Determination Of The Equilibrium Constant

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Introduction
A complex ion has a metal ion in its center. The metal ions surrounding the center is a ligand. All ligands function as a Lewis base. When the ligand forms bonds with the metals a complex formation forms. The interaction between an iron (III) ion (Fe3+) and a its ligand thiocyanate ion (SCN- ) will form a blood-red complex ion. The equilibrium constant can be expressed form the concentrations of the three components. Thus, the purpose of this experiment is to experimentally find the equilibrium constant, Kc.
Fe3+ + SCN- → FeSCN2+
When the two reactants are combined, equilibrium is establishes between these two ions and the product ion. To calculate Kc it is necessary to know the concentrations of all ions at equilibrium. According to Le Chatelier’s principle the high concentrations force the reaction far to the right, using almost all of the thiocyanate ions From the resulting reaction the blood red color makes it easy to carry out quantitative determination by spectrophotometry. From the spectrophotometer the absorbance of the equilibrium system and standard solution will be obtained to determine [FeSCN2+] eq. The absorbance at 470 nm will correlate to the complex ion and a calibration curve can be obtained. This gives the relationship between absorbance and wavelength. An equilibrium constant can then be determined for each mixture; the average should be the equilibrium constant value for the formation of the FeSCN2+ ion.

Materials and methods
Preparing standard and equilibrium solutions
1.2 grams of Fe (NO3)3 were mixed into 50 ml of water and 1.486 grams of KSCN were mixed in 50 ml of water also. The two beakers were combined to make a solution. Five test tubes were obtained and different amounts of nitric acid, water and solution to find concentrations below 1.

Using a spectrophotometer, the optimal wavelength was determined at 464. At the optimal wavelength, each test tube was used to find the absorbance. This gives the calibration curve.

Results

Concentration of FeSCN2+ (M) Absorbance
5.0 x 10-5 5.4 x 10-2
7.0 x 10-5 9.7 x 10-2
1.0 x 10-4 1.2 x 10-1
1.5 x 10-4 1.8 x 10-1
2.0 x 10-4 2.6 x 10-1

Table 1: Values for the calibration plot.

Graph 1: Plot of absorbance vs concentration

From the calibration data, a least squares analysis was performed and the best straight line fitting the data was drawn. The equation for the line is given at the top of the plot: y = 1312.9x-.0061 r2= 0.9845

The concentration of FeSCN2+ can be determined by substituting the value of the absorbance into the equation for the least squares line and solving for the concentration. x = (.144+.0061)/1312.9 = 1.14 x 10-4M
The correlation coefficient indicates how well the data fits the equation for a straight line. A value of 1.000 indicates a perfect fit. The farther the correlation coefficient is from 1.000 indicates that the data is less well represented by a straight line. From the equation above, the correlation coefficient of 0.9845 indicates that the data is linear. Concentration of
Iron (III) Concentration of thiocyanate Absorbance of
Thiocyantoiron (III) Kc Keq
1.0 x 10-3 1.0 x 10-3 1.5 x 10-1 1.2 x 10-4 1.5 x 102
1.1 x 10-3 1.1 x 10-3 2.0 x 10-1 1.6 x 10-4 1.8 x 102
1.2 x 10-3 1.2 x 10-3 2.1 x 10-1 1.7 x 10-4 1.6 x 102
1.6 x 10-3 1.6 x 10-3 3.6 x 10-1 2.8 x 10-4 1.6 x 102
1.7 x 10-3 1.7 x 10-3 4.0 x 10-1 3.2 x 10-4 1.7 x 102

Table 2: An ICE table was used to find the equilibrium
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