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Topic: Voltaic pile question  (Read 15556 times)

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Offline somebodyy

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Re: Voltaic pile question
« Reply #15 on: April 10, 2013, 12:16:17 PM »
Why doesnt the zinc give its electrons to hydrogen at zinc electrode itself but sends them  to copper so that hydrogen receives then there ?

Offline Borek

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Re: Voltaic pile question
« Reply #16 on: April 10, 2013, 12:53:45 PM »
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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Offline somebodyy

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Re: Voltaic pile question
« Reply #17 on: April 10, 2013, 01:18:12 PM »
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 ??

Offline Borek

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Re: Voltaic pile question
« Reply #18 on: April 10, 2013, 01:55:38 PM »
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.

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professor says the overall reaction is:  Zn + 2H20  ->  Zn(OH)2 + H2

This is equivalent to the net ionic reaction you listed earlier.

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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.
« Last Edit: April 10, 2013, 02:17:47 PM by Borek »
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Offline somebodyy

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Re: Voltaic pile question
« Reply #19 on: April 10, 2013, 02:14:08 PM »
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.

Offline Borek

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Re: Voltaic pile question
« Reply #20 on: April 10, 2013, 02:34:03 PM »
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.

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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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Offline somebodyy

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Re: Voltaic pile question
« Reply #21 on: April 10, 2013, 02:50:14 PM »
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?

Offline Borek

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Re: Voltaic pile question
« Reply #22 on: April 10, 2013, 04:08:26 PM »
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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Offline somebodyy

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Re: Voltaic pile question
« Reply #23 on: April 10, 2013, 04:37:17 PM »
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.

Offline Borek

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Re: Voltaic pile question
« Reply #24 on: April 10, 2013, 05:24:02 PM »
ok then put copper alone in water, doest it also react?

No, it doesn't.

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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.

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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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Offline somebodyy

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Re: Voltaic pile question
« Reply #25 on: April 10, 2013, 07:37:14 PM »
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...

Offline Borek

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Re: Voltaic pile question
« Reply #26 on: April 11, 2013, 03:18:20 AM »
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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Offline somebodyy

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Re: Voltaic pile question
« Reply #27 on: April 11, 2013, 07:42:24 PM »
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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Re: Voltaic pile question
« Reply #28 on: April 12, 2013, 02:58:33 AM »
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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Offline somebodyy

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Re: Voltaic pile question
« Reply #29 on: April 12, 2013, 04:42:02 PM »
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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