[johnny90]

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!

[Astrokel]

What are your half cells?

[johnny90]

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?

[Astrokel]

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

[johnny90]

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...

[Astrokel]

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.

[johnny90]

That makes perfect sense! Thanks a lot!

[Borek]

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.

[enahs]

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?

[Astrokel]

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.

[johnny90]

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

[Borek]

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

[enahs]

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.