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Voltaic Cells and electron flow

  1. Nov 8, 2011 #1
    In a voltaic cell, why do electrons flow at all? If I place a piece of zinc metal into a zinc ion solution, nothing happens, right? Likewise if I place a copper electrode into a copper ion solution, nothing happens. When I attach the half cells with something that allows electrons to flow through it, then you get voltage from the electrons of the zinc going to the copper electrode and the copper ions becoming attracted to the electrons. (This is assuming I've attached a salt bridge as well).

    My questions is: why does attaching something that allows for the flow of electrons actually mean that any electrons will flow? Why doesn't the zinc half cell and the copper half cell just sit there and nothing happens? The electrons can't be attracted enough to the copper to flow through a wire just to get to the copper ions, can they?

    I'm a bit confused. Any help would be great!
     
  2. jcsd
  3. Nov 9, 2011 #2

    Borek

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    Think about it this way - if you put piece of zinc into solution of copper ions, you will see reaction, right? Copper will get reduced on the zinc surface. In the voltaic cell exactly the same happens, we just artificially separate places where both processes (oxidation of zinc, reduction of copper) take place. So the reason behind the reaction is the same in both cases.
     
  4. Nov 9, 2011 #3
    It's still a bit odd though. The copper directly interacting with the zinc makes sense, because the actually atoms are interacting, but the voltaic cell is indirect. Yeah the anode has a higher electric potential and such, but just because I stick a wire between them means that electrons will start flowing between them? Obviously that is what's happening, in a sense, but it seems weird.

    If I have a zinc electrode placed into zinc ion solution, and stick a wire into it (not connected to anything else, just a wire straight up) do electrons start traveling up the wire then? If so, why? If not, then the there needs to be an attraction that spans the wire or travels through it. I can't seem to get a good grasp of this.

    Thanks!
     
  5. Nov 9, 2011 #4
    Yep. You can test this out for yourself. Take a look at this:

    if you attach the negative terminal of a battery to the lead of the electroscope then electrons travel up the wire and when they reach the aluminum foil, the pieces of spread out cuz electrons repel each other. Similarly if you dip the lead of the electroscope into your zinc electrode/zinc ion half cell, it should spread out a bit. Thats a guess though, I've never tried this before, I'm gonna build one of these electroscopes one of these days and start experimenting with it.

    To answer your question, the electrons travel up the wire cuz its not as negatively charged up there. Charge always tries to neutralise itself. If you hook a positive charge up to a wire then electrons from the wire will travel towards the positive charge. The purpose of a battery is it regenerates the charge as soon as its neutralised. The only problem with using a half cell (such as a zinc electrode submerged in a zinc ion solution) is that the build up of positive charge in the solution (which happens because for electrons to leave the zinc metal, zinc cations first have to break off the zinc metal and into the solution) will attract the electrons back into the solution. This is the purpose of the salt bridge. The salt bridge contains anions that can flow into the half cell and keep it neutral. Sorry for giving such a bad explanation there. Its tricky to explain. Took me a good while to get my head around these concepts too. Keep at it and it will all fall into place for you. I learned this by watching youtube videos. I'll try and find the videos I used for you cuz animations can explain these concepts far better than words can.
     
    Last edited by a moderator: Sep 25, 2014
  6. Nov 9, 2011 #5

    Borek

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    System will reach some kind of equilibrium. Either Zn2+ from the solution will try to deposit on the surface (charging the wire with +2e), or Zn atoms from the surface will try to get into solution (as Zn2+ - so electrode will be left with 2 excess electrons, that is -2e charge). In fact both things will happen at the same time, just one will dominate (which one depends on the metal and concentration of ions in the solution). That will mean electrode will get slightly charged - and this charge is a source of potential difference. And yes, this charge travels throughout the wire - after all, metals are good conductors.
     
  7. Nov 9, 2011 #6

    Borek

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    Staff: Mentor

    You will need extremely sensitive electroscope for that to work. But you are right in principle.
     
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