Equilibrium Reaction

In this experiment, equilibrium will be examines in the reaction beteween the iron (III) ion and the thiocyanate ion:
Fe3+ (aq) + SCN- (aq) ------ FeSCN2+ (aq)

The FeSCN2+ complex ion has a blood red color while the iron and the thiocyanate ion are colorless. Therefore, the shift in the reaction can followed by noting a change in the intensity of the clood red color, which indicates a change in the concentration of the complex ion FeSCN2+. If the reaction shifts to the right, the blood red color will get darker because there is more FeSCN2+ present after the shift. If the reaction shifts to the left, the color will get lighter because there is less FeSCN2+.

In an equilibrium reaction, the ratio of the products to the reactants is a constant.
Consider the reaction:

AA+bB-------cC

The equilibrium constant expresion, Kc for this hypothetical reaction would be:

[C]^c/[A]^a[B]^b=Kc
(the square brakets [C] indicates the concentration of C expressed in molarity.

Therefore, for the reaction of iron (III) and SCN- the equilibrium expression would be:

Kc= [FeSCN2+]/[Fe^3+][SCN-]

The value of Kc is a constant at a given temperature.
This means that mixtures containing Fe^3+, SCN-, and FeSCN2+ will react until the equilibrium constant, Kc is reached. If the reaction is stressed by changing the concentration of either of the reactants or product, the reaction will shift until the value of Kc is again achieved. If the temperature is changed, the value of Kc will change causing the reaction to shift.
To determine the concentration of the complex ion formed, a spectrophometer will be used. The red complex absorbs light in the visible region of the spectrum. It absorbs most strongly at a wavelength of 445 nm, I.e. it absorbs in the blue region and transmits mostly in the red region. The absorbance, A, of the solution is directly proportional to its concentration in the molarity, M, of the FeSCN2+ complex ion. Thus the absorbance obtained from the spectrophotometer can be used as a direct measure of the concentration of the FeSCN2+ ion.

Equilibrium calculations
Dilution:
When you add water, or nitric acid for this lab, to a solution you can calculate the new, diluted molarity using the
formula:
M1V1=M2V2

Remember that V1 is the volume of the solution before the dilution and V2 is the total volume after dilution, not the volume of the solvent added.
Equilibrium concentration:

When Fe^3+and SCN- ions are placed together in solution, they react to form FeSCN2+ according to the reaction below:

Fe3+ (aq) + SCN- (aq) ------ FeSCN2+ (aq)
As stated earlier, this reaction does not proceed to completion but achieves an equilibrium state. So, only some of the iron and thiocyanate ions react. In order to calculate the equilibrium concentration, you need to use an ICE table as shown below.

Fe3+ SCN- ------
FeSCN2+
I 0.0030 .0002 --
C -x -x +x
E .0030-x .0002-x x

I, stands for the initial concentration. Since you re adding Fe^3+ and SCN- together, there is an imaginary moment before reaction proceeds where the concentration of the product is still 0. C, stands for change and since you don’t know how much of the Fe^3+ and SCN- react, we use the x for the unknown amount. However, the stoichiometry indicates that if x Fe^3+ reacts with xSCN-, then xFeSCN^2- would form. Since the reactants disappear, they are shown as -x and since the product forms, it is shown as +x in the table. E, stands for the equilibrium concentrations of each chemical species in the reaction. Since x is the equilibrium value of FeSCN^2+, its value is calculated using absorbance values in this experiment using the equation you found in the prelab experiment.