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Topic: redox equilibrium (Is their such a thing?) Quick questions!!  (Read 14792 times)

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Offline sameeralord

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Re: redox equilibrium (Is their such a thing?) Quick questions!!
« Reply #15 on: August 29, 2008, 05:56:52 PM »
I have already explained details on August 25th. Water (or H+) will react with solid metal, dissolving just enough to create the equilibrium. If you put salt into the solution, the same equilibrium between water and solid will exist, it will be just shifted by the presence of excess ions.

Thanks a lot for your response Borek  ;). You have expressed it very precisly. So my understanding is that the equilibrium that is setup is as result of reacting with water's half equation. Zn + 2e- <----> Is the 2nd half equation right?

One more question what happens if there is postive gradient between water and the metal in electrochemical series. Then this spontanteous reaction would not occur. So how can daniel cell be used in a situation like this.

Also as Hunt has mentioned if there is a platinum electrode withe Fe2+ and Fe3+. Is this the equlibrium caused by water's half equation or by Fe2+ and Fe3+. There is no metal to dissolve in this case.

Thanks  for taking your time to help me so far  ;)  ;)  ;)  ;)  ;)

Offline Borek

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Re: redox equilibrium (Is their such a thing?) Quick questions!!
« Reply #16 on: August 29, 2008, 06:27:14 PM »
Thanks a lot for your response Borek  ;). You have expressed it very precisly. So my understanding is that the equilibrium that is setup is as result of reacting with water's half equation. Zn + 2e- <----> Is the 2nd half equation right?

If I understand you correctly - yes. But keep reading.

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One more question what happens if there is postive gradient between water and the metal in electrochemical series. Then this spontanteous reaction would not occur. So how can daniel cell be used in a situation like this.

First of all, activity series is only an approximation. Even so called noble metals react to some extent with H+, although usually their equilibrium concentrations are so small that they can be safely neglected.

Second, Daniell cell uses solutions of both ions needed, so ions present are not from the reaction with water, they are added by hand.

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Also as Hunt has mentioned if there is a platinum electrode withe Fe2+ and Fe3+. Is this the equlibrium caused by water's half equation or by Fe2+ and Fe3+. There is no metal to dissolve in this case.

You don't a solid metal for the redox reaction to occur, Fe2+ can be oxidized to Fe3+ just like Zn can be oxidized to Zn2+.
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Offline sameeralord

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Re: redox equilibrium (Is their such a thing?) Quick questions!!
« Reply #17 on: August 29, 2008, 06:41:50 PM »
Thank you once again for your quick replies Borek  ;)

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Second, Daniell cell uses solutions of both ions needed, so ions present are not from the reaction with water, they are added by hand.

Do you mean that by adding ions that we can enhance the equilibrium of noble atoms.  For example to make these equilibrium more considererable.

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You don't a solid metal for the redox reaction to occur, Fe2+ can be oxidized to Fe3+ just like Zn can be oxidized to Zn2+.

So in this case is this the result of water's half equation or Fe simply turn s into Fe3+. If that is the case we'll have two half equations that cancel out.

Thank a lot once again for your help  ;) I have improved my chemistry since the day I joined this site. As my test marks indicate  ;)


Offline sameeralord

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Re: redox equilibrium (Is their such a thing?) Quick questions!!
« Reply #18 on: August 29, 2008, 07:12:30 PM »
After looking at the electrochemical series,

I have found some metals that do not react with H+ (not sponateously)

1. Cu electrode
2. Fe2+
3. Ag
4.I-

I have seen many pracs using Cu electrode. How can an equilibrium created then.

Also we just did a prac  with Carbon inert electrode and KI solution in one half cell.

Reduction occured in that cell. I2 + 2e ------> 2I-
Now there was no I2 to begin with so how did this reaction occur. I checked as you have said with H+ but no spontaneous reaction occurs.

These are truly the final questions I have in this section.  ;)

Looking forward to your reply Borek  ;)

Offline Borek

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Re: redox equilibrium (Is their such a thing?) Quick questions!!
« Reply #19 on: August 29, 2008, 07:35:56 PM »
Do you mean that by adding ions that we can enhance the equilibrium of noble atoms.  For example to make these equilibrium more considererable.

