The Spectrophotometric Determination of an Equilibrium Constant

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The Spectrophotometric Determination of an Equilibrium Constant.


The report presents determination of equilibrium constant for the formation of a complex ion FeSCN2+. This was accomplished using a colorimeter to measure absorbance of some known concentration solutions in order to generate the calibration curve. The equation of the graph was used to compute the equilibrium concentrations of the reactants and products, needed to calculate the equilibrium constant for the reaction. I. Introduction:

The purpose of the experiment is to determine the equilibrium constant for the formation of a complex ion iron (III) thiocyanate (FeSCN2+). This ion is formed in the reaction of iron (III) ion with thiocyanate ion SCN-): Eq.1 Fe3+ (aq) + SCN- (aq) FeSCN2+ (aq,)

The equilibrium constant is the ratio between the reactants and products when the forward and reverse reaction rates are equal. It is calculated from the mass action expression: Eq. 2Kc = [FeSCN2+] / [Fe3+][SCN-]

In order to calculate the equilibrium constant it is necessary to know the concentrations of all ions at equilibrium. The concentration of FeSCN2+ ions can be determined colorimetrically as the FeSCN2+ ions are red colored (the reactant ions are colorless), and therefore they are the primary absorber in the mixture. According to Beer’s law the higher the concentration of the FeSCN2+ ion in the solution the more intense the red color (The Columbia Encyclopedia, 6th ed., 2008). By pushing the reaction in equation 1 to completion using LeChatelier’s principle with different volumes of the reactants, the Beer’s curve of absorbance versus concentration can be generated and used to determine the concentration of FeSCN2+ in an equilibrium mixture. If the initial concentrations of the reactant ions are known, their equilibrium concentrations can be calculated using the ICE table, and then the equilibrium constant can be calculated (Kotz, Townsend, & Treichel, 2008). II. Experimental:

A. Chemicals and Equipment:

Chemicals used in the experiment include: 0.00200M potassium thiocyanate (KSCN), 0.200M iron (III) nitrate (Fe(NO3)3) in 0.1M nitric acid, 0.00200M iron(III) nitrate in 0.1M nitric acid, and 0.10M nitric acid. Equipment used in the experiment includes: LabPro unit with the colorimeter probe, 1 cm cuvettes, Kimwipes, graduated cylinders, and pipettes. B. Experimental Procedure:

The LabPro unit was obtained and set for the wavelength of 470nm. Then a 0.1M nitric acid solution was added into a curvette and placed in the spectrophotometer to calibrate it. Five solutions of known FeSCN2+ concentrations were then prepared in separate beakers by mixing appropriate amounts of 0.00200M KSCN, 0.200 M Fe(NO3)3 in 0.1M HNO3, and 0.10M HNO3 solutions for a total volume of 10mL, according to the table below: Table1

| |0.200M Fe(NO3)3 in 0.1M HNO3 (mL) |0.00200M KSCN (mL) |0.10M HNO3 (mL) | |Experiment 1 |5.0 |1.0 |4.0 | |Experiment 2 |5.0 |2.0 |3.0 | |Experiment 3 |5.0 |3.0 |2.0 | |Experiment 4 |5.0 |4.0 |1.0 | |Experiment 5 |5.0 |5.0 |0.0 |

The five solutions were allowed to react and then they were added to separate cuvettes and placed in a spectrophotometer to measure their absorbance. The absorbance versus concentration was then plotted on a graph using Excel and the equation of the...
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