Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The Initial Reaction of a Voltaic Cell

  1. Mar 8, 2015 #1
    I just want to start by saying that I have found many questions very similar to this one using the search bar, but I still don't understand exactly what happens initially in a simple voltaic cell with a salt bridge (Daniel cell).

    For example, I am trying to figure out what initially happens in a cell with a zinc anode in zinc sulfate solution connected with an external circuit to a separate jar with a copper cathode in a copper sulfate solution. The salt bridge contains potassium chloride.

    Because I am concerned about the very beginning of the flow of electrons in the voltaic cell, sources say that the salt bridge does not come into effect yet due to the lack of positive zinc ions. So the charges do not make it through the full circuit yet. Okay, that's fine.

    So what is the initial reaction that starts this initial one-way flow of charges: zinc is oxidized, copper is reduced, both simultaneously or some other reason?
     
  2. jcsd
  3. Mar 8, 2015 #2

    Bystander

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    ... , and, the salt bridge maintains the charge balance (zero) in both sides of the cell.
     
  4. Mar 8, 2015 #3
    But I am missing the reason why the electrons flow from the zinc to the copper in the first place. Even if they oxidize and reduce simultaneously, what initially causes the copper to be reduced and at the same time that the zinc to be oxidized. I don't understand the initial driving forces.
     
  5. Mar 8, 2015 #4

    Bystander

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Copper ions oxidize zinc metal; or, zinc metal reduces copper ions, either way you care to think about it. You have been introduced to free energies of chemical reactions?
     
  6. Mar 9, 2015 #5
    I think I understand. Just to be sure, if I touched solid copper to solid zinc instead of connecting them with a conducting wire, would the copper oxidize some of the zinc atoms?

    I don't think so.
     
  7. Mar 9, 2015 #6

    Bystander

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Just looked at previous threads, and, you're being "eased" into the concept. It's pretty much the same thing as is going on with the plating out of silver on the copper strip, or this thread https://www.physicsforums.com/threads/oxidizing-and-reducing-agents.799828/ , and hopefully, they'll get to the "bottom line" and give you a few rules and a table of redox potentials to use for this type of problem fairly soon. Copper is a less reactive metal than zinc, and that means that copper ions are more reactive toward zinc metal (corrosive, likely to oxidize zinc, whatever) than zinc ions are toward copper metal.
     
  8. Mar 9, 2015 #7

    DrDu

    User Avatar
    Science Advisor

    Entropy
     
  9. Mar 9, 2015 #8

    Borek

    User Avatar

    Staff: Mentor

    To be honest, I don't see how it addresses the problem.

    We remove ion on the copper side from the solution and put it on the electrode surface. Entropy goes down.

    We remove an atom on the zinc side from the electrode and put it into the solution. Entropy goes up.

    As a first approximation final entropy doesn't change, so it can't be a driving force.
     
  10. Mar 9, 2015 #9

    DrDu

    User Avatar
    Science Advisor

    Sorry, I didn't have the possibility to write much. As soon as you bring an electrode into the corresponding solution, some of the metal will dissolve and some get deposited. Which of the two reactions predominates depends on the relative stability of the ions vs. metal. This stability changes then when the electrodes start to charge up, which is a very fast process. As long as there is no external current, there will also be no internal current through the salt bridge, as each half cell will stay overall neutral.
     
  11. Mar 9, 2015 #10
    I think I found the answer. My textbook seems to say that the copper atoms of the anode will actually oxidize the zinc atoms of the cathode with or without the solution.

    Does anyone know if this is accurate?
     
  12. Mar 9, 2015 #11

    Borek

    User Avatar

    Staff: Mentor

    I am trying to imagine what it means. I have two pieces of metal, I touch them - and what? Zinc cations jump out into the air?

    Definitely there will be some potential difference created by the junction, that's the thermoelectric effect at work.
     
  13. Mar 9, 2015 #12
    I am not sure. It seems to be the only way thing that makes sense to me. I don't know what else could cause the flow of electrons?
     
  14. Mar 10, 2015 #13

    Borek

    User Avatar

    Staff: Mentor

    For the record: zinc cations will NOT jump into the air.
     
  15. Mar 10, 2015 #14
    So then what else would be the initial reaction?
     
  16. Mar 10, 2015 #15

    Bystander

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    If you're asking what initiates the reaction, it's when the circuit is closed and electrons are able to flow. Open circuit, no flow, no reaction. Closed circuit, electrons flow, copper is reduced and zinc is oxidized.
     
  17. Mar 10, 2015 #16

    Borek

    User Avatar

    Staff: Mentor

    No idea, that's why it caught my attention.
     
  18. Mar 10, 2015 #17

    Borek

    User Avatar

    Staff: Mentor

    There is a potential difference even in the open circuit, so there is _some_ initial reaction involved. Electrodes become charged the moment they are immersed in the solution, even if the circuit is otherwise open.
     
  19. May 10, 2015 #18

    James Pelezo

    User Avatar
    Gold Member

    Check out this post.
    https://www.physicsforums.com/threads/galvanic-cells-and-salt-bridges.812134/
     
  20. May 10, 2015 #19
    Hey,

    a little bit of hijacking here, but anyways: In Daniell cell, why SO4 ion does not get bonded with Zinc at the electrode interface? I guess it has to do something with reduction potential or is it because its a soluble salt? But in that case, would that mean that solvent bondings are keeping SO4 ion so tightly? Or would SO4 ion get initially attached and then solvated again?
     
  21. May 11, 2015 #20

    Borek

    User Avatar

    Staff: Mentor

    Solubility is the only thing that matters here.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: The Initial Reaction of a Voltaic Cell
  1. Voltaic (Daniell )cell (Replies: 3)

Loading...