Spectrophotometric Determination of Equilibrium Constant of a Reaction

Topics: Concentration, Chemical reaction, Chemical equilibrium, Chemistry / Pages: 11 (2594 words) / Published: Jan 7th, 2014
SPECTROPHOTOMETRIC DETERMINATION OF EQUILIBRIUM CONSTANT OF A REACTION

UNIVERSITY OF THE PHILIPPINES, DILIMAN QUEZON CITY, PHILIPPINES
August 2, 2013

ABSTRACT 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.

INTRODUCTION 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).



References: [1] Cobb, C.L., Love, G.A. (1998) Journal of Chemical Eduation Volume 75, Number 1. [2] Dodd, R.E. (1925). Chemical Spectroscopy. Elsevier Publishing Company. [3] 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 [4] Wikipedia. Beer-Lambert Law. Retrieved from: http://en.wikipedia.org/wiki/Beer%E2%80%93Lambert_law [5] Bilbo. Kf Table. Retrieved from: http://bilbo.chm.uri.edu/CHM112/tables/Kftable.htm

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