[lillybeans]

Hi everyone, I know this question sounds very stupid, but I don't understand what PURPOSE an electrolyte serves.

Please take a look at this diagram I've drawn:

http://i55.tinypic.com/99zy9z.jpg

As you can see in the diagram, the circuit starts at the negative terminal (anode) and ends at the positive terminal (cathode). Electrons NEVER FLOW THROUGH THE ELECTROLYTE solution. The entire time they are transferred outside of the solution through an EXTERNAL circuit. So WHY do we need an electrolyte to conduct electricity if it's not even part of the circuit? Isn't the E-cell (electrical potential difference between Zinc and Copper) all that's needed to allow the spontaneous reaction to occur? How does dipping them in an electrolyte and not say, water, help conduct electricity?

Correct me if I'm wrong. In fact I'm dying to know where I went wrong. Much appreciated.

[UG]

Well if you think about it, if the zinc is giving electrons to copper, doesn't that make the zinc more positive and the copper more negative? So why should electrons 'want' to move to the copper since it is getting more and more negative. It is the ions inside the electrolyte that is taking the electrons away from copper (and similarly giving them to zinc) so that the electrons can continue to flow in the external circuit.

[lillybeans]

Well if you think about it, if the zinc is giving electrons to copper, doesn't that make the zinc more positive and the copper more negative? So why should electrons 'want' to move to the copper since it is getting more and more negative. It is the ions inside the electrolyte that is taking the electrons away from copper (and similarly giving them to zinc) so that the electrons can continue to flow in the external circuit.

Hi there,

You mean like how a salt bridge works to maintain electrical neutrality? As in... the anions migrate toward the zinc side and sodium migrate toward the negative terminal? I'm pretty sure Na+ and Cl- are not involved in electron transfers, because that would make it electrolysis not a galvanic cell?

Thanks for your help once again

Lilly

[UG]

Yes, you're right, why did I think it was electrolysis 
But the fact remains, zinc is losing electrons so something must be getting reduced at the cathode.

[lillybeans]

Yes, you're right, why did I think it was electrolysis 
But the fact remains, zinc is losing electrons so something must be getting reduced at the cathode.

Haha, thank you so much. Since you are here, I was wondering if you could please help me with another question that I have? It's about voltaic pile and how voltage increases when you connect the cells in series. So let's say we have a voltaic pile in this configuration. so a layer of copper, then a cardboard soaked in an electrolyte, then a layer of zinc.

Zn (anode)
Electrolyte
Cu
Zn
Electrolyte
Cu
Zn
Electrolyte
Cu (cathode)

I read on wikipedia that how voltaic pile works is this. At the anode, zinc loses two electrons, which travel through the external circuit all the way to the cathode so that copper gains the two electrons. This part is fine and makes perfect sense, I understand. However, in the mean while, the zinc ion that has lost the two electrons travel through the electrolyte layer, and meet the copper. Zinc then steals two electrons from copper to replace the ones it lost through oxidation. This applies at every level in between the terminals.

BUT HOW CAN THIS HAPPEN? Isn't Zinc more likely to be OXIDIZED than REDUCED? HOW does it manage to gain electrons by stealing coppers? Isn't Copper's reduction potential greater than Zinc's? Shouldn't the electron flow in the opposite direction, from Zinc to copper, not copper to zinc?

Please explain. Thanks!!

[UG]

At the anode, zinc loses two electrons, which travel through the external circuit all the way to the cathode so that copper gains the two electrons. This part is fine and makes perfect sense, I understand

Copper gains two electrons? and forms....Cu^2-? I haven't seen such a species. It be something elso that is getting reduced (perhaps something in the electrolyte.

[lillybeans]

At the anode, zinc loses two electrons, which travel through the external circuit all the way to the cathode so that copper gains the two electrons. This part is fine and makes perfect sense, I understand

Copper gains two electrons? and forms....Cu^2-? I haven't seen such a species. It be something elso that is getting reduced (perhaps something in the electrolyte.

Ahh, well this is the page i was referring to.

http://simple.wikipedia.org/wiki/Voltaic_pile

under "How the Voltaic Pile Works"

Maybe I have comprehension issues, but I'm pretty sure that's what happens. Cu2- I'm not so sure. Maybe it was already in the ion form (Cu2+) when it was about to receive electrons?

