electrons rather than the movement of charge.

Also the situation above would be slightly different in terms of current flow if the electrodes were made from differently reactive metals (eg copper and zinc) as opposed to just 2 inert electrodes?

Electricity travels through the wire in the form of electrons, but in the electrolyte as charged particles.

All electrons are charged particles but not all charged particles are electrons?!

As an example...

If I was to stick my finger into the electrolyte of a strong enough electrolytic cell I might get an electric shock. However, the shock would not be coming from the electrons of the battery per se but from the flow of negatively charged aqueous ions, that upon contacting my finger would discharge their negative charge. Voltage and current are proportional  the larger the battery  the greater the current  the bigger the shock!
Hence why if I stand in a bath of pure water and something electrical falls in, I wont get electrocuted as there are no charged particles present.

Thanks for any feedback.....and apols if I'm making any basic errors!

[mjc123]

The anode battery end (oxidation of electrons) leads to the cathode electrode (reduction of chemical species)
    The cathode battery end (reduction of electrons) leads to the anode electrode (oxidation of chemical species)

You don't oxidise or reduce electrons. You oxidise a chemical species by taking electrons from it, and reduce it by adding electrons to it.
At the battery anode, some species inside the battery is oxidised, and the electrons released flow to the cathode of the electrolytic cell. Likewise at the battery cathode, the electrons from the cell anode reduce some battery species.

Also the situation above would be slightly different in terms of current flow if the electrodes were made from differently reactive metals (eg copper and zinc) as opposed to just 2 inert electrodes?

If you have two differently reactive metals they can create a potential difference. The direction of current flow depends on the applied voltage relative to the cell potential. If the applied voltage is sufficient it can cause the electrodes to react (e.g. oxidation of Zn and reduction of Cu²⁺ - or the reverse, if the voltage is the other way round). If not, but there is something in the solution that can be oxidised/reduced, the electrodes can act as current carriers like inert electrodes.

However, the shock would not be coming from the electrons of the battery per se but from the flow of negatively charged aqueous ions

Current in solution is carried by both positive and negative ions, moving in opposite directions.

Hence why if I stand in a bath of pure water and something electrical falls in, I wont get electrocuted as there are no charged particles present.

Do not try this at home! The conductivity of pure water is low, but not zero as it does contain charged particles by virtue of its autoionisation. Water from the bath tap is not pure, it will contain various ions, and have a significant conductivity. Once you get in the bath, salts etc from your skin will increase the conductivity further. This is not a safe experiment to try!

[PFScience]

Thanks for the reply

You don't oxidise or reduce electrons. You oxidise a chemical species by taking electrons from it, and reduce it by adding electrons to it.
At the battery anode, some species inside the battery is oxidised, and the electrons released flow to the cathode of the electrolytic cell. Likewise at the battery cathode, the electrons from the cell anode reduce some battery species.

Yes, my apologies, poor choice of words, I was trying to communicate the loss and gain of electrons from the respective ends.

If you have two differently reactive metals they can create a potential difference. The direction of current flow depends on the applied voltage relative to the cell potential. If the applied voltage is sufficient it can cause the electrodes to react (e.g. oxidation of Zn and reduction of Cu²⁺ - or the reverse, if the voltage is the other way round). If not, but there is something in the solution that can be oxidised/reduced, the electrodes can act as current carriers like inert electrodes.

Yes, commonly, electrolytic cells are used to drive non-spontaneous reactions. If you have 2 sufficiently differently reactive metals, then you dont need an external power source to achieve electrical current.... hence the voltaic cell.

Current in solution is carried by both positive and negative ions, moving in opposite directions.

Right... so when someone is electrocuted in water, it is done via the potential difference between their body and the negative and positive charges. When a negatively charged ion encounters the body there will be a potential difference and likewise the same for the positively charged ion, except this time the current will flow from the body to the ion.

The current flows:

Negatively charged ion  body  positively charged ion  ??

So the current in this case does flow through the body


Lastly, could you give me a run through of the workings of the original Voltaic Pile? I've tried going through it myself and a couple of online references, but I havent seen anywhere that gives a good visual explanation of how the current transitions through the multiple elements (zinc electrolyte copper/silver.... repeat). And in fact a few of the online explanations were a bit contradictory as to what exactly was going on.... I 've seen ones with the electron flow going from the top of the pile to the bottom (via the wire) and other versions with the current flowing the other way!

Much appreciated

[mjc123]

You are aware that conventional current flows from positive to negative, unlike electrons? That may be why some versions differ from others. Otherwise, I'm not sure of your difficulty. Could you perhaps say what you don't understand about the explanation in the Wikipedia article https://en.wikipedia.org/wiki/Voltaic_pile, for example?

Current in an electrolyte does not flow from negative to positive ions (that would be a redox reaction, not the flow of current). Current flows by positive and negative ions moving in opposite directions through the electrolyte towards the respective electrodes, and by electrochemical reactions at the electrodes.

Why are you so interested in electrocuting people?

[PFScience]

You are aware that conventional current flows from positive to negative, unlike electrons? That may be why some versions differ from others. Otherwise, I'm not sure of your difficulty. Could you perhaps say what you don't understand about the explanation in the Wikipedia article https://en.wikipedia.org/wiki/Voltaic_pile, for example?

Yes I saw the Wiki page. My problem is really to do with the general functioning of the pile and in particular the role of the copper electrode:

-The top of the pile has a zinc plate which is oxidised, releasing Zn²⁺ into the salt water electrolyte. The electrons left behind then travel along the conducting wire to the bottom of the pile which consists of a copper plate. The Zn²⁺ bonds with the Cl^- to form ZnCl₂

- The copper plate appears to serve as purely a conducting electrode (in a similar way to a normal galvanic cell) - obviously with the Voltaic Pile there is no salt bridge present. The copper is functioning as the cathode due to H⁺ from the water being reduced - thus forming H₂ gas at the copper surface.

