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Homework Help: Electrochemistry; redox potential

  1. May 19, 2008 #1
    1. The problem statement, all variables and given/known data

    I have to calculate the redox potential of this equation:
    Cr2(SO4)3 + 4K2SO4 + 3J2 +7H2O <=> K2Cr2O7 + 6KJ +7H2SO4
    I used the nernst equation to calculate the potentials of
    2Cr(3+)+7H2O -> Cr2O7(2-) +14H(+) +6e(-) and
    I2 +2e(-) ->2J(-)
    which were -1.42V and 0.51V, all ion concentrations were 0.1

    Is the overall redox potential the sum of these potentials?
  2. jcsd
  3. May 19, 2008 #2

    Are these J's typos, meaning I instead?

    So you used the Nerst equation to calculate the half cell potentials? The Nerst equation is used to find the overall redox potential for the overall redox reaction, not half cell potential as implied here.

    The standard half cell potentials for these half reactions are used (look up in a table) to calculate Eo, find the number of electrons transferred and use the 0.1 concentrations in the ions for the overall reaction.
  4. May 20, 2008 #3


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    Staff: Mentor

    Interesting. J was iodine symbol up to around 1980, it is used this way in older books.

    It works both ways.

  5. May 20, 2008 #4
    Yes the J was typo, it should be I. DonĀ“t know what was I thinking. Could you explain me, how the Eo is calculated, or what should I be looking for? Is the reaction standard potential calculated between the pair which actually exchanged electrons Cr(3+) and I2, or somehow between the chrome and iodide pairs.
    Last edited: May 20, 2008
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