spectrophotometric determination of the equilibrium constant of a reaction

J.B. BATOdepartment of chemical engineering, college of engineeringuniversity of the philippines, diliman, quezon city, philippinesdate performed: 27 february 2015, 4 march 2015Instructors Name: Mariecris banez

ABSTRACTThe equilibrium constant of the reaction involving the formation of Iron (III) thiocyanate was determined by computing for the absorbance and component concentrations via spectrophotometric analysis. Solutions of varying concentrations of and KSCN were formulated and allowed to reach equilibrium before being analyzed for their absorbance of a certain wavelength. The molar absorptivity determined from calibration and the equilibrium concentrations from the unknown solutions were all computed using equilibrium relationships and Beer-Lamberts Law. A molar absorptivity value of 1099.2 M-1cm-1 was obtained which led to an equilibrium constant of 704.8, which was lower than the accepted literature value. The error, however, was determined to be less than 20.8% of the theoretical value, which would indicate the existence of errors to skew the results.

CONCLUSION AND RECOMMENDATION

Both the molar absorbance and the equilibrium concentrations were found to be moderately accurate as

The experimental molar absorbance of the complex was found to be inaccurate, showing a deviance of 69.0% from the theoretical value at 3550 M-1cm-1.The molar absorbance value of 1099.2 M-1cm-1 computed from the plot of standard solutions was used to calculate the equilibrium constant in the unknown solutions.

The experimental equilibrium constant was found to be moderately accurate, having a mean value of 704.8 and showing only a deviance of 20.8% from the literature value of 890. From this, the experiment could be considered slightly successful because of the low deviation of the constant values.

More accurate measurements and a way to stabilize the temperature and pressure during measurement may improve the accuracy of the results. Other improvements are in the handling and rinsing of the cuvette, where the methods used in this experiment might not be adequate enough to clean the cuvette of any previous solution and/or contaminants, which may factor into the measured absorbance of each subsequent solution. The procedure can also be improved by the measurement of absorbance immediately after the reagents are mixed together in a test tube so that the complex formed won't settle down.

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