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It is a simple Delta G naught reaction problem

  1. Jul 25, 2015 #1
    1. The problem statement, all variables and given/known data
    Calculate Delta G naught rxn and K for the reaction:

    2 Fe 3+ +2I- <--> 2 Fe 2+ +I2

    2. Relevant equations

    3. The attempt at a solution
    I came up with -.24 V for E naught and plugged it in to ( -2*96.5*-.24) and got Delta G as 46.32 KJ but I don't know if this is right and i'm not sure if it is delta G naught rather than just delta G.

    Then from that answer ( if it is right ) I put Delta G into ( Keq = e^-delta G naught/ RT) and got 8*10^-9

    Please let me know if I am doing this right. It is due Monday and I need to make sure I am doing it right for final exam on Tuesday. Thank You
  2. jcsd
  3. Jul 25, 2015 #2


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    Try asking this in the chemistry section ... I don't have any tables (and you ignore units except for 46 kJ, which should be kJ/mole)
    ... it's been a long time, but 46 kJ/mol sounds suspiciuosly high for an iron 2 - iron 3 reaction. what the heck is 2*96.5 ???
  4. Jul 25, 2015 #3

    -2*96.5*-.46 is nFE Moles of electrons *Faraday's constant * E rxn and Im prett sure Volts and Moles cancel leaving

    The Answer however does sound high to me as well though.

    Is there a way to move this to chemistry thread ( I am new to this),
    or should I just go there and start new?

    (Mentor Note: Moved)
    Last edited by a moderator: Jul 25, 2015
  5. Jul 25, 2015 #4


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    ok, a Faraday is a mole of electrons, with charge 6E23e/mol*1.6E-19C/e = 96 kC/mol
    ... Energy = charge * voltage = 2e/molecule * 96 kC/mol *0.24V = 46 kJ/mole.
    the R is 8.3 J/mole*Kelvin, or =0.0083 kJ/mol*K ... so the moles cancel in the exponent, as they should.
    I get 9.5E-9 at 300 Kelvin, meaning that the "weak" ¼ volt drives the reaction essentially to completion
    (equilibrium is Temperature-dependent, but would not approach 50/50 until 5,500 Kelvin).

    I don't know how to move a thread, either. sorry ; best I can do.
  6. Jul 25, 2015 #5
    From my CRC Handbook:
    DeltaG0 in kcal/gmol:

    Fe3+........ -2.52

    Fe2+........ -20.3

    I-.............. -12.35

    I2.............. 4.63

    Using these, I did not get your answer.
    Last edited: Jul 25, 2015
  7. Jul 25, 2015 #6
    Do you mean completion to the right in the reaction as stated? With Keq = 9.5E-9?
  8. Jul 25, 2015 #7


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    Gold Member

    State the reaction conditions.
    Do NOT use intuition.
    Reread the problem statement as frequently as is necessary to avoid sidetracking yourselves.
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