The equilibrium system is prepared from known concentrations of Fe3+ and SCN-, carefully measured with pipets and mixed in volumetric flasks. Since the Fe(SCN)2+ formed is an intense red-brown colored complex ion with an absorption maximum at about 447 nm, its concentration can be determined with a spectrophotometer. By knowing the initial concentrations of Fe3+ and SCN-, and by measuring the Fe(SCN)2+ equilibrium concentration, the equilibrium concentrations of Fe3+ and SCN- can be calculated. Using these quilibrium concentrations, the K for several solutions in which the ratios of reactant and product species are varied can be determined.
*In aqueous solutions, the ferric ion is octahedrally coordinated to six water ligands. The actual red-brown species in this experiment is the pentaaquothiocyanate iron(III) ion, Fe(H2O)5(SCN)2+, and in the actual reaction the thiocyanate ion replaces a water molecule:
Dillute this solution with distilled water to the 50 mL mark and mix well. Pour this solution into a clean, dry Erlenmeyer flask and label as Solution 1. This sample will be used as the "blank" in the spectrophotometric analysis.
Prepare the remaining solutions for determining Kc as indicated in Table 18.1. Use a pipet for the measurement of the ferric nitrate and sodium thoicyanate solutions. For solutions 2 through 4, add 20.0 mL of ferric nitrate to each of the remaining 50 mL volumetric flasks. Then add a different volume of NaSCN to each flask, varying the amounts between 10 and 20 mL for each sample.
Remember to rinse out the volumetric flask after each sample is prepared, as well as the pipet when changing solutions.
Table 18.1
| Solution | 0.003 M Fe(NO3)3 | 0.003 M NaSCN | 6 M HNO3 | |
|---|---|---|---|---|
| 1 | 20 | 0 | 4 | |
| 2 | 20 | ? | 4 | |
| 3 | 20 | ? | 4 | |
| 4 | 20 | ? | 4 |
Although the solutions are reasonably stable, they are slowly decomposed by light. Do not store the solutions more than one lab period.
Turn on the Spec 20 and set the wavelength to 447 nm. Then obtain two clean cuvettes, one for the blank and one for the samples. After the Spec 20 has warmed up for 20 minutes, zero it, and then using Solution 1 as the blank, set the %T at 100% (and absorbance should be 0).
Determine, and record the absorbance of each of the three remaining samples, Solutions 2 through 4.
| Concentration / M | Absorbance |
|---|---|
| 5.00 x 10-5 | .200 |
| 10.0 x 10-5 | .400 |
| 15.0 x 10-5 | .600 |
The equilibrium concentrations of Fe3+ and SCN- are then obtained by subtracting the concentration of Fe(SCN)2+ from the initial Fe3+ and SCN- concentrations. Use a table as you have in class and include OC, Change, EC for each species involved.
Calculate the equilibrium constant for each of these solutions using the equation presented earlier, and then determine the mean value for Kc for this system.