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why do anions travel to the anode if the anode is negatively charged in galvanic reactions?

Discussion in 'MCAT Study Question Q&A' started by mrh125, Jan 14, 2014.

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  1. mrh125

    mrh125 Banned Banned Account on Hold

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    after going through this stuff a few different times it still trips me up. In galvanic cells the anode is negatively charged (-) and the cathode is positively charged, so why do anions go to anion and cations go to the cathode? I don't get it.

    I know current goes from cathode to anode and electrons go from anode to cathode, but i'm lost with this idea.
     
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  3. Lifeman

    Lifeman 7+ Year Member

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    It's not really negatively charged, just at a lower potential. Since oxidation happens at the anode, like Ag -> Ag+ + e-, we are gaining positive charge in solution so in order to balance it out we need to put negative ions in solution; otherwise if too much positive charge builds up in the solution, Ag will not oxidize anymore.
     
  4. IlyaR

    IlyaR 2+ Year Member

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    I could see how the ambiguity can get annoying

    In a cell, + charges move to the anode terminal of an EMF source, and - charges move to the cathode terminal.

    an anode PLATE however, accumulates a positive charge from all of the + charges moving to it from the connected cathode terminal

    [​IMG]
     
  5. mrh125

    mrh125 Banned Banned Account on Hold

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    hmm, im still a bit confused. dont the signs of anodes and cathodes plates change in electrolytic cells? I keep seeing anodes listed with negative charges and cathodes with + positive signs in galvanic cells. Does what you wrote change in chem and physics or something?
     
  6. IlyaR

    IlyaR 2+ Year Member

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    Anode PLATE keep in mind. The term can change meanings from chem/physics/biochem etc, since you can think of the cathode PLATE*** core as + charged, or wholly - charged due to the build up of - charges traveling from the anode to it.

    This is why in Gel Electrophoresis, the negatively charged DNA molecules move towards the anode plates


    In my drawing, the EMF source on top is Anode (which is negative) on the left, and cathode (which is positive) on the right. Should have labeled
     
  7. Odi

    Odi

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    When it comes to galvanic cells don't think of charges for anodes and cathodes or it gets confusing. You really have see the bigger picture and think oxidation/reduction.

    Let's say you have ZnSO4 in the anode solution and CuSO4 in the cathode solution. Your salt bridge is KCl (draw this out if it helps).

    Oxidation occurs in the anode. Zn is oxidized to Zn2+: and these electrons flow up the anode. The solution therefore builds up a positive charge (thanks to all the left over Zn2+ ions). If this (+) charge builds up too high the reaction will stop. Thus the salt bridge releases anions (Cl-) into the anode solution to balance this build up of Zn2+ charges from all the oxidation reactions. This is why negatively charged anions flow towards the anode.

    Reduction occurs in the cathode. Cu2+ is reduced to Cu(s) on this side. The solutions thus becomes negatively charged thanks to the left over SO4^2- ions that are now dominating the solution. If this (-) charge builds up too high the reaction will stop. Thus the salt bridge releases cations (K+) into the cathode solution to balance this build up of SO4^2- charge from all the reduction reactions. This is why positively charged cations flow towards the cathode.
     
  8. neurodoc

    neurodoc Neurologist 10+ Year Member

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    If you think about the flow of electrons into or out of the "cell" it makes more sense. This is a case where knowing the
    etymology (Greek) of the words "cathode" and "anode" can help. The "ode" comes from the Greek, odos for "road." Ana
    and kata are prepositions that respectively mean "up" and "down." In this case, up and down are in reference to the
    electrolytic or galvanic cell, and regard the flow of electrons. In batteries the electrons take the road "up" and in electrolytic
    cells, the road "down" into the cell.
     

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