SPECTROPHOTOMETRIC DETERMINATION OF EQUILIBRIUM CONSTANT OF A REACTION
UNIVERSITY OF THE PHILIPPINES, DILIMAN QUEZON CITY, PHILIPPINES August 2, 2013
The objective of this experiment is to determine the equilibrium constant, denoted Keq, for the formation of [Fe(SCN)]2+ complex which is a product of the reaction between the ions Fe3+ and SCN-. In performing this experiment, solutions containing FeCl3 and KSCN, diluted in HCl, were measured for their absorbance using a UV-Vis spectrophotometer. Upon computing for the molar concentrations of the reagents, determining absorbance, and noting the length pathway of the instrument (cuvette), the Beer-Lambert’s law was applied to find the molar absorptivity constant, denoted ε. Having found ε, the equilibrium concentrations of all species were determined, and those values were used to find the equilibrium constant. Another objective of the experiment is to compare the experimental result with the literature result, and evaluate the efficiency and accuracy of this experiment and experiment performers. The calculated Keq was 411, with the literature value being 890, yielding a 53.82% error. The experiment was successful in demonstrating and validating the Beer-Lambert law concept that absorbance is in fact directly proportional to molar concentration, but is a failure in that the experiment incurred a very large % error. The methods or the tools utilized, or the experiment doers themselves performed poorly and attained a very inaccurate answer, but the concepts and relationships involved were validated.
Chemical equilibrium is a balanced state within a system of chemical reactions. At chemical equilibrium, the forward and backward movements of the reaction happen at the same rate. In this case, both product(s) and reactant(s) are present, and there is no tendency for the reaction to move any further in either direction (since the rate of both forward and backward are equal). In this experiment, the class was tasked to determine the equilibrium constant of the formation of [Fe(SCN)]2+ in the reaction between Fe3+ and SCN-. Keq can be defined in more than one way, but for this experiment, it will be defined as such: Given the chemical reaction:
aA + bB cC + dD
where A B C D are chemical species, while a, b, c, and d represent the respective coefficients of those chemicals, Keq will then be defined as: Working Equation to solve for Keq
In order to solve for Keq using this equation, one just needs the balanced chemical equation, and the molar concentrations at equilibrium of all the species involved in the reaction. For this experiment, spectrophotometry was used to determine the absorbance of FeSCN2+ in a solution, a product in the chemical reaction between Fe3+ and SCN-. Balanced Chemical Equation:
FeCl3 + 3KSCN FeSCN2+ + 3KCl + 2SCN-
Net Ionic Equation:
Fe3+ + SCN- FeSCN2+
Spectrophotometry is the measurement of reflection or emission of a property, in this case visible light, as a function of wavelength. A photometer is used in spectrophotometry to measure intensity as a function of light source wavelength. The importance of finding the absorbance of a solution in this experiment is that it is an essential step in determining the equilibrium constant. The Beer-Lambert Law explains the relationship between absorbance of light and the properties of the material/medium that light passes through. Properties include molar absortivity constant, length pathway and molar concentration (which is required to find Keq!). Beer-Lambert law:
A = εbc
A is absorbance of the species.
ε is the molar absorptivity constant.
b is the length pathway (in cm).
c is the analyte molar concentration.
In this experiment, 5 Standard solutions, a Standard blank, 3 Unknown solutions, and an Unknown blank were prepared in 10 different test tubes. All contained (varying) amounts of FeCl3, KSCN and diluted in HCl to a total of 10mL solution in each...
References:  Cobb, C.L., Love, G.A. (1998) Journal of Chemical Eduation Volume 75, Number 1.
 Dodd, R.E. (1925). Chemical Spectroscopy. Elsevier Publishing Company.
 Pettruci, R.H., Harwood, W.S., Herring, F.G.. (2004) General Chemistry: Principles and Modern Applications 8th Edition. Pearson Education South Asia Pte. Ltd. Singapore
 Wikipedia. Beer-Lambert Law. Retrieved from: http://en.wikipedia.org/wiki/Beer%E2%80%93Lambert_law
 Bilbo. Kf Table. Retrieved from: http://bilbo.chm.uri.edu/CHM112/tables/Kftable.htm
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