Chem 17 Expt 5 Spectrophotometry

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Topics: Chemistry
Spectrophotometric determination of the equilibrium constant of a reaction
K.F.P. Boado
Department of Chemical Engineering, College of Engineering
University of the Philippines, Diliman, Quezon City, Philippines
4 March 2015
Jade Nazareno

ABSTRACT
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[2][3], 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

techniques may be used to find the Keq



References: [1] Brown T.(2012) Chemistry, The Central Science. Chemistry(2013) Formation Constants.

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