electrochemistry

pratik7

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  1. Attending Physician
    hey guys

    I am having trouble understanding which electrode is positive or negative in both the electrolytic cell and galvanic cell. I know in the electrolytic cell the cathode is positve and anode is neg; the opposite is true with the galvanic. But I cant seem to figure out why.

    Any help is appreciated
     

    ASDIC

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      hey:

      In the electrolytic cell, oxidation occurs at the anode and reduction occurs at the cathode. So the anode is rich with electrons as the element is oxidized. So the anode is negative and the cathode is positive.

      In a galvanic cell, oxidation occurs at the anode and reduction occurs at the cathode. However here, the reaction is spontaneous, so any electrons released at the anode are immediately used up in the reduction at the cathode. So the cathode accumulates the electrons. Therefore, the cathode is negative and the anode is positive (as it rapidly loses its electrons).
       

      jhk43

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        current always flows from + to -. but of course, electrons flow from - to +.

        in a galvanic cell, its easy to see that electrons flow from - to +.

        in an electrolytic cell, G>0, so you need a battery to pump the electrons. the sign convention comes from the +/- orientation of the battery supply. At the anode, you still have oxidation (releasing electrons) but it must flow to a lower potential (less negative or +). At the cathode, you still have reduction (absorbing electrons), so u need to supply that side with electrons from the battery (-).

        look at the diagram, try flipping the battery terminals around, and see why it wouldnt work.
         
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        Drakensoul

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          Originally posted by thekegalman
          hey:

          In the electrolytic cell, oxidation occurs at the anode and reduction occurs at the cathode. So the anode is rich with electrons as the element is oxidized. So the anode is negative and the cathode is positive.

          In a galvanic cell, oxidation occurs at the anode and reduction occurs at the cathode. However here, the reaction is spontaneous, so any electrons released at the anode are immediately used up in the reduction at the cathode. So the cathode accumulates the electrons. Therefore, the cathode is negative and the anode is positive (as it rapidly loses its electrons).

          Pretty sure that's backwards, and JHK has it right.. Galvanic cells are the normal cells, and the anode is negative, cathode is positive..

          Electrons flowing to the cathode doesn't make it negative; the electrons are taken up by whatever the ions are in the solution. If it's something like Cu+2, the copper will plate out on the cathode, but the cathode is still positive. If you look at it from a field-perspective in the wire, the field goes from the cathode side to the anode side, and electrons are accelerated opposite the direction of the field within the wire.
           

          willthatsall

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            Originally posted by pratik7
            thanks guys.....i think i understand

            but i have one more followup question. Assume that the anode is negative... will a positive ion always be oxidized at that anode, and is the opposite also true??


            thanks again


            From what I've read so far, I think oxidation always occurs at the anode, reduction always occurs at the cathode. In a galvanic cell, the solid at the anode is oxidized into a cation, and the electrons produced from the oxidation are what make the anode negative. In the electrolytic cell, anions are oxidized by the positive anode. In both cases, electrons flow from anode (oxidation) to cathode (reduction) but the charges are different on the anode and cathode. Hope that helps.
             

            Drakensoul

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              Originally posted by pratik7
              thanks guys.....i think i understand

              but i have one more followup question. Assume that the anode is negative... will a positive ion always be oxidized at that anode, and is the opposite also true??


              thanks again

              Need to straighten this out. The ions are not oxidized, unless you're dealing with a non-reactive electrode. In a simplistic galvanic cell, the electrode is what is oxidized. Example, if you have a Zinc electrode as the anode, the Zinc electrode itself will oxidize, and drop zinc ions into the solution of the anode compartment. The electrons travel to the copper cathode, which is resting in a copper ion solution. The copper ions take up the electrons from the Zn anode, and plate out as copper metal onto the copper cathode.

              So you have the anode electrode oxidizing, which allows electrons to flow (through some wire, and load-resistor like a lightbulb or amp meter) to the cathode. To keep the charges balanced, anions flow from the cathode compartment to the anode compartment, and cations flow from the anode compartment to the cathode compartment, both of which occur through the salt-bridge.

              If you're dealing with non-reactive electrodes, then it is still the anode reaction that oxidizes. Whatever that reaction is.
               
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