Chemical Forums
Chemistry Forums for Students => Inorganic Chemistry Forum => Topic started by: somebodyy on April 06, 2013, 07:16:19 PM
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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
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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?
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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&ere 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.
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Nobody ever answered this question for last year. Is this a very hard question?
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Here I made a drawing the voltaic pile for clearance. What is the overall reaction in this pile ?
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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).
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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'. :-(
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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
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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?
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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.
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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.
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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.
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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 ?
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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
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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.
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Why doesnt the zinc give its electrons to hydrogen at zinc electrode itself but sends them to copper so that hydrogen receives then there ?
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Overall reactions dont have ions in them, learn that.
Have you ever heard about net ionic reactions?
Why doesnt the zinc give its electrons to hydrogen at zinc electrode itself but sends them to copper so that hydrogen receives then there ?
Good question. My guess is that it is because of the kinetics - H+ gets reduced much more faster on the copper surface than on zinc. Zinc reacts with acids easily - but slowly.
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But why ? electron is electron, what difference it makes to get it from copper or zinc.
professor says the overall reaction is: Zn + 2H20 -> Zn(OH)2 + H2
Now that we know the overall reaction the question remains same as last one. Zinc by itself immersed in water doesnt reac with water, why do we need copper there to make zinc react with water, why doesnt it react with water like sodium or potassium or lithium ??
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But why ? electron is electron, what difference it makes to get it from copper or zinc.
On some surfaces reactions go faster, on some slower. This the kinetic element I have already mentioned.
professor says the overall reaction is: Zn + 2H20 -> Zn(OH)2 + H2
This is equivalent to the net ionic reaction you listed earlier.
Now that we know the overall reaction the question remains same as last one. Zinc by itself immersed in water doesnt reac with water, why do we need copper there to make zinc react with water, why doesnt it react with water like sodium or potassium or lithium ??
Zinc does react with water, but slowly. To speed the reaction up we can add some copper salt to the solution - copper will get reduced on the zinc surface creating electrochemical cells and speeding up the reaction.
Kinetics is independent on the thermodynamics - just because we know zinc will react with water, doesn't mean it has to do it fast, these are two different things. Yes, often they work in the same direction, but not always.
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You mean hydrogen gets reduced at more noble metals easily that on less nobles. I dont think you answer to this question is correct but thanks for answering. It is still a question for me:
Why hydrogen doesnt get electrons from zinc electrode but gets them from copper electrode ? Somebody knows other explanation ? I aksed this to professor also, hope he knows the answer.
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You mean hydrogen gets reduced at more noble metals easily that on less nobles. I dont think you answer to this question is correct but thanks for answering.
No, that's not what I meant, I was referring to copper only, not to all noble metals (even if they usually have low overpotential, as their surface is not passivated). Every metal has specific properties in this regard. A lot depends on the surface finish and on oxide passivation. Wikipedia article on overpotential has some numbers listed.
See also here (http://books.google.pl/books?id=Qmf4VsriAtMC&lpg=PA46&ots=3Xc9j2O3gD&dq=hydrogen%20reduction%20overpotential&pg=PA46#v=onepage&q=hydrogen%20reduction%20overpotential&f=false).
Why hydrogen doesnt get electrons from zinc electrode but gets them from copper electrode ? Somebody knows other explanation ? I aksed this to professor also, hope he knows the answer.
Nobody ever said hydrogen doesn't get reduced on the zinc surface. It reacts on both surfaces, just faster on the copper, as it requires lower overpotential there.
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Ok put zinc alone in water. now hydrogen has no way but to get reduced on zinc. why doesn't this happen ? you will probably say due to protective layer that prevents further reaction. but doesn't protective layers form on copper also? and since there's protective layer on zinc how zinc ions get into solution?
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Ok put zinc alone in water. now hydrogen has no way but to get reduced on zinc. why doesn't this happen ?
Who told you it doesn't happen? Have you read my posts? I told you several times that it reacts, just slowly.
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ok then put copper alone in water, doest it also react? if what you are saying is true then hydrogen gets reduced on copper better than zinc then this means copper alone in water corrodes away faster than zinc. this also means that gold/platinum/silver are best hydrogen reducers and they alone in water corrode away fastest. but that is not so.
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ok then put copper alone in water, doest it also react?
No, it doesn't.
if what you are saying is true then hydrogen gets reduced on copper better than zinc then this means copper alone in water corrodes away faster than zinc.
No, it doesn't mean that.
this also means that gold/platinum/silver are best hydrogen reducers and they alone in water corrode away fastest. but that is not so.
It doesn't mean it either. There is a difference between metal reacting with water and metal serving as an inert electrode. Zinc does react with water directly, all other metals you have listed above serve as inert electrodes - hydrogen gets reduced on their surface only if the electrons are delivered from the outside, without metal being oxidized. We don't split redox reactions into half cells without a reason.
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Hmmm. I don't feel the strength in your answer. Because I think, for a reaction to occur two water molecules has to meet(collide/touch/come very close/whatever) with zinc atom on zinc surface. when this happens : Zn+2H2O>Zn(OH)2+H2 the reaction should happen at the place where they met. why would Hydrogen travel all the way to copper. Ohhhhhhhhhhh, I have a headache from this, but I want to understand this thing. sleep...
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Hmmm. I don't feel the strength in your answer. Because I think, for a reaction to occur two water molecules has to meet(collide/touch/come very close/whatever) with zinc atom on zinc surface. when this happens : Zn+2H2O>Zn(OH)2+H2 the reaction should happen at the place where they met. why would Hydrogen travel all the way to copper.
You think wrong, reaction is split into two parts which occur in different places. That's what is happening in all chemical batteries, regardless of their exact make (be it acid/lead, Leclanché, CdNi, Li polymer, whatever). Charge flows through the wire closing the circuit, reaction products diffuse/migrate between the electrodes. Overall reaction is a sum of half reactions which are separated and occurring in different places.
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Borek, does this mean that if put electrically connected aluminium and graphite electrodes in sodium hydroxide solution, the hydrogen bubbles will appear on graphite instead of aluminium ? Because normally aluminium reacts with sodium hydroxide solution with evolution of hydrogen gas. I was going to do this test today but my sodium hydroxide that I bought a lot of time ago has absorbed CO2 from air and became something else. I'll do it tomorrow probably.
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I am not 100% sure, but my bet is that Al reaction will be fast enough to be not kinetically separated (so gas will evolve directly on Al surface).
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Borek, I just did the experiment with aluminium/graphite/sodium hydroxide and as you predicted nothing changed, hydrogen bubbles formed on aluminium. But how can I test the explanation that you said ? It must be like this: electrically connected electrodes X and Y immersed in a solution Z. X normally reacts with Z with evolution of gas. but when I connect X and Y and put them in Z, bubbles produced on Y instead of on X. What X,Y and Z must be to prove the explanation you proposed ?
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I think it must a metal that reacts with a solution not very vigorously but still at observable rate. Do you think zinc/copper/sulfuric acid solution will work ? I think zinc reacts slowly with sulfuric acid solution but still at observable rate with production of hydrogen gas.