The overall goal of this caper is the determination of the equilibrium constant for the reaction
You will do this using the Beer-Lambert law in a somewhat modified form.
In the experimental procedure, you are instructed to dilute 10.0 mL of 0.002M KSCN (along with 25 mL of 2.0M HNO3) to 100 mL. We are interested in the concentration of the thiocyanate ion, SCN-. We calculate this concentration using the solution dilution formula
where C1 and V1 are the concentrations and volumes of the stock solution (the more concentrated solution) and C2 and V2 are the concentration of the diluted solution. For example, if we diluted 10.0 mL of 0.002 M KSCN to a volume of 100 mL, then the concentration (C2) of the dilute solution is
Now, we add 0.1 M Fe3+ in 1 mL increments (we do this a total of 10 times.) We need to calculate the concentrations of SCN- and Fe3+ after each 1 mL addition of iron (III). We use the solution dilution formula again, and to make things simple, we note that the moles of thiocyanate do not change, but the total volume of the solution, and the moles of iron(III), *does change* because we are adding 1 mL increments of 0.1M Fe3+. Therefore, we can calculate the total concentrations of thiocyanate ( we call this Cs) and iron(III) (we call this Cf) as follows:
Cf = (0.1 M)(total volume of iron(III), L) / total solution volume, L
After each 1 mL addition of iron(III), you will measure the absorbance of the solution at 445 nm on a Spectronic-20. You should make a table of the following data:
| mL 0.1 M iron(III) added | Cs, M | Cf, M | Absorbance (445 nm) |
Notice that Cs and Cf for each addition of iron(III) are calculated as described above. After you have calculated Cs and Cf for each addition, calculate A/CsCf and A(Cs + Cf)/CsCf; notice that A is the absorbance measurement from the Spectronic-20. Make a plot with A/CsCf on the y-axis, and A(Cs + Cf)/CsCf on the x-axis; use mathcad or excel to fit a regression line. The slope of this line = -Keq, the equilibrium constant for the reaction.
Here is a sample plot....the slope of the line (multiplied by -1) gives the equilibrium constant.
Fig. 1. Modified equilibrium constant expression plotted in linear form.
The literature1 value for the equilibrium constant is K = 138 at 25oC. Calculate the per cent error between your value and the literature value, and discuss error.
References
1. Ramette, R.W. J. Chem. Ed. 40, 71 (1963).
