Spectrophotometric Determination of the Equilibrium Constant of a Reaction Abstract

DATE PERFORMED: JANUARY 6, 2011 SPECTROPHOTOMETRIC DETERMINATION OF THE EQUILIBRIUM CONSTANT OF A REACTION ABSTRACT The objective of the experiment was to determine the equilibrium constant of the reaction forming ferric thiocyanate through the use of Spectrophotometry. For the calibration, five standard solutions were prepared, then their respective absorbance values that were obtained through the use of the spectrophotometer, were plotted versus the concentration of the analyte so that a calibration curve would be obtained. The calibration curve was then used to determine the molar absorptivity coefficient. The unknown solutions were then tested, resulting in absorbance values. The molar absorptivity coefficient obtained in the calibration curve and the absorbance values were used to determine the equilibrium concentrations of all the species involved in the reaction. These concentrations were then used to calculate the equilibrium constant. An equilibrium constant of 54.0 was found. Compared to the literature value of 890 there was a 177.11% difference. Even if there was a high percent difference, this was enough to prove that spectrophotometry can be used to determine the equilibrium constant of a given reaction. INTRODUCTION In this experiment spectrophotometry was used which involves an instrument called a spectrophotometer. A spectrophotometer is a light measuring device that measures intensity as a function of the light source wavelength. The amount of light that had passed through the system is called the transmittance. The amount of light absorbed by the system is called the absorbance. However, the absorbance of [Fe(SCN)]2+ is the one that the experiment is concerned with. The concept of the Q, the reaction quotient is needed. There are infinitely many Q's for a given reaction. The reaction quotient at state where the rate of the forward reaction and the reverse reaction are equal is called the Keq. The equal reaction rates means equal production of