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Anode vs cathode?

  1. Jul 10, 2008 #1
    anode vs cathode???

    Hello everyone :)

    I am having a problem understanding the difference between the anode and cathode in electrochemistry.

    What I know so far is that the anode is the electrode at which oxidation takes place (and hence donates electrons) and the cathode is the electrode at which reduction takes place (and hence accepts these electrons). Is this true? Please correct me if not.

    Then, I am very confused regarding the charges on the each of the electrodes. Which is positive and which is negative? In my chem book (Chemistry: The Central Science by Brown, LeMay, and Bursten) it is says that "although the anode and cathode are labeled with - and + signs respectively, you should not interpret the labels as charges on the electrodes. The labels simply tell us the electrode at which the electrons are released to the external circuit (the anode) and received from the external circuit (the cathode). The actual charges on the electrodes are essentially zero."

    Then I read on Wikipedia that the anode and cathode are charged. It states that "anode polarity is not always positive but depends on the device type, and sometimes even in which mode it operates, as determined by the above electric current direction-based universal definition." and that "cathode polarity is not always negative but depends on the device type, and sometimes even in which mode it operates, as determined by the above current direction based universal definition." They say that the charges of the anode and cathode differ depending on whether they are a part of a Galvanic cell or an electrolysis cell.

    I just don't get it! I am totally confused. Can anyone help me out please?

    Another question: in a conventional (dry-cell) battery, does the positive sign refer to the anode or cathode?

    Thank you so much and sorry for my looooong question.
     
  2. jcsd
  3. Jul 10, 2008 #2

    Borek

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    Staff: Mentor

    Re: anode vs cathode???

    Cathode and anode are charged, but their charges are very small. Small enough that statement "essentially zero" is correct.

    OIL RIG - Oxidation Is Loss, Reduction Is Gain. Reduced substance Gains electrons. For that to happen electrode where reduction goes must be able to donate electrons.

    Cathode is the electrode that attracts cations. Cations are positively charged, they will be attracted by negativey charged electrode. Negatively charged electrode is the one with excess electrons that it can donate, thereby reducing cations.
     
  4. Jul 10, 2008 #3
    Re: anode vs cathode???

    Thank you Borek

    So the electrodes are charged (slightly), but I still don't get which is negative and which is postitive :shy: I would suppose that the anode is positive (because it looses electrons and attracts anions) and the cathode is negative (because it gains electrons and attracts cations). Is this right? In some textbooks I find the opposite :confused:

    And does the charge change depending on the type of cell (galvanic or electrolytic)?
     
  5. Jul 10, 2008 #4

    Borek

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    Re: anode vs cathode???

    What I have described earlier is an electrolytic cell - cation attracted by cathode, cation positive, so cathode must be negative and so on. That's my starting point, you better don't move ahead till it sinks :wink:

    For the galvanic cell, you have to reverse the situation. Cations that were reduced now want to get back to the solution, so they will "oxidize themselves" giving away electrons. During electrolysis electrons were transferred to the electrode to be put into cations, now they are running back - so cathode becomes an anode.
     
  6. Jul 10, 2008 #5

    turbo

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    Re: anode vs cathode???

    The terminology "anode" and "cathode" are applied to describe the functions of these elements of a circuit. The function can change depending on the situation, so we can't necessarily assign these names based on the electrical potential at these elements. If you will Google on the two terms, you will probably get more information than you can easily digest, but hopefully there is a web-page somewhere that treats the distinction in detail without being overly technical.
     
  7. Jul 11, 2008 #6
    Re: anode vs cathode???

    OK so the terms "anode" and "cathode" are used to describe the function of the electrode. What they are saying on wikipedia means that the function of the electrode changes when the type of cell is changed. Right?

    Last question: which is the anode and the cathode in a dry-cell battery? (Do we use the + or - sign for the anode/cathode?)

    Thank you so much everyone :smile:
     
  8. Jul 11, 2008 #7

    symbolipoint

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    Re: anode vs cathode???

    Here, you are describing a battery:
    So that is galvanic - the system has stored energy and if the circuit could be completed, the system would like to discharge. Electrons (negative) will flow from the negative electrode, where you find the "minus" sign. The electrons will flow to the positive electrode with the "plus" sign.
     
  9. Jul 24, 2008 #8
    Re: anode vs cathode???

    Thank you everyone. I got an idea now.

    Best regards :smile:
     
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