What is the Formation Constant of Iron(III) Thiocyanate?

[phizuks]

Hello,

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

I've looked through my textbook 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.

Thanks.

 

[Borek]

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)

 

[NoName013]

H

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. Both of my parents are from Poland and I can speak Polish fluently.

 

[NoName013]

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.