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Topic: Increasing concentration of solution in a simple galvanic cell  (Read 19192 times)

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Offline johnny90

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Increasing concentration of solution in a simple galvanic cell
« on: February 28, 2009, 10:22:33 AM »
Hello!

For a chemistry lab we were required to make a simple galvanic cell (both metals are placed into the same beaker containing KNO3) and increase the concentration of the solution both metals are placed in. My group found that an increase in the concentration of the KNO3 solution leads to a decrease in the voltage produced. Is this correct? And if so, can anyone please explain why this happens.

Thanks in advance!

Offline Astrokel

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Re: Increasing concentration of solution in a simple galvanic cell
« Reply #1 on: February 28, 2009, 05:15:06 PM »
What are your half cells?
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Offline johnny90

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Re: Increasing concentration of solution in a simple galvanic cell
« Reply #2 on: March 01, 2009, 03:38:18 AM »
That's the thing, there are none. I have a single beaker filled with KNO3. Then I have two metals (copper and iron) which are connected to a voltmeter and both of them are placed into that beaker with the KNO3 solution (they are not touching). My question now is: will the voltage increase or decrease if the concentration of the KNO3 solution inside the beaker is increased? And why?

Offline Astrokel

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Re: Increasing concentration of solution in a simple galvanic cell
« Reply #3 on: March 01, 2009, 04:30:38 AM »
The voltage will decrease as concentration of KNO3 increases due to molar conductance of your electrolytic solution increases.

http://en.wikipedia.org/wiki/Molar_conductivity
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Offline johnny90

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Re: Increasing concentration of solution in a simple galvanic cell
« Reply #4 on: March 01, 2009, 05:02:16 AM »
Exactly what I needed! Thank you so much!

EDIT: Hold on, are you sure the molar conductivity increases when the concentration increases? I just found this: http://answers.yahoo.com/question/index?qid=20080815093230AAM41x8 - they guy states that as the concentration increases, the molar conductivity decreases. Otherwise I wouldn't see how an increase in the molar conductivity leads to a decrease in the voltage...
« Last Edit: March 01, 2009, 05:12:23 AM by johnny90 »

Offline Astrokel

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Re: Increasing concentration of solution in a simple galvanic cell
« Reply #5 on: March 01, 2009, 06:57:43 AM »
You are right i made a mistake. Should be..

The voltage will decrease as concentration of KNO3 increases due to molar conductance of your electrolytic solution decreases. Besides basing on the definition like the guy mentioned, you could think it as when concentration of KNO3 increases, the oppositely charged ions K+ and NO3- exerts greater ionic influences on each other so it hinders the ease of electricity flowing because the ions will 'stick' with themselves better, thus voltage decreases.
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Offline johnny90

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Re: Increasing concentration of solution in a simple galvanic cell
« Reply #6 on: March 01, 2009, 09:16:25 AM »
That makes perfect sense! Thanks a lot! :)

Offline Borek

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Re: Increasing concentration of solution in a simple galvanic cell
« Reply #7 on: March 01, 2009, 09:21:35 AM »
Unless I am missing something - it doesn't make sense.

If you increase concentration of the ionic salt dissolved in the solution, solution resistivity decreases and conductivity increases.

Molar conductivity has the greatest value for the infinitely diluted solution, but it is molar - per mole of dissolved substance. When you increase concentration molar conductivity goes down, but the real, observed conductivity of the solution goes up, as increase of the ions concentration is much more important.
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Offline enahs

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Re: Increasing concentration of solution in a simple galvanic cell
« Reply #8 on: March 01, 2009, 09:27:25 AM »
Maybe I am reading it wrong. But you have the two electrodes in the same beaker and solution? You should not measure a voltage at all. They need to be separated, with a salt bridge to keep them neutral.

Are you measuring the "voltage" of the solution, or leads connecting the two electrodes?


Offline Astrokel

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Re: Increasing concentration of solution in a simple galvanic cell
« Reply #9 on: March 01, 2009, 09:40:52 AM »
Borek, do you mean electrical conductivity when you mentioned real observed conductivity?

enahs, I agree with you there will be no reaction. i think what TS is doing is its electrolytic solution.
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Offline johnny90

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Re: Increasing concentration of solution in a simple galvanic cell
« Reply #10 on: March 01, 2009, 09:47:33 AM »
Maybe I am reading it wrong. But you have the two electrodes in the same beaker and solution? You should not measure a voltage at all. They need to be separated, with a salt bridge to keep them neutral.

Are you measuring the "voltage" of the solution, or leads connecting the two electrodes?

Yeah, I do have both electrodes in the same beaker and solution. I am getting a voltage, which is what I'm measuring, so it must work ???

Offline Borek

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Re: Increasing concentration of solution in a simple galvanic cell
« Reply #11 on: March 01, 2009, 11:52:22 AM »
Borek, do you mean electrical conductivity when you mentioned real observed conductivity?

Yes.

But you have the two electrodes in the same beaker and solution? You should not measure a voltage at all.

If these electrodes are made of different metals (like Zn and Cu) you will measure nice voltage. That's not much different from the lemon battery.

http://www.youtube.com/watch?v=AY9qcDCFeVI
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Offline enahs

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Re: Increasing concentration of solution in a simple galvanic cell
« Reply #12 on: March 01, 2009, 12:34:05 PM »
I was wrong in saying you should not measure a voltage.

But this is not a battery nor a galvanic cell, this is just galvanic corrosion.

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