Equilibrium concentration question

[Latsabb]

It has been a bit since I took chemistry, but I have been wondering something, and can't seem to remember. Let's say that I have the following:

Fe³⁺ + SCN^- <-> FeSCN²⁺

How can I calculate the point where Fe³⁺ concentrations become effectively zero? That would say, if I know that I have 1M Fe³⁺ in a solution, and I plan to add SCN^- in large amounts to put pressure on the left side, forcing the reaction to the right, how can I calculate the concentration of SCN^- needed to make the Fe³⁺ run out?

Thanks in advance.

 

[Borek]

Define "effectively zero".

Fe³⁺/SCN^- is a particularly difficult case, as there are six consecutive equilibria involved.

Even for huge excess of SCN^- you can expect concentrations of Fe³⁺ (actually some hydrated forms) to be present. In 0.01M FeCl₃ with 1 M KSCN (so a hundredfold excess of the ligand), concentration of Fe³⁺ is in the 10^-9 M range.

 

[Latsabb]

Basically, the reason I am asking is due to a question that was asked of me by someone. They are trying to determine the concentration of Cl- in a solution by using Hg(SCN)₂, with Fe(NO₃)₃ added into the mix. He told me that the professor stated that Fe was present in "large amounts."

So I pointed out that since FeSCN will be formed regardless of Cl- being present, with a high enough Fe³⁺ concentration, all that SCN is going to get used up with no help of Cl-. (The Hg binding to Cl drives the reaction further to the right, but if high concentrations of Fe³⁺ is there, eventually the Cl- won't drive the equation anymore)

He then asked me what sort of concentration would be needed for that. And so here I am.

 

[Borek]

Not enough data for a reasonable answer. Without seeing what the initial concentrations are it is not clear what can happen.

 

[Latsabb]

Ok, well if we have the two following:
Cl^-(aq) + Hg(SCN)₂(aq) <-> HgCl₂(aq) +2SCN^-(aq)

SCN^- (aq) + Fe³⁺ <-> Fe(SCN)²⁺

This is how it was presented to me. It caught my eye that the end reaction doesn't use Cl at all. Being that Hg(SCN)₂ is listed as aq, and he confirmed that there is no solid present, this tells me that A) the top reaction shouldn't even be in an equilibrium, as all of the Hg(SCN)₂ should be broken up into ions to begin with. (if memory served correctly) And B) "large amounts" of Fe³⁺ seems to me like it would drive the right side of the top equation to exhaustion no matter what in an equilibrium. And since they are measuring the absorbance of the Fe(SCN)²⁺ complex, it seemed like an odd way of testing for Cl.

 

[Borek]

It caught my eye that the end reaction doesn't use Cl at all

Any reason why it should?

this tells me that A) the top reaction shouldn't even be in an equilibrium

It definitely is.

all of the Hg(SCN)₂ should be broken up into ions to begin with

No, SCN^- is a ligand and it complexes dissolved mercury, this happens in parallel to precipitation. Which process dominates depends on the concentrations. Not knowing them we won't get top any conclusions.

"large amounts" of Fe³⁺ seems to me like it would drive the right side of the top equation to exhaustion

That's not how the equilibrium works.

Can't say I understand the idea behind the procedure you described, and not knowing expected/used concentrations of all ions involved it is impossible to say what is happening in the solution, but your analysis doesn't look correct to me.