Spectrophotometric determination of the equilibrium
constant of a reaction
Department of Chemical Engineering, College of Engineering
University of the Philippines, Diliman, Quezon City, Philippines 4 March 2015
The experiment aims to determine the Keq of the formation of Fe(SCN)2+ through the use of UV-Vis spectrophotometry. The solutions used in the study were allowed to equilibrate days before calibration in a UV-Vis spectrophotometer which determined the absorbance and thus the molar absorptivity of Fe(SCN)2, which was found to be 1.942 x 10-3 M-1cm-1. Given the molar absorptivity and absorbance, Beer-Lambert’s Law gave the equilibrium concentrations for the product and reactants and from that, the Keq of the formation of Fe(SCN)2+ for each trial. The average experimental Keq of the formation of Fe(SCN)2+ was then found to be 355.05 . Upon comparison to the literature value, which was 890, the Keq had a 60.12% difference.
CONCLUSION AND RECOMMENDATIONS
The experiment failed in providing accurate results
for the determination of the Keq of formation of
Fe(SCN)2+. The experimental values for the molar
absorptivity and Keq were 1942 M-1cm-1 and 355.05,
respectively. In comparing the theoretical and
experimental values for Keq, the percent difference
was found to be 60.12%. This indicates that the
experiment was subject to numerous errors. The
large percentage of difference can be attributed to
the highly sensitive nature of the instrument and
the conditions in which the solutions were
prepared. Given that the solutions were prepared
from stock reagents, small deviations in the
concentration would have led to faulty readings.
The sources of error include human error,
improper handling of equipment, and inherent
machine error. The determination of the
equilibrium constant is limited to the formation of
Fe(SCN)2+ complex and the use of a UV-Vis
spectrophotometer. Other solutions and...
References:  Brown T.(2012) Chemistry, The Central Science.
Chemistry(2013) Formation Constants.
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