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### Topic: Where can I find the formation constant (Kf) of SCN- + Fe3+ <----> FeSCN2+  (Read 39307 times)

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#### Altered State

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##### Where can I find the formation constant (Kf) of SCN- + Fe3+ <----> FeSCN2+
« on: April 28, 2013, 06:27:23 PM »
I've just finished data treatment of my first practice, in which I determinated the value of Kf to the equilibrium reaction SCN-  +  Fe3+  <---->  FeSCN2+
And my result was an average of K = 485.3
It is logical, but I do not know where can I find the accepted mean value for this. I've tried Googleing before going to check any book out, to see if I am close or not, and on the internet I've found vaules from 100 to 800. So the result appears to be logical, but I do not know where can I find the "accepted" value to compare with in my report discussion.

For my other 2 lab experiments I'm gonna need aswell the dissociation acetylsalicylic acid constant (Ka), I think this one is gonna be easier to find, but if you can tell me where and how can I find it, would be nice.

My third experiment was about determinating the reaction rate constant of this hydrolisis reaction:

Ethyl acetate + NaOH  ---> Ethanol  +  Sodium acetate

I guess that, like my teacher said, Its almost impossible to compare the result of this experiment with any literature given value. Is this true? I suppose I can do a similarity comparision with any value, if its the case, would you enlighten me about how can I find that value?

Regards!

#### Borek

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##### Re: Where can I find the formation constant (Kf) of SCN- + Fe3+ <----> FeSCN2+
« Reply #1 on: April 29, 2013, 03:08:07 AM »
http://www.chemicalforums.com/index.php?topic=21816.msg83326#msg83326

These were taken from the Handbook of Chemical Equilibria in Analytical Chemistry, Kotrly and Sucha, Ellis Horwood Ltd. 1985.
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#### Altered State

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##### Re: Where can I find the formation constant (Kf) of SCN- + Fe3+ <----> FeSCN2+
« Reply #2 on: April 29, 2013, 06:43:15 AM »
http://www.chemicalforums.com/index.php?topic=21816.msg83326#msg83326

These were taken from the Handbook of Chemical Equilibria in Analytical Chemistry, Kotrly and Sucha, Ellis Horwood Ltd. 1985.

So the value that I should have got in this experiments is log K = 3.02, so K =~1000?
In all the mixtures the concentration of SCN- was lesser than Fe3+, so the first equilibrium should take place...
How can I justify my k = 485 result? I can't figure it out, some help?

Furthermore, other experimental values that I found on the internet and some of my classmates told me, are about K=100-300, is that even worse? What is causing this? Just experimental error? Do not think so...

My proceedure was similar to this one: http://www.jackson.k12.ga.us/teachers/rbryan/AP_Chemistry_Online/LabReport%20ExampleKeq.pdf

My exact concentrations are:

[SCN-] inicial (mol/l)   [Fe3+] inicial (mol/l)

0.0002                     0.001
0.0004                     0.001
0.0006                     0.001
0.0008                     0.001
0.001                             0.001

Since the bigger the coordination complex is, the higher is the reaction favored, shouldn't it form bigger complex in my mixtures?
« Last Edit: April 29, 2013, 06:53:46 AM by Altered State »

#### Borek

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##### Re: Where can I find the formation constant (Kf) of SCN- + Fe3+ <----> FeSCN2+
« Reply #3 on: April 29, 2013, 07:49:29 AM »
I believe there are several sources of the values if these constants, and they differ substantially - there is not much that you can do about it. Best approach is to quote the source of constant values you are using.

I believe there were several papers in Journal of Chemical Education where it was discussed. Sorry, I don't remember issues, but they should be easy to google.
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#### Altered State

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##### Re: Where can I find the formation constant (Kf) of SCN- + Fe3+ <----> FeSCN2+
« Reply #4 on: April 29, 2013, 08:10:29 AM »
I believe there are several sources of the values if these constants, and they differ substantially - there is not much that you can do about it. Best approach is to quote the source of constant values you are using.

I believe there were several papers in Journal of Chemical Education where it was discussed. Sorry, I don't remember issues, but they should be easy to google.

What would you suggest me to put in my lab report discussion?
It's the first time I'm writing one, and it's about 50% percent of this subjetct's calification, so I really need to do it well.

Should I quote these values you gave me, and just say that my result of "485" is close enough to the accepted value of ~k=1000? I guess I cannot simply do that..
Should I mention the other equilibra with the different possible complex?

Could you provide another source in which I can get values that differ substantially from those ones, so I can state that the exact value is not clear?

Sorry for the amount of questions, but I really need help.

Thank you