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Author Topic: A 10 euro (overrated) Daniel cell experiment part II  (Read 4957 times)

Offline sorincosofret

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A 10 euro (overrated) Daniel cell experiment part II

Materials for the experiment:
Cu and Zn metals strips;
CuSO4 as 1M solutions;
H2SO4 as 1  up to 3 M solution.

The purpose of experiment is to dissect how a  ,,formal” Daniel cell works and the importance of every electrode in the process.

For beginning the simplest electric cell is build using CuSO4 as electrolyte and a strip of Cu and Zn inserted into solution.


At immersion the quite normal potential (1.054) is displayed. After few seconds from immersion, something strange begins to manifest. The Zn strip starts to become dark (even black). I considered this only a visual effect due to the dissolution of Zn atoms from strip. But, I was wrong, because at a detailed analysis it can be observed that Cu ions from solution form a deposit on the Zn strip. At the Copper electrode there is no visual sign of any chemical reaction.
This is a picture of cell after one hour of working with voltmeter connected to it.



After another 2 hours the electrodes in glass appear as follows:



After about 4 hours the Zn electrode is completely disintegrated, but in its region a brown-dark fluffy compound remain.
When this effect was observed quite long time ago ( about 3 years), I was thinking in a experimental error.
But, numerous repetition of experiment convinces me about the reality of this effect.
Of course the effect is the same if the compounds are changed. For example putting in a H2SO4 solution a Cu and Zn strip connected to a voltmeter, the hydrogen is developed at Zn electrode.
Faced with this problem, in evident contradiction with known percepts of electrochemistry -oxidation and reduction took place at different electrodes-, I was trying to find a reasonable explanation.
The deposit remaining in the Zn region was separated, washed with diluted nitric acid in order to eliminate the traces of Zn and after that reacted with concentrated nitric acid. As anyone can suppose, the deposit dissolves in concentrate nitric acid, so the deposit is formed by copper.
Other question arise…
If the electric current is produced due to the electrons movement and both processes took place at the same place, how is possible to appear a difference of potential ?
Again, I will leave to the ,,high authorities” a reasonable explanation of this phenomena in the frame of actual physics; of course after they learn how to connect two wires in order to obtain a electrochemical cell.
In order to have an intuitive explanation of what’s happened in a formal Daniel cell, the relief of next experiment is colossal.
As observed in the following picture two cells are formed, one with a metallic conductor between Cu and Zn electrodes  (cell 1) and another with a salt bridge between electrodes (cell 2).



Cell 1.



Cell 2

As is observed in the pictures, after immersion in the CuSO4 solution, the Zn electrodes become darkened in both cells.
The cells are leaved for one day and electrodes consume in every cell is figured out in the following pictures.





Cell 1. View from edgewise

« Last Edit: 09/08/2008 00:00:12 by sorincosofret »


 

Offline sorincosofret

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A 10 euro (overrated) Daniel cell experiment part II
« Reply #1 on: 09/08/2008 00:02:52 »
and further....


Cell 2. View from edgewise



Cell 2 Upper view

As is observed, in case of a simple metallic conductor between electrodes, the phenomena are evolving, in principle, according to actual interpretation. More precisely, without taking into consideration the direct reaction between Zn and CuSO4, it can be considered that Zn is dissolved and Cu is deposited on the Cu electrode.
There is also a direct reaction between Cu SO4 and Zn, and a deposition of Cu as secondary reaction.

The strange comportment of the cell manifests in case of salt bridge connected between electrodes. In this case both reactions (oxidation and reduction take place at the same electrode). The situation is similar with the case when voltmeter is connected to the circuit.

I live again the interpretation of these results in the frame of actual physics in the custody of ,,high authorities” who read the post.
The proposed interpretation will be found in the book.

There will be a last 3rd post regarding Daniel cell; the subject of post will be: how is working a non-workable Daniel cell?
After that … only the subject will be changed…. But the ,,Show most go on ….”
« Last Edit: 09/08/2008 00:05:22 by sorincosofret »
 

Offline Bored chemist

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A 10 euro (overrated) Daniel cell experiment part II
« Reply #2 on: 09/08/2008 18:56:09 »
"Faced with this problem, in evident contradiction with known percepts of electrochemistry -oxidation and reduction took place at different electrodes-, I was trying to find a reasonable explanation. "

Once again, Sorin finds a contradiction where ther isn't one. Here's the reasonable explanation.
If you put a piece of zinc in copper sulphate solution a reaction takes place.
CuSO4 + Zn --> Cu + ZnSO4
This happens wherther or not there's another electrode present connected via a meter or whatever.

To make a Daniel cell you need some sort of salt bridge to keep the copper sulphate solution away from the zinc.
 

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A 10 euro (overrated) Daniel cell experiment part II
« Reply #2 on: 09/08/2008 18:56:09 »

 

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