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Author Topic: Galvanic cell  (Read 1373 times)

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mariusz504

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Galvanic cell
« on: December 05, 2012, 03:27:04 AM »

Welcome, I'm student from Poland (my English isn't very well) and I've got some problem. I've built galvanic cell. There were 2 test-tubes (with extra orifice) linked with rubber pipe (there were any salt bridge). Then, I filled this system with cuprum (II) sulfate (molar concentartion is 1 mol/dm3). Next, I put 2 cooper wires in these 2 test tubes and linked with a voltmeter. The voltage was 0V. Afterwards, I heated one half-cell almost to boiling and it tuned out, that this cell generated electric current. Electrode, which was being submarged in heated semi-cell, was covered with red sediment. To my mind, this is a cathode. Do you agree? Can you explain me how is the influence of temperature on voltage in this cell. Does any equation exist, which is connected with this experience. I Think, that the Nernst equation isn't about that, since the molar concentartion of solution was 1 mol/dm3.
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Borek

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Re: Galvanic cell
« Reply #1 on: December 05, 2012, 04:29:11 AM »

Write the Nernst equation for a half cell.
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mariusz504

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Re: Galvanic cell
« Reply #2 on: December 05, 2012, 04:55:18 AM »

E=E° + 2,303RT/n log[Cox], but log[Cox] is 0 because C=1mol/dm3 but I'm not sure.
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Borek

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Re: Galvanic cell
« Reply #3 on: December 05, 2012, 05:37:30 AM »

While concentration is 1M, activity of Cu2+ is not 1 - 1M CuSO4 solution has a high ionic strength. Hence the RT/F can't be ignored.
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mariusz504

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Re: Galvanic cell
« Reply #4 on: December 05, 2012, 05:44:44 AM »

So, Does the Nernst equation correct in this experience? Can we count concentration of ions Cu 2+?
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Borek

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Re: Galvanic cell
« Reply #5 on: December 05, 2012, 07:57:36 AM »

Yes & yes.

And on this and several other forums, I am probably the only person able to understand what you meant  ::)
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mariusz504

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Re: Galvanic cell
« Reply #6 on: December 05, 2012, 08:30:04 AM »

probably yes :D
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mariusz504

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Re: Galvanic cell
« Reply #7 on: December 05, 2012, 08:36:48 AM »

So, could you explain me what is a method of counting concentartion of ions 2+.
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Borek

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Re: Galvanic cell
« Reply #8 on: December 05, 2012, 09:26:38 AM »

Now you've lost lost me.

(note - you want to calculate concentration, you can count fingers, or coins, or any other objects, but you calculate concentration, logarithm, speed an so on; some things are countable, some are not)

You wrote concentration is known and equals 1M, no need to calculate it.
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mariusz504

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Re: Galvanic cell
« Reply #9 on: December 05, 2012, 09:41:47 AM »

so logC=0 because 10^0=1 (C=1M), yes?
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Borek

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Re: Galvanic cell
« Reply #10 on: December 05, 2012, 11:20:15 AM »

No.

While usually we write Nernst equation using concentrations, in fact it should contain not concentrations, but activities of ions. In fact everywhere you use concentrations (like all equilibrium calculations) you should use activities, not concentrations. For very diluted solutions, with ionic strength close to 0, activity equals concentration, but the higher the ionic strength, the higher the difference (to make things more difficult, the difference between activity and concentration doesn't change in an easy to predict and calculate way).

Sure, there exist a standard solution with copper(II) activity equal 1, and then e0=1 would hold. However, even then it doesn't mean potential is temperature independent, as changing temperature of the solution means changing the volume, changing the concentration, changing the activity - and it is will be no longer 1.
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mariusz504

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Re: Galvanic cell
« Reply #11 on: December 06, 2012, 06:04:58 AM »

Now I understand. So in fact, we arent't able to calculate (using easy methods) potential of electrodes in this situation, are we?
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Borek

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Re: Galvanic cell
« Reply #12 on: December 06, 2012, 06:54:37 AM »

No way to get the exact result starting calculations from the first principles. There are some possible approximations and methods that will get you a reasonably correct result mixing theory and experimental data.
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mariusz504

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Re: Galvanic cell
« Reply #13 on: December 06, 2012, 07:31:05 AM »

Do you know them? Are they complicated?
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Borek

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Re: Galvanic cell
« Reply #14 on: December 06, 2012, 07:51:08 AM »

Not easy enough for a single post. At least a chapter in a book (if not a book itself).
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