Chemical equilibrium refers to a steady state of reaction, it does not mean the reaction has stopped but the reaction between reactants and the formation of substances has reached a balanced state. A dynamic equilibrium can occur, when the reaction continues but the quantity of the reactants remain constant which goes hand in hand with Le chatelier’s principle

This experiment is to determine the equilibrium constant between the reactions of Fe3+ and SCN- using a spectrophotometer. When SCN- ions react with Fe3+ ions in aqueous solution, the following reaction occurs

Fe3+ (aq) + SCN- (aq) ↔ Fe(SCN)2+ (aq)

Therefore the equilibrium (K) is given by the formula reactants over the products as shown below:

K= [ Fe (SCN)2+]

[ Fe3+]eq [SCN]eq


Both iron and thiocynate are colorless, but the product (Fe(SCN)2+ has a deep red color with maximum absorption of light occurring at 450 nm so the reaction can be monitored at this wavelength of the spectrophotometer.

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Equilibrium constant, equilibrium concentration and the molar apsorptivity constant

The equilibrium constant is attained by measuring the concentration of the equilibrium using a spectrophotometer and Beer’s law- (A=EAC)

Beer’s law states that the rate of absorbance is directly proportional to the molar apsorptivity constant (E), the path length of the sample cell (A) and the concentration of the absorbing molecules(C)

The value of K is calculated by using the equilibrium concentration of FeSCN2+ given by X, and dividing it by the reactants and giving account to the dilution that occurs when SCN- ions is added to the solution of Fe3+

K= X

( [ Fe3+]-x[ SCN-]-x)

According to Beer’s law absorbance (A) at 450nm will be directly proportional to the concentration of Fe(SCN)2+ ions, so we use the following formulae get the concentration of the equilibrium.

A= EA [ Fe(SCN)2+]

Molar apsorptivity constant (E) is calculated by finding the absorbance concentration of Fe(SCN)2+ and then calculating using Beer’s law.


The first part of the experiment is to determine E, the equilibrium concentration and the length of the cell. The second part is determining the constant K. Since the value of E is known, K can be determined by mixing different concentrations of the reactants. Absorbance can be measured, and Beer’s law used to calculate the concentration. Each value will allow you to calculate the value of the constant K, and if all values are identical then the experiment was precise.

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