Adding KNO3

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  • #1
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I have two aqueous solutions---Fe(III)(NO3)3 and KSCN (no precipitate in either, both are fully dissociated into ions). I react five drops of the ferric nitrate solution with one drop of KSCN (assume that we end up with excess Fe[3+])

The net ionic reaction Fe[3+] + SCN[-] (equilibrium arrows!) FeSCN[2-] forms a blood-red complex ion FeSCN[2-].

I add a small amount (10mg) KNO3 and observe a change (more brown or lighter red). Well, the equilibrium shifted;

Why do I notice a significant color change???
 

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  • #2
chem_tr
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Adding KNO3 to the medium increases the ionic strength, so solubilities show a general lowering in value.

However, the more important fact is that you start with a nitrate-containing compound, i.e., iron(III) nitrate. According to La Châtelier's principle, the equilibrium shifts to the right until the equilibrium is established again. That's why a more intense color is observed. The result is that more thiocyanate complex is formed.
 
  • #3
Borek
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This is not only FeSCN2-, but every form up to Fe(SCN)6 3-.

Do you observe the same thing when adding other salts?

Fe3+ is slightly complexed by NO3- ions, you added them so perhaps more FeNO3 2+ complex is formed and less FeSCN2- is present. IIRC the latter has about 100 times larger stability constant, so there is a lrge difference in concentrations needed.

What were the starting concentrations of Fe(III)(NO3)3 and KSCN?


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  • #4
Hello Gentlemen...

I 've this aqueous solution with probably it complex formed, but I would like to Know How could I to separate these ones salts?
Fe(NO3)3 + 3 KSCN -------> 3KNO3 +Fe(SCN)3


Paulo
 
  • #5
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K = [A]i[B ]j/[C]k[D]l , and concentration of A or B is increased?
 
  • #6
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It's been a year and a half since I started this thread
(alotropia_entropia, you may want to make a new thread...)
 
Last edited:
  • #8
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Hello I am doing the same lab. I have reacted KSCN with Fe(NO3)3, then after that I added NaF (sodium fluoride) and it turned colourless.

Why? What happened? How has the concentration of the complex ion or Fe(SCN)3 been affected?
The fluoride ions do not react directly with Fe(SCN)3 but they react with iron(III) ions to give colourless, soluble product. What does the effect of fluoride ions show about the reaction of iron(III) ions and thocyanate ions to form the complex?

Thanks.
 
  • #9
Borek
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You have two reactions in equilibrium - Fe/SCN and Fe/F.

Color disappears - what does it tell you about concentration of Fe/SCN complex?

Think in terms of Le Chatelier's principle - what has to happen to lower Fe/SCN complex concentration?
 
  • #10
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Wait, so NaF reacted with both Fe(NO3)3 and Fe(SCN)3 ?
F- ions displace both NO3- ions and SCN- ions? Or only SCN- ions?

I sort of get it now, the complex ion's concentration has decreased that's why it turned colourless, yeah?
 
  • #11
Borek
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You are right that concentration of complex decreased, but seems to me like you are missing something important. There is no Fe(NO3)3 in the solution - that was salt that was put in, but you can safely assume it was just dissociated. On the other hand, Fe/SCN complexes are real molecules/ions existing in the solution.
 
  • #12
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Adding KNO3 to the medium increases the ionic strength, so solubilities show a general lowering in value.
I knew that solubility increases when the ionic strenght increases.
 

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