Spectrophotometric Determination of Equilibrium Constant for a Reaction

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DATE PERFORMED: JULY 20, 2007

SPECTROPHOTOMETRIC DETERMINATION OF EQUILIBRIUM CONSTANT FOR A REACTION

ABSTRACT

UV-VIS spectrophotometry is one of the most widely-used methods for determining and identifying many inorganic species. During this experiment, this spectrophotometry was used to determine the equilibrium constant, Keq, of the Fe3+(aq)+SCN-(aq)↔ FeSCN2+(aq) reaction. By determining the amount of light absorbed, the concentration of the colored FeSCN2+ solution was also quantitatively determined. From that data, the concentrations of the reagents at equilibrium may also be determined. This experiment should thus provide a Keq value without computing for the concentration of each of the species in the reaction. This experiment will only deal with the aspect of chemical equilibrium, particularly the aforementioned equilibrium constant, and not with associated topics such as thermodynamics or kinetics.

INTRODUCTION

Reactions strive to attain equilibrium or stability. In kinetics, stability is attained when the rate of formation of the products is equivalent to the rate of reactant re-formation. Rate is determined by the rate expression, r=k[A]x, where A is the reactant, x the order of reaction with respect to the reactant and also to the coefficient of the reactant if it is an elementary reaction, and k, the rate constant. In equilibrium, there is an equilibrium constant determined by kf[A]x=kr[B]y, which is equivalent to the aforementioned parameter of equilibrium, rate of formation (forward) is equal to the rate of reformation (reverse). Therefore, Keq=(kf/kr)=([B]y/[A]x). Concentration may be determined by taking the number of moles of the reagent or product and dividing it by the total volume of the solution (dimension: M, molarity). There is, however, another method of determining the equilibrium constant, Keq, of a reaction. Spectrophotometric analysis results in the quantitative determination of the concentration of the product, through the



References: [1] Skoog, D.A., West, D.M., et al. Fundamentals of Analytical Chemistry 8th edition. Brooks/Cole, Singapore. 2004. [2] Lothian, G.F. Absorption Spectrophotmetry. Hilge&Watts, London. 1958. [3] Kolthoff, I.M, Sandell E.B, et al. Quantitative Chemical Analysis: 4th Edition. Macmillan Co., New York. 1969. [4] Atkins, P.W., et al. Physical Chemistry 8th edition. Oxford University Press, United Kingdom. 2005.

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