- No mention is made of the interaction between the Na⁺ ions and OH^- ions? If they interact to form NaOH, then I'm assuming the electrolyte is now rapidly becoming less efficient?

- So essentially that appears to be it. Each cell (zinc-electrolyte-copper) is its own redox reaction site. But then how does the charge transition from the bottom of the pile to the top? It says that each cell generates +0.76 V and that this is cumulative  so that the entire pile generates +4.56 V. But then how does that work? Each cell is its own reaction, the electrons generated are used in the reduction to H₂ gas - I dont see how the charge can build up at the anode so as to create the combined potential difference?

Why are you so interested in electrocuting people?

I'm not per se (although I do have an interest in forensics!). I just finds it helps me understand what the current is and how it interacts. It seems v.peculiar that Na⁺ and Cl^- ions can kill you! I understand they are moving very fast and in great numbers but even so its seems very peculiar. How grabbing a piece connected conducting wire can be harmful seems like common sense and intuitive, but the situation in an electrolyte seems counterintuitive since in this case the current from the power source isn't touching you at all.

It almost seems like electrocution water is akin to a colligative property in that it doesnt seem to matter what the type of ions are, just the quantity!

I hope that makes sense.... I'm not morbid I promise! 

[Corribus]

Not so hard to understand. Flowing current damages human tissue via: resistive heating (moving charged particles collide with other particles, creating heat), interference of nerve impulses (which utilize the motion of ions to propagate), and (the big one) disruption of electrical signaling of the heart.

[PFScience]

Not so hard to understand. Flowing current damages human tissue via: resistive heating (moving charged particles collide with other particles, creating heat), interference of nerve impulses (which utilize the motion of ions to propagate), and (the big one) disruption of electrical signaling of the heart.

Thanks... I've also been doing some additional hunting around and came along this which I thought was informative

http://amasci.com/amateur/elsafe.html

Could you assist at all in my Voltaic pile query?

[mjc123]

-The top of the pile has a zinc plate which is oxidised, releasing Zn2+ into the salt water electrolyte. The electrons left behind then travel along the conducting wire to the bottom of the pile which consists of a copper plate. The Zn2+ bonds with the Cl- to form ZnCl2

OK so far, except that Zn²⁺ doesn't bond with Cl^-. ZnCl₂ is soluble, and an aqueous solution of zinc chloride contains separate Zn²⁺ and Cl^- ions.

The copper plate appears to serve as purely a conducting electrode (in a similar way to a normal galvanic cell) - obviously with the Voltaic Pile there is no salt bridge present. The copper is functioning as the cathode due to H+ from the water being reduced - thus forming H2 gas at the copper surface.

OK here too. Note that the electrons don't complete the circuit - this is not necessary for a current to flow.

No mention is made of the interaction between the Na+ ions and OH- ions? If they interact to form NaOH, then I'm assuming the electrolyte is now rapidly becoming less efficient?

They don't. Like ZnCl₂, NaOH is fully ionised in solution. Zn(OH)₂, however, is insoluble, and if the Zn²⁺ ions meet the OH^- ions in sufficient concentration, the solid will precipitate out.
Current flows through the electrolyte as positive ions migrate from the anode to the cathode and negative ions the other way. Most of the current is carried by Na⁺ and Cl^-, as these are the ions present in largest concentration, even though they don't participate in the electrochemical reactions.

So essentially that appears to be it. Each cell (zinc-electrolyte-copper) is its own redox reaction site. But then how does the charge transition from the bottom of the pile to the top? It says that each cell generates +0.76 V and that this is cumulative  so that the entire pile generates +4.56 V. But then how does that work? Each cell is its own reaction, the electrons generated are used in the reduction to H2 gas - I dont see how the charge can build up at the anode so as to create the combined potential difference?

Electrons flow from the zinc at the top of one cell to the copper at the bottom of the next. As these are in electrical contact, they have the same potential (well, difference is IR, assume R is very small). So if zinc(1) is at -0.76V relative to copper(1), so is copper(2). Zinc(2) is at -0.76V relative to copper(2), thus at -1.52V relative to copper(1). And so on. Remember these are potential differences, not absolute values. The potential difference across a zinc/copper cell is always 0.76V (well, E° anyway; it will change during discharge), and if you have several cells in series the potential differences are additive. (What about in parallel?)

[PFScience]

Thanks for this - I'll go through it now to check everything makes sense.

[PFScience]

OK so.. lets have another go! 

1) The Zinc plate at the top of the pile is oxidised to Zn²⁺ which passes into the solution. The 2e pass through the conducting wire to the bottom copper plate.

2) The 2e pass through the bottom copper plate as it is functioning as a conducting electrode. The flow of current continues upwards as negatively charged aqueous Cl^- ions (and some OH^- ions) flow towards to the next Zinc plate above due to the repulsive effect of the electrons. H₂ gas is formed at the surface of the bottom copper plate (2H⁺ + 2e  H₂(g) )

3) Likewise at the zinc plate above, the positively charged Zn²⁺ ions along with Na⁺ ions create a flow of positive charge downwards in the opposite direction through the solution towards the bottom copper plate. The 2e left behind on the zinc plate are pushed up through the adjoining copper plate above due to the repulsive effect of the flow of negatively charged Cl^- and OH^- ions.

The above process repeating as you go up through the pile and the current is maintained...

H₂ gas is formed instead of Na (s) at the cathode due to the reaction being more electropositive