[somebodyy]

Electrically connected zinc and copper electrodes immersed in salt solution (table salt, NaCl) make a voltaic pile. what is the overall reaction in this pile?


Mod edit: CAPS demands removed from title. Please read the forum rules. Dan

[Arkcon]

Could we possibly trouble you, Instead of making demands, to try and figure a bit of this out for yourself?  That is part of the Forum Rules.  You know what a voltaic pile does, to send electrons down a conductor.  Where do they come from?  Where are they going as they travel down the conductor?  How can you represent that as a (hint: pair of) chemical equations?

[somebodyy]

Firstly thank you very much for answering. I respect that. I have done a lot of research about this question for a very long time before coming to here bur I still can not answer this question and I need help about it. Of course while doing research I learned a lot. For example: I know that positive zinc ions get into the solution and leave behind electrons. these electrons travel from zinc electrode to copper electrode due to potential difference. at copper electrode these electrons are given to hydrogen ions and hydrogen gas evolves there. I have conducted this experiment many times at home, measured the voltage&ampere but even that did not help. I got zinc from zinc-carbon battery and copper from electronics shop. But I still dont know the answer and I need help about this. please help.

[somebodyy]

Nobody ever answered this question for last year. Is this a very hard question?

[somebodyy]

Here I made a drawing the voltaic pile for clearance. What is the overall reaction in this pile ?

[Borek]

What sources have you checked?

I started with the most obvious one and it contains an explanation that is good enough to write the overall reaction (assuming you have a HS understanding of the electrochemistry).

[somebodyy]

I checked every webpage on the first 2-3 pages of google search results while searching for many different keywords related, including the obvious one. Only half reactions are written there and just adding them doesnt result in overall reaction, theres something missing. Nobody ever really answers this question, you too. everybody acts like they know the answer but nobody answers, people keep blaming me, like : 'hey this is sooo simple, are you really that stupid'.   :-(

[somebodyy]

Is it shame that I dont know the answer? That is why I am here asking for help.  Check the wikipedia page for "zinc-carbon battery". there are you can see half reactions and overall reaction:

Zn(s) + 2MnO2(s) + 2NH4Cl(aq) → Mn2O3(s) + Zn(NH3)2Cl2 (aq) + H2O(l)

But no such thing is written for "voltaic pile", only half reactions:

Zn → Zn2+ + 2 e−

2H++ 2 e− → H2

Believe me right now I have four graphite-zinc-salty water  voltaic piles stacked in series to increase the voltage,  lighting two LEDs,  in my room next to me. I dont know am I doing this again. the voltages for each pile ranges from 0.8 to 1.1

[Arkcon]

That's pretty nice, you've found the half reactions for some other battery.  That's what I wanted you to learn for yourself, even if you couldn't find it online or in a book for your specific case.  Now, you have the half reactions for some other battery, and one of the metals, zinc, matches yours.  Can you now write the half reaction for your other metal, copper?

[somebodyy]

Nothing happens to copper. The less 'noble' metal corrodes to more 'noble' metal. This is called galvanic corrosion. They protect the iron ships this way, by attaching small zinc plates to the surface of the ship that is exposed to sea water. The zinc corrodes away but the ship is protected. the zinc plates are replaced from time to time. So since copper does not react, there is no half reaction for copper. It just gives the electrons that are coming from zinc to hydrogen ions and hydrogen gas bubbles up from the surface of copper electrode. Just like I see them right now on my graphite electrodes.

[Arkcon]

Sorry.  But your explanation is illogical.  A battery is a reduction-oxidation reaction, which means something gains electrons, and something loses electrons, and we just put those electrons to work while it happens.  Your sacrificial anode protection is a good example of this in action, but not applicable to a battery -- otherwise, they'd never wear out.

[somebodyy]

Not only it's logical but it's the fact. You asked the half reaction for copper, I say nothing happens to copper, absolutely nothing. Zinc loses it's electrons and hydrogen gains them. galvanic corrosion is exactly the same thing as voltaic pile. I think even you guys don't know the answer. I should have known that, because I asked this question to some chemistry professors, both at my school and at USA ( I called them ;-) )  even they didn't know.

[somebodyy]

Hey I just got an email from a professor in USA. As you can guess there's nothing new in his email, the same thing as in wikipedia, only half reactions. But I suddenly came up with a good way of asking my question, maybe then he and you guys will be able to answer. Here it is:

After all of the zinc is dissolved in the solution and the battery is dead, if I boil the solution and get rid of water, what will I be left with?

NaCl(table salt) and what else ??  Do you know what I mean ?

[Borek]

You will be left with ZnCl₂ mixed with Zn(OH)₂ (as well as NaOH and NaCl).

When you have two half reactions all you have to do is to add them together to get a full reaction. This is really taught to 15 yo kids. See for example http://www.chembuddy.com/?left=balancing-stoichiometry&right=half-reactions-method

[somebodyy]

Zn → Zn2+ + 2 e−

2H++ 2 e− → H2

add these:

Zn + 2H++  ---> Zn2+ + H2

You mean this is the overall reaction then ?

I know the addition of half reactions dont worry about that, but this is not an overall reaction. Overall reactions dont have ions in them, learn that.