No idea what you mean by "enhancing the equilibrium". Adding ions you simply shift the equilibrium that exists in the solution.

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So in this case is this the result of water's half equation or Fe simply turn s into Fe3+. If that is the case we'll have two half equations that cancel out.

PLease elaborate, I have no idea what you are talking about.

I have seen many pracs using Cu electrode. How can an equilibrium created then.

Copper in fact does react with H+, but the equilibrium concentration of Cu2+ is in the range of 5x10-12M. That's the original equilibrium that is created automatically when you put pure copper wire into pH 0 solution. But you may as well put copper wire into 1M Cu2+ solution, in boith cases copper wire will be in th eequilibrium with the copper solution, just the potential of the wire (as measured against some other electrode) will be different.

Quote
Also we just did a prac  with Carbon inert electrode and KI solution in one half cell.

Reduction occured in that cell. I2 + 2e ------> 2I-
Now there was no I2 to begin with so how did this reaction occur. I checked as you have said with H+ but no spontaneous reaction occurs.

Yes, it occurs spontaneously, just the equilbrium concentrations is so low, that it is below the detection limit. So to simplify things we say that the reaction doesn't occur - but as you have spotted it leads to inconsistencies.
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Offline sameeralord

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Re: redox equilibrium (Is their such a thing?) Quick questions!!
« Reply #20 on: August 30, 2008, 12:03:29 AM »
Thanks heaps for you help once again Borek  ;)

I think I got it now. Just want to clarify some points.

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you may as well put copper wire into 1M Cu2+ solution, in boith cases copper wire will be in th eequilibrium with the copper solution, just the potential of the wire (as measured against some other electrode) will be different.

Cu2+  + 2e-  <-------> Cu

So you mean when you add Cu2+ ions the equilibrium would be moved to the right so less electrons deposit on the electrode. So the potential of the wire differs.

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So in this case is this the result of water's half equation or Fe simply turn s into Fe3+. If that is the case we'll have two half equations that cancel out.

PLease elaborate, I have no idea what you are talking about.

Sorry for wayward question  ;). So it is not only metals that can dissociate in water right? I mean Fe2+ can react with H+ as well. Just to clarify  ;) This brings me to the sense that almost all aqeuous solution would be in equilibrium after reacting with H+

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Yes, it occurs spontaneously, just the equilbrium concentrations is so low, that it is below the detection limit. So to simplify things we say that the reaction doesn't occur - but as you have spotted it leads to inconsistencies.

Yeah that makes sense. So basically I- ions in the KI would just shift the equilibrium right?

So basically is the reason why most of the time electrode and solution are from the same element is to create a equilibrium with larger concentrations.

Thanks a lot for you help so far Borek  ;) It has being very valuable.





Offline Astrokel

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Re: redox equilibrium (Is their such a thing?) Quick questions!!
« Reply #21 on: August 30, 2008, 01:00:14 AM »
Quote
Also we just did a prac  with Carbon inert electrode and KI solution in one half cell.

Reduction occured in that cell. I2 + 2e ------> 2I-
Now there was no I2 to begin with so how did this reaction occur. I checked as you have said with H+ but no spontaneous reaction occurs.

Wow, that's really cool. I always wanted to do many chemistry experiments but so far to it is zero. have fun!!!  :)
No matters what results are waiting for us, it's nothing but the DESTINY!!!!!!!!!!!!

Offline sameeralord

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Re: redox equilibrium (Is their such a thing?) Quick questions!!
« Reply #22 on: August 30, 2008, 03:09:51 AM »

EDIT: So basically I- ions in the KI would just shift the equilibrium right?

They cause it I mean not shift . I- and H+ ions react to cause this equilibrium. Right?

Quote
Also we just did a prac  with Carbon inert electrode and KI solution in one half cell.

Reduction occured in that cell. I2 + 2e ------> 2I-
Now there was no I2 to begin with so how did this reaction occur. I checked as you have said with H+ but no spontaneous reaction occurs.

Wow, that's really cool. I always wanted to do many chemistry experiments but so far to it is zero. have fun!!!  :)

  ;D We don't too much. Just few pracs. Don't worry Astrokel you'll do lots of pracs in the future as I'm sure you'll do a good career in chemistry  ;)

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