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