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Homework Help: Kf of Iron(III)Thiocyanate?

  1. Oct 20, 2005 #1

    Does anyone know the formation constant for the reaction
    Fe3+ + HSCN <--> H+ + FeSCN2+

    I've looked through my text book with no luck, and I have been googling for a while. I'm writing a lab writeup right now, and I need it to compare my values to the accepted values.

  2. jcsd
  3. Oct 21, 2005 #2


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    Staff: Mentor

    Six constants, as there are six steps of complex formation:

    log K1 = 3.02
    log K2 = 4.64
    log K3 = 5.0
    log K4 = 6.3
    log K5 = 6.2
    log K6 = 6.1

    (these are overall constants, like:

    K5 = [Fe(SCN)5(2-)]/([Fe(3+)][SCN-]^5)

    and so on)
  4. Oct 23, 2005 #3

    I guess this lab is pretty common for freshman chem. I had the report for this due last week. My lab manual gave the expression [H+][FeSCN2+]/[Fe3+][HSCN] but my professor gave the other one(without H).

    The reaction was carried out in a .5M nitric acid solution to prevent this reaction:Fe3+(aq) + 3OH-(aq) --> Fe(OH)3. You don't have SCN- ions in solution b/c of the large amount of H+ and the small amount of HSCN. (HSCN <-->H+ + SCN-, shifted to the left by addition of H+) You have Fe3+ in solution, not Fe(NO3)3 b/c that compound dissociates to a large degree but HSCN is a weak acid.

    I think my thoughts here make sense. I guess I'll find out when I get my grade.

    PS Nice to see someone from Poland, Borek.:smile: Both of my parents are from Poland and I can speak Polish fluently.
  5. Oct 23, 2005 #4
    On one website, it said that the H+ makes no difference in the equilibrium..but that doesn't make much sense. Someone please explain. Thanks.
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