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Electrochemists brain teaser-copper electrolysis

  1. Feb 14, 2012 #1
    Hi all,
    I have some history in practical electrochemistry, and have, I think, a reasonable uderstanding of the basics of Redox reactions and ½ cells. Recently, I have been thinking about a project, and it has me in a complete quandry. There’s probably something obvious that I’m missing, but here goes.

    To set the scene, take the classic galvanic cell situation in which we have 2 separate compartments. One contains Zn with a molar electrolyte, and the other Cu with same. The Zn will be oxidised, and the Cu reduced with the appropriate potential. We use a salt bridge to connect the 2 compartments in order to combat the build up of Zn ions in one cell, and the depletion of Cu in the other.

    Now imagine an electrolytic cell pair which both contain Cu, and a molar electrolyte. Now, we have an external power supply which supplies the necessary p.d. to cause the reduction of Cu in one cell, and the corresponding oxidation in the other. If we link the 2 with a salt bridge, this should keep going for quite a while- it is a valid and stable situation.

    The power supply ‘expects’ to see electrons leaving the negative pole, and this does indeed happen. They are consumed in reducing the Cu ions. Also, electrons are arriving at the positive pole due to being donated by the oxidising Cu at the anode. This doesn’t need to be a theoretical type cell in which we are looking for full reversibility, and therefore and almost zero current flow, as we can afford to push a bit to get some volume transacting. The requirements are being met. The current flow is being fully maintained by the redox reactions, and is provided by nF electrons flowing, directly connected to the Cu redox rate.
    Instead of a salt bridge, we can use a small pump to circulate the electrolyte to even out the concentrations, because the 2 solutions don’t need to be separated for chemical reasons. The reason for the separation is an operational one.

    Comparing this to a commercial copper electrolyser, how is it that I can’t get out of my head that current density is all about electrode spacing, and that the above won’t happen. I can’t find a problem with running this reaction in the above way. Your arguments would be greatly appreciated.
     
    Last edited: Feb 14, 2012
  2. jcsd
  3. Feb 15, 2012 #2
    Edit: second thoughts for improved clarity.

    My problem is that I think I have argued soundly that the separate copper electroylsis reaction will work continuously. However, instinctively, and reading articles on copper refining, there is no evidence of such a cell operating, and all the information seems to indicate that unless the electrodes are in close proximity, there will be little if any current flow. Current flow seems to be taken as a phenomenon which is controlled by a potential gradient between 2 opposite electrode plates, and governed by solution resistance. It is taken as a flow 'through' the electrolyte, like a resistor in a normal circuit. Thus, if 2 separate tanks were linked by a pipe, the resistance would be so high that it would effectively not work. I think I have argued that the current flow is not through the electrolyte, but by the 2 redox reactions at the electrode surfaces. I can't find a mechanistic argument which will resolve this issue.

    I hope this clarifies the issue, as this is driving me nuts ! I think I,m just going to need to set up the experiment and see what happens. Although that is a perfectly valid scientific thing to do, it seems a little crude that it will be because I can't put a conclusive looking hypothesis together.

    Thanks in advance.
     
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