Determination of an Equilibrium Constant

Introduction

The purpose of the experiment was to determine the equilibrium constant for the reaction of iron (III) with thiocyanate (eq 1).

Fe⁺³(aq) + SCN^-(aq) FeSCN²⁺ (1)

The equilibrium constant K_eq of this solution is represented by eq 2.

(2)

According to the Beer-Lambert law (eq 3) the intensity of the color directly depends on the concentration of the solution.

A=abc (3)

In eq 3, A is the absorbance, a is the absorptivity, b is the path length of the cuvette, and c is the concentration [FeSCN^2+] of the solution.

Procedure

10.00 mL of 2.00 x 10^-3 M potassium thiocyanate and 25 mL nitric acid were measured and diluted to 100 mL. 1.00 mL of 0.100 M iron (III) nitrate was added to the solution. A sample of the solution was added to a spectrophotometer cuvette and the absorbance was measured at 445 nm with a Spectrum SP-1105 spectrometer that was adjusted to zero absorbance (100% T) with diluted water.  The solution sample in the cuvette was poured back into the solution and another 1.00 mL of 0.100 M iron (III) nitrate was added to the solution and the absorbance of the new solution concentration was measured. There were a total of ten additions of iron and absorbance was measured after each. The data was put into Microsoft Excel where calculations done and a plot was created.

Results

The data was plotted in linear form in Figure 1 where the x axis is 1/(mol/L) and the y axis is 1/(mol/L)². The first three data points were omitted from the plot because they were obvious outliers.  

Figure 1. Modified equilibrium constant expression plotted in linear form.

The line of best fit in Figure 1 had the equation of y = -235.27x + 794687. The equilibrium constant was determined from the slope of the line to be K= 235.27.

Discussion

The equilibrium constant was found to be K=235.27, which has a 70% error when compared to the literature¹ value of K=138. Reasons for this error may be because the experiment was carried out at 20° C and the literature value was carried out at 25 ° C.  There were large outliers in the data because two different spectrometers were used in this experiment, which caused inconsistent data.

References

1. Ramette, R.W. J. Chem. Ed. 40, 71 (1963).