[UG]

From what I can gather, the zinc loses 2 electrons to the acid, so 2H⁺ is reduced to H₂ and then as the zinc moves through the pile, 'steals' electrons from copper. This isn't as absurd as it sounds, since the reduction potentials you are given are all under standard conditions (so all solutions are 1 mol L^-1). When you have non standard conditions, the reduction potentials change, and the new values can be calculated with the Nernst equation

[lillybeans]

From what I can gather, the zinc loses 2 electrons to the acid, so 2H⁺ is reduced to H₂ and then as the zinc moves through the pile, 'steals' electrons from copper. This isn't as absurd as it sounds, since the reduction potentials you are given are all under standard conditions (so all solutions are 1 mol L^-1). When you have non standard conditions, the reduction potentials change, and the new values can be calculated with the Nernst equation

Thank you for the clarification! But if hydrogen ions are reduced, then what about copper? Isn't the redox reaction supposed to take place between Zn/Cu and not Zn/H+? Or do both occur in the voltaic pile?

[DevaDevil]

The zinc/copper battery only works in a reaction between these two metals if ions of copper are present:

Zn + Cu²⁺ --> Zn²⁺ + Cu (E⁰ = 0.91V)

if they are not present, the copper (or any other more noble metal than zinc) will basically be used as a conductor to evolve hydrogen

Zn + 2H⁺ --> Zn²⁺ + H₂ (E⁰ = 0.76V)

[lillybeans]

But let's say there is a voltaic pile. If the reaction is Zn + 2H+ --> Zn2+ + H2, between zinc and hydrogen, what is copper used for? Why do we need it at all if it's not reduced, and hydrogen ions are the ones that are reduced? If the whole point is so that the electrons given off can travel through an external circuit to meet the copper electrode ... then does that mean copper is an INERT electrode in this case? since it does not participate in redox reactions and only acts as an.. electron transfer-er to the elctrolyte?

(Please be patient with me. I know I am confused and ignorant beyond belief, but I just have to understand this!)

Also, in my chemistry class, we stacked a voltaic pile like this

Al
Na2CO3 (paper towel soaked in it)
Cu
Al
Na2CO3
Cu

etc..

And it conducted electricity! What's going on? I know that Aluminum lost 3 electrons to become Al3+, and that travelled through the electrolyte... but Na2CO3 is no acid, there's no H+. So if the reaction is between Aluminum and hydorgen ions, does that mean the hydrogen ions come from water? (Which does not have a great amount of supply of it). Or does it react with something else?

[DevaDevil]

But let's say there is a voltaic pile. If the reaction is Zn + 2H+ --> Zn2+ + H2, between zinc and hydrogen, what is copper used for? Why do we need it at all if it's not reduced, and hydrogen ions are the ones that are reduced? If the whole point is so that the electrons given off can travel through an external circuit to meet the copper electrode ... then does that mean copper is an INERT electrode in this case? since it does not participate in redox reactions and only acts as an.. electron transfer-er to the electrolyte?

it is not entirely inert, some copper will corrode due to the acidic environment as well and supply the Cu2+ cations, which will be reduced by the zinc as well. It speeds up the zinc corrosion in the same way an iron nail in an old zinc roof drainpipe will cause the pipe to get a hole faster. (you could also say the zinc is a sacrificial metal)

It is also the way to separate the electrons from the zinc ions in order to gain useful electricity. a small block of zinc would corrode (dissolve) by itself in acidic solution; the copper -separated by the salt bridge- makes the external circuit and thus electricity harvesting possible.



as for the second part of your question: the sodium carbonate was still in water, which means there are still protons (H⁺), as well as hydroxyls. (remember: H₂O <--> H⁺ + OH^-)
The salt just makes an abundance of charge carriers needed for conduction of electricity.

[lillybeans]

Great answer. Thank you so much for your help and patience. I really appreciate it!

[Bert95]

In addition:

If you have a neutral electrolyt like in the picture in your first post, oxygen can also act as the electron acceptor:

0.5 O₂ + H₂O + 2 e- --> 2 OH^- (E⁰=-0.2 V)