Spectrophotometric Determination of Equilibrium Constant for a Reaction

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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.


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 Beer-Lambert’s Law. The absorbance of light by a colored solution, such as FeSCN2+, is displayed when the solution is placed in the spectrophotometer. By using standard solutions to determine the average molar absorptivity of the solution, and given the path length, the concentration of the product of the reaction, the red FeSCN2+ solution may be determined. From that data, and from the initial concentrations of the reactant species, the concentrations of the reactants at may also be determined. With the concentrations of the products and reactants solved for, the equilibrium constant, Keq, of the reaction may be determined using Keq=(kf/kr)=([B]y/[A]x). In this experiment, the aim is to determine the equilibrium constant, Keq, of the Fe3+(aq)+SCN-(aq)↔ FeSCN2+(aq) reaction by using the Beer-Lambert’s Law and the Keq expression.



Figure1. Calibration Curve

The calibration of the sample was done so that the value of the molar absorptivity approximates as closely as possible, the overall composition of the samples and covers a reasonable range of the concentrations of the analyte. The linear relationship between the absorbance and the concentration signifies that Beer’s law holds in our standard. Looking at the literature value of the molar absorptivity of [Fe(SCN)]2+ at 447 nm which is around 4500 cm-1M-1, we can observe that it is a bit far from our experimental molar absorptivity. This is because there are many factors such as solvent, solution composition, and temperature that all affect the value of the absorptivity. The absorption spectrum is also influenced by the pH of the solution, the concentration of electrolytes in the solution, and the presence of interfering substances. Thus, it is not wise to use literature values in quantitative analyses because the absorptivity varies with certain conditions. In the calibration, we have considered [Fe(SCN)]2+ to be the absorbing medium since Fe3+ is...
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