Title Page (1pt)
Name:
Exp : Determine of Kf for FeSCN2+ (aq) (#15)
Date:
Unknown # C
Concentration (5pts):
Report Grade (15pts):
Experiment
Determine of Kf for FeSCN2+ (aq)
I. Objective:
To determine the equilibrium constant for the formation of a complex ion, FeSCN2+.
Fe3+ (aq) + SCN- (aq) FeSCN2+ (aq)
II. Observations:
Part A. Constructing a Beer’s Law Plot
Five solutions are prepared according to the table below. We assume in this first part, that the SCN- determines the concentration of FeSCN2+ formed (because Fe3+ is in excess and SCN- is limiting). So we can calculate the FeSCN2+ concentration just by using the moles of SCN- that we started with.
Data Table 1.
Beaker Number
0.200 M Fe(NO)3 (mL)
0.0020 M KSCN (mL)
H2O (mL)
1
5.0
5.0
40.0
2
5.0
4.0
41.0
3
5.0
3.0
42.0
4
5.0
2.0
43.0
5 (Blank )
Data not needed
Data not needed
Data not needed
The samples are run on a visible spectrometer and an example of the data is shown below.
Using the given data file in this experiment, construct your own plot of Absorbance versus wavelength with all 4 spectra on the same plot. Use the example of data above as a guide. Your plot should look similar, but not exactly the same. (Spectra Plot, 1 pt)
Be sure to include a chart title and axis titles (with units if applicable)
Include a chart title and include a legend that labels each spectrum with a different color and the concentration of the solution
Restrict the x axis to be between 400-600 nm
Select the wavelength that has the highest Absorbance (A) [highest point on the peak]. This is the λmax. Mark this point on your plot with a label.
Insert an image of your spectra graph below:
Fill out the table below and determine the concentration of FeSCN2+ in each sample. Remember for Part A the limiting reactant is SCN- (Calculations 2 pts)
Table 1: Concentration of Solutions and Absorbance Values
Show calculations on the next page
Beaker
Moles of SCN-
Moles of FeSCN2+
Concentration of FeSCN2+ (M)
1
2
3
4
Insert your sample calculation here:
Using the [FeSCN2+] concentrations from above and the Absorbance values from the plot generate a Beer’s Law plot (which is Absorbance vs. Concentration of FeSCN2+). You will need the equation of the line from the Beer’s Law plot for Parts B and C.
(Beer’s Law Plot 1 pt)
See an example of a Beer’s Law plot in the Introduction to this experiment in your lab manual.
Be sure that your axes are labeled properly, and your plot includes a title.
You must include and display a trendline on your plot. To do this using Excel, start by right clicking on a data point. A list should pop up, click on “Add Trendline”. Under Trendline Options, select Linear. At the bottom of the Options be sure to check the boxes for both “Display Equation in Chart” and “Display R-squared value on Chart”
Insert an image of your Beer’s Law Plot below:
Part B. Run an unknown FeSCN2+ solution (5pts).
An unknown is issued from your instructor. The visible spectrum of your unknown is measured and the absorbance is shown below. From the Absorbance, what is your unknown concentration using the Beer’s Law Plot (use the equation of the line and show the calculation under the table)
Unknown
Absorbance at
max
Concentration of FeSCN2+ in unknown (M)
A
0.975
Unknown
Absorbance at
max
Concentration of FeSCN2+ in unknown (M)
B
0.835
Unknown
Absorbance at
max
Concentration of FeSCN2+ in unknown (M)
C
0.656
Unknown
Absorbance at
max
Concentration of FeSCN2+ in unknown (M)
D
0.423
Insert your calculation here:
Part C. Calculation of Kf.
Four solutions are prepared according to the table below.
Table 2.
Test tube
0.00200 M Fe(NO)3 (mL)
0.0020 M KSCN (mL)
H2O (mL)
1
3.0
2.0
5.0
2
3.0
3.0
4.0
3
3.0
4.0
3.0
4
3.0
5.0
2.0
The data from the UV-vis spectrometer for the solutions is shown below.
Table 3.
Test tube
max (nm)
Absorbance at λmax
Temperature (oC)
1
450
0.369
22.5
2
452
0.574
23.5
3
450
0.742
22.0
4
453
0.852
22.5
Use the Equation of the Line From the Beer’s Law plot in Part A to determine the equilibrium FeSCN2+ concentrations. Use an ICE table to determine the concentrations of the SCN- and Fe3+ at equilibrium (what is left over at equilibrium). Fill out the table below. Show one example calculation using an ICE table below Table 4. Use those concentrations to determine Kf (4pts).
Table 4: Determination of Kf based on Absorbance values
Test Tube
Absorbance at
max from Data Table 3.
Equilibrium Concentration of FeSCN2+ (M)
Equilibrium Concentration of Fe3+
(M)
Equilibrium Concentration of SCN-
(M)
Kf
1
0.369
2
0.574
3
0.742
4
0.852
Report an average Kf (with temperature in Kelvin)
Insert sample calculation on the next page
Insert your sample calculation here:
Be sure to include a sample of each type of calculation that was performed.
III. Conclusion and Comparison to literature value (1pt).
Report an average Kf and compare to the literature value. Find the Kf for the formation of FeSCN2+ and cite your reference. The error is likely to be high. Explain any sources of error.
Please type your response here.
IV. Questions and Calculations similar to the lab (5pts)
1. In Part A, you measure the absorbance of four FeSCN2+ solutions and prepare a Beer’s law plot (Absorbance vs. concentration) similar to the one below.
You use this plot as a standard curve in Part B to determine the concentration of an unknown. If an unknown FeSCN2+ solution gives an absorbance reading of 0.334, use the equation of the line shown above to determine its concentration. Show all your work below (2pts).
2. In Part C, you prepare four solutions (starting concentrations are given below) in test tubes and measure the Absorbance after the solutions have reached equilibrium. Shown below is an example of just one solution (Absorbance data also below). Using this data and the standard curve from number 1. (on previous page), calculate the Kf (3pts).
Starting Concentrations
Test tube
0.00200 M Fe(NO)3 (mL)
0.0020 M KSCN (mL)
H2O (mL)
1
3.0
2.0
5.0
Absorbance Data
Test tube
Absorbance at max
[FeSCN2+] at Equil
Kf
1
0.118
11