[defencegrid]

I've setup the experiment below to confirm Avogadro's number by using a galvanic cell.

https://mse.engineering.ucdavis.edu/wp-content/uploads/2014/06/GAANN-lab-2011.pdf

The experiment involves setting up a galvanic cell using a zinc and copper strip both immersed in one beaker of Copper Sulphate, and then connecting both the zinc and copper strips to a multimeter to measure the current.

The experiment worked fine and I was able to determine Avogadro's number.

What I don't understand is why the copper strip lost mass?  As far as I understand, the zinc electrode loses electrons, which flow across the multimeter to the copper electrode, where Cu2+ ions in solution pick up the electrons to form solid copper which deposits on the zinc electrode.  So why is the copper electrode losing mass? 

[KungKemi]

I've setup the experiment below to confirm Avogadro's number by using a galvanic cell.

https://mse.engineering.ucdavis.edu/wp-content/uploads/2014/06/GAANN-lab-2011.pdf

The experiment involves setting up a galvanic cell using a zinc and copper strip both immersed in one beaker of Copper Sulphate, and then connecting both the zinc and copper strips to a multimeter to measure the current.

The experiment worked fine and I was able to determine Avogadro's number.

What I don't understand is why the copper strip lost mass?  As far as I understand, the zinc electrode loses electrons, which flow across the multimeter to the copper electrode, where Cu2+ ions in solution pick up the electrons to form solid copper which deposits on the zinc electrode.  So why is the copper electrode losing mass?

I'm not quite sure about how you came to that conclusion. I took a look at the pdf myself and it states that the copper is reduced (gains mass), and the zinc is oxidised (loses mass.) Besides, if you look at the half-cell potentials of both copper and zinc you will notice that in order for the redox reaction to be spontaneous then copper will have to be reduced, and zinc oxidised.

I hope this helps clear some confusion,
KungKemi

[defencegrid]

Yes, that's what I don't get.  The copper is reduced, so it should not lose mass, but when I weighed the copper electrode after it had been in the copper sulphate solution for five minutes, it had lost 0.01g. 

[KungKemi]

Did you repeat the experiment more than once and see similar trends? If not, it could just be that either this is an anomalous error, there was an equipment error, or some other external error at play. I'm not quite sure, but did you make sure that you followed the procedure carefully?

EDIT: On the contrary, in theory it would be expected that the copper electrode would have increased by ~0.020 g in mass after the given amount of time (300 s) under the influence of 200 mA.

KungKemi

[Borek]

If you can repeat the experiment, try to weigh both electrodes.

[defencegrid]

If you can repeat the experiment, try to weigh both electrodes.

I repeated the experiment as suggested and got the same result.  The Cu electrode lost 0.01g.  I also weighed the Zn electrode, which gained 0.06g.  The Zn electrode was covered in copper. 

It seems as though the Cu electrode is being oxidised and forming Cu2+ which dissolves in solution and is then attracted to the Zn electrode where it is being reduced to Cu which then coats the Zn electrode.  It's basically behaving like an electrolytic cell, even though there's no power source.  Don't know how.

[KungKemi]

I am still very much a beginner in the field of electrochemistry, however, I believe that what you have created is an electrolytic cell. This is due to the application of an external current which is driving the redox reaction to work in a non-spontaneous manner. The process which is occurring is known as electrolysis, and essentially the copper strip is acting as the anode in this system, and the zinc strip (the article being electroplated) acts as the cathode per say.

Faraday wrote a physical law which describes this process: Q = It = zFn, where Q is the charge, I is the current, t is the time, z is the change in oxidation state (2), F is Faraday's constant, and n is the amount of substance oxidised or reduced.

KungKemi

[Borek]

This is due to the application of an external current

Pure nonsense. Where is the source of this applied external current?

[defencegrid]

I believe that what you have created is an electrolytic cell. This is due to the application of an external current which is driving the redox reaction to work in a non-spontaneous manner. The process which is occurring is known as electrolysis, and essentially the copper strip is acting as the anode in this system, and the zinc strip (the article being electroplated) acts as the cathode per say.

Well my results are consistent with an electrolytic cell.  I don't know how that's possible without an external current.  The only explanation I can think of is that the Zn electrode is indeed acting as the cathode.  That would explain why the Zn electrode is covered in copper. 

[KungKemi]

This is due to the application of an external current

Pure nonsense. Where is the source of this applied external current?

Hmm...yes, I quite agree. Sorry for the misconception, the current arises from the resistance against the flow of the electromotive force, where I = V/R. As such, since the current is dependent upon the electromotive force of the cell, it would be expected that the current would change over time (is that correct?) What if there was a constant current in the circuit? Would it be expected that this is the result of an external source or some other means?

EDIT: Would this be a case when the current is constant; since the reduced cupric ions from solution are replaced by the oxidised copper ions from the copper strip? Just out of curiosity.

KungKemi

[Borek]

Hmm...yes, I quite agree. Sorry for the misconception, the current arises from the resistance against the flow of the electromotive force, where I = V/R. As such, since the current is dependent upon the electromotive force of the cell, it would be expected that the current would change over time (is that correct?) What if there was a constant current in the circuit? Would it be expected that this is the result of an external source or some other means?

EDIT: Would this be a case when the current is constant; since the reduced cupric ions from solution are replaced by the oxidised copper ions from the copper strip? Just out of curiosity.

KungKemi

You are again inventing some new physics and chemistry. It doesn't matter whether the current is constant or variable. An electrolytic cell requires an external source of energy - be it a battery, or power supply. No such thing here.

[Borek]

Well my results are consistent with an electrolytic cell.  I don't know how that's possible without an external current.  The only explanation I can think of is that the Zn electrode is indeed acting as the cathode.  That would explain why the Zn electrode is covered in copper.

If there were no copper electrode, Zn would reduce Cu²⁺ from the solution, and that would be perfectly understandable. Presence of the external copper electrode means that the reduction can take place in other places (other that the zinc surface) as well.

To be honest I don't understand the setup nor the result.