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(gen chem) whats the difference ?

Discussion in 'MCAT Study Question Q&A' started by doxycycline, Jan 6, 2009.

  1. swamprat

    Physician Classifieds Approved 10+ Year Member

    Jul 25, 2008
    Likes Received:
    Attending Physician
    I remember seeing this formular: EMF = Ecat -Eanod

    But now I also see EMF = Eox + Ered.

    Did I just make that first equation up? I def saw it somewhere I can't find it in my review book though.
  2. futuredoctor10

    Classifieds Approved 10+ Year Member

    Jan 25, 2007
    Likes Received:
    Resident [Any Field]
    These are just two conventions for writing the EMF formula but both work.

    If you write the two equations (half-reactions) so that one is undergoing reduction and one is undergoing oxidation, and you change the value of E by inverting the sign, then you can just add the electric potentials.
    ** if reactions are written so that they are undergoing reduction (convention), you would flip the reaction [so it will undergo oxidation from L-->R] AND flip the sign.

    If you identify two half-reactions but DO NOT flip the reaction/sign, then you need to use EMF = Ecat -Eanod.

    Does that make sense?

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