Determination of an Equilibrium Constant of a Reaction Using Spectrophotometric Techniques

Lab 2: Determination of an Equilibrium Constant of a Reaction using Spectrophotometric Techniques.

Anh Le*, Luqman

Chemistry 163, section B

07/10/2016

Introduction        

               All chemical reaction starts with different concentrations of reactants and reacts until it reaches its equilibrium state. A reaction at its equilibrium is expressed by a particular rate law which includes the equilibrium constant Kc which is a ratio of the concentration of the products to the concentration of the reactants. This number, as it is called, is constant for each reaction. When there is a stress pushed on the system (addition amount of products or reactants), the reaction flow shifts back and forth to maintain this ratio.

Kc = [pic 1]

          The main purpose of this experiment is to use the spectrophotometric techniques method to find the equilibrium constant Kc of a reaction

Fe+3(aq) + SCN-(aq) ↔ FeSCN+2(aq) (1)

             When these two reactants are mixed, one mole of Fe+3 reacts with one mole of SCN- and form a red-orange aqueous solution that contains FeSCN+2 ions. When the concentrations of Fe+3 and SCN- are equal, there is no react activity occurs and the reaction reaches its equilibrium. Then, the equilibrium constant of this equation is calculated by the fraction:

Keq = [pic 2]

        The techniques used to determine the concentration of the product FeSCN+2 is spectrophotometric techniques. Different volumes of solutions are mixed in each trial. When the solutions reach their equilibrium states, the Vernier Spectrophotometer measures the absorbance and the amount of light absorbed by the solution at λmax. A graph will be plotted using the data of concentrations and absorbance for each trial. The relationship between absorbance and concentration of a colored species is expressed in Beer’s Law equation:

A = bc[pic 3]

(ε is a constant that depends on the wavelength of light and on the substance that is absorbing the light; b is the distance that the light travels through the sample of the absorbing substance.)

        The concentration of FeSCN- will be calculated using the linear equation of the concentration vs. absorbance graph. The equilibrium constant of the reaction will be the same in every trial since it occurs in a same room temperature. However, some errors might affect the precision of the results and therefore averaging the constant will give a more correct constant for this chemical reaction.