[velinos]

 To determine the Equilibrium concentrations of [Fe3+] and [SCN-] you will need to use the following two equations where x = 1,2 or 3:

Fe3+ Equilibrium Concentration


and

SCN Equilibrium Concentration


where you simply subtract the initial concentration from the equilibrium concentration of [Fe(SCN)x(3-x)]
---


Now here's the thing that I don't understand.

I see that I'm going to get 3 different answers for each one, but where do I plug in the x for the first equation? Is it the subscript after SCN? I'm stuck on that part!

What's confusing me most is how the equations are set up. One of them has an "x" in front of [Fe(SCN)] while the other one does not, so I'm unsure whether or not where I need to place "x".


For example: my value for [Fe(SCN)_x^(3-x)] is 0.000145602 M
Initial of Fe⁺³ = 0.002 M

So x = 1 the equation will be: 0.002 M - (0.000145602)² M
Correct? I only place it in the superscript?

Then for the second equation involving SCN, is it:

[SCN^- (aq)]_equilibrium = 0.002 SCN^- (aq) M_initial - 1 (0.000145602 M)² Fe(SCN)_x^(3-1)


Correct?

(Please note that both have the same initial concentration: 0.002 M)

Just a bit unsure, hopeful that I am correct!

Are there any mistakes?

[Borek]

I don't like these equations.

They pretend to be a simple mass balance equations, based on mass conservation.

But they assume there is only one complex present in the solution at any given time, that's incorrect. All forms are present and mas balance equation should take them all into account. So for example:

[Fe³⁺]_initial = [Fe³⁺]_eq + [FeSCN²⁺]_eq + [Fe(SCN)₂⁺]_eq + [Fe(SCN)₃]_eq + ...

(there are more complexes that can be present in this solution).

For SCN^- equation is similar, but it have to take into account fact that there are more then one SCN^- in complex molecule (ion) so

[SCN^-]_initial = [SCN^-]_eq + 1x[FeSCN²⁺]_eq + 2x[Fe(SCN)₂⁺]_eq + 3x[Fe(SCN)₃]_eq + ...

Now, after analysis of complex formation constants it may became obvious that at some concentrations of iron and thiocyanate some complexes dominate the solution and others can be ignored, thus the equations you were given are approximately correct - but as I read your post these are given as absolute truths. They are not.

[velinos]

Sorry for the double post, but I believe what I did was correct.

I just have one final question though.

After determining the concentrations at equilibrium determine K for each x value and reaction solution using the equation:



Finally for each x value or set of data calculate the average K and the standard deviation in K. The x value with the smallest variation in K is the value you want. Be sure to state that value and plug it into the reaction equation.

Now, here's a screenshot of what I have from my data (probably with a couple of experimental errors since I heard my lab partner say "oops" too frequently when I wasn't looking =/ )



My question is: In the first equation (of this post, not the one above), do I use the equilibrium data or the initial data that I got to use in the equilibrium equation? It kind of makes sense to use the equilibrium data, but then again my values that I got for x = 1 and 3 were repeated and the only variability is with x = 2.

[Borek]

You should use equilibrium concentrations.

Hopefully this time you will not ignore the answer given.