Introduction:
The equation of the reaction is Fe3+(aq) + SCN- (aq)  Fe(NCS)2+ (aq).
The objective of this lab was to determine the equilibrium constant (K) for the formation
of the ferric thiocyanate. This was done by creating the solutions and putting them into a
spectrophotometer. This determines the absorbance at each wavelength. The maximum
wavelength gives the absorbance at equilibrium. Equilibrium is the point where the amount of
products equals the amount reactants. Equilibrium is needed to solve the K value. The equation
for the K value is, K = [C]c [D]d / [A]a [B]b. A, B, C, and D are the concentrations of the
reaction and the lower case a, b, c, and d are the molar ratios of the different reactants.
To start the lab a blank solution has to be made and then poured into a cuvette and
put into the spectrophotometer make sure to wipe off the outside of the cuvette so the maximum
amount of light can go through. This solution is created by adding both solution one and solution
four. Next the maximum wavelength has to be determined by using the solution of the highest
concentration of Fe(NCS)2+ . Also record the absorbance for this concentration every 10 nm
beginning at 400 nm and ending at 500 nm. Next all the other solutions have to run through the
spectrophotometer; however the maximum wavelength is the only thing that needs to be
recorded here. Figures 1 are the stock solutions needed. Figure 2 are the solutions for the
standard curve. Figure three are the solutions for equilibrium. All of the following solutions were
put into the spectrophotometer and the absorbances were recorded.
Using the varying molarities of ferric thyocianate was used in a chart to find the standard
curve. The standard curve is determined by plotting absorbance versus concentration of
Fe(NCS)2+ which can be seen in Figure two. A trend line is then added to a graph this gives a
y=mx+b equation. Y is equal to absorbance and x is equal to the concentration of Fe(NCS)2+.
This equation can then be used to determine unknown concentrations. The concentrations are
then plugged into the equation previously stated in paragraph one to solve for K. The K avg then
needed to be determined which can be done by adding all the K values together and dividing by
the number of K values.
Fig. 1
Solution #
Solution Contents
I
0.20 M Fe(NO3)3
II
2.0 x 10-3 M Fe(NO3)3
III
3.0 x 10-3 M KSCN
IV
0.5 M HNO3
Fig.2
[KSCN]
KSCN (sol III)
Fe(NO3)3 (sol I)
HNO3 (sol IV)
Total Volume
3.0 x 10-4 M
2.5 mL
12.5 mL
10 mL
25 mL
6.0 x 10-4 M
5.0 mL
12.5 mL
7.5 mL
25 mL
9.0 x 10-4 M
7.5 mL
12.5 mL
5.0 mL
25 mL
1.2 x 10-3 M
10.0 mL
12.5 mL
2.5 mL
25 mL
1.5 x 10-3 M
12.5 mL
12.5 mL
0 mL
25 mL
Fig.3
2.0 x 10-3 M Fe(NO3)3
3.0 x 10-3 M KSCN
0.50 M HNO3
(Solution II)
(Solution III)
(Solution IV)
1
10 mL
2.0 mL
8.0 mL
2
10 mL
4.0 mL
6.0 mL
3
10 mL
6.0 mL
4.0 mL
4
10 mL
8.0 mL
2.0 mL
5
10 mL
10.0 mL
0 mL
Beaker #
Data Tables
Solution 5
and graphs :
Wavelength (nm)
Absorbency
400
1.102
410
0.999
420
0.960
430
0.966
440
0.968
450
0.941
460
0.891
470
0.822
480
0.742
490
0.645
500
0.560
Absorbance for the standard curve
Solution #
Absorbency
I
0.641
II
0.358
III
0.213
IV
0.105
Absorbance vs wavelength
1.1
1
0.9
Absorbance vs
wavelength
0.8
0.7
Linear (Absorbance vs
wavelength)
y = -0.0047x + 2.9771
R² = 0.8972
0.6
0.5
400
420
440
460
480
500
Equilibrium Solutions
Solution #
Absorbency
I
0.108
II
0.205
III
0.378
IV
0.628
V
1.295
Concentration vs Absorbance
0.7
y = 1217.7x + 0.0492
R² = 0.9951
0.6
0.5
Concentration vs
Absorbance
0.4
0.3
Linear (Concentration
vs Absorbance)
0.2
0.1
0
0
0.0002
0.0004
0.0006
Absorbance
0.105
0.213
0.358
0.641
Fe(NCS)
0.00005
0.00012
0.00027
0.00048
The Volume Used in Each Solution For Equilibrium
Fe(NO3)3
I
II
III
IV
V
Initial mL
0
10
0
10
15
Final mL
10
20
10
20
25
Change in
Volume
10
10
10
10
10
3.0 x10-3
KSCN
I
II
III
IV
V
Initial mL
0
2
6
12
15
Final mL
2
6
12
20
25
Change in
Volume
2
4
6
8
10
HNO3
I
II
III
IV
V
Initial mL
0
8
14
18
20
Final mL
8
14
18
20
20
Change in
Volume
8
6
4
2
0
Calculations Separate Paper:
Conclusion:
The goal of this lab was to determine the K value of Fe(NCS). This goal was
accomplished. By creating all the solutions from the chart and putting them in the
spectrophotometer. This gives the absorbance of all the solutions. The trend line created from the
data obtained gave the equation to solve for the concentrations. Once the concentrations were
determined they could be put into the [C]c [D]d / [A]a [B]b to determine the K values. Then the
average K value was found by adding all the K values together and dividing by the number of K
values used. The K avg value determined was 6.025x102.
There were many errors that could of occurred in this lab. One error could have been that
the same spectrophotometer was not used for all solutions. Another error could have occurred
from the cuvette was not the same for all solutions and the cuvette not being clean. A more major
error is that for the concentration vs. Absorbance Graph the point (.105, .00005) is not correct.
The point should be (.105,.00003) this would of changed the equation of the trend line which in
result would change all the concentrations, resulting in changing all the K values.