Determining of the equilibrium constant for the formation of FeSCN2+

Topics: Chemical equilibrium, Chemistry, Equilibrium constant Pages: 5 (611 words) Published: October 17, 2013

General Chemistry II
Determining of the equilibrium constant for the formation of FeSCN2+ Introduction
The objective of this experiment was to determine the equilibrium concentration and then determine Kc. A dilution calculation was formed to determine the concentration of SCN- and Fe(SCN)2+. Each cuvette was filled to the same volume and can be seen in table 1. Then the absorbances were recorded from each cuvette and can be seen in table 1. A Beer’s law plot was made from the data that was recorded from the optical absorbance. During the second part of the experiment Fe (NO3)3 was added and diluted with HNO3 . All of the cuvettes were mixed with the same solutions in the second part of the experiment, which can be seen in table 2. A dilution calculation was made to determine the initial concentration of Fe3+and SCN-. Then the formula Abs + b/ slope was used to determine the equilibrium concentration which lead to the calculation of each Kc per trial. Chemical reaction

Fe3+ + SCN- FeSCN2+

Data presentations and analysis
Table 1. Absorbances vs. concentrations of Beer’s law plot Volume of 0.10 Fe3+ 0.10
Volume of 0.0020 M SCN-
Volume of water
Concentration of SCN-
Concentration of Fe(SCN)2+
Optical absorbance
2.00E+00
3.00E-01
7.00E-01
2.00E-04
2.00E-04
6.00E-02
2.00E+00
5.00E-01
5.00E-01
3.30E-04
3.30E-04
1.52E-01
2.00E+00
7.00E-01
3.00E-01
4.70E-04
4.70E-04
2.37E-01
2.00E+00
1.00E+00
0.00E+00
6.70E-04
6.70E-04
3.47E-01
0.00E+00
0.00E+00
3.00E+00
0
0
0.00E+00

Figure 1. Optical absorbance Beer’s law plot

Table 2. Part b. trials and concentrations
Volume of 0.006 M Fe+3 (mL)
Volume of 0.0020 M SCN- (mL)
Volume of water (mL)
Initial Concentration of Fe+3 (M)
Initial Concentration of SCN- (M)
Equilibrium Concentration of Fe(SCN)2+ (M)
Optical absorbance
1.70E+00
7.00E-01
6.00E-01
3.40E-03
4.67E-04
8.97E-05
2.80E-02
2.00E+00
4.00E-01
6.00E-01
4.00E-03
2.67E-04
1.21E-04
4.50E-02
1.60E+00
8.00E-01
6.00E-01
3.20E-03
5.33E-04
1.68E-04
7.00E-02
1.30E+00
1.00E+00
7.00E-01
2.60E-03
6.67E-04
2.01E-04
8.80E-02
1.00E+00
1.30E+00
7.00E-01
2.00E-03
8.67E-04
1.87E-04
8.00E-02

Table 3. Equilibrium concentrations
Eq. Fe3+
Eq. SCN-
3.31E-03
3.77E-04
3.88E-03
1.45E-04
3.03E-03
3.65E-04
2.40E-03
4.65E-04
1.81E-03
6.80E-04
Table 4. Five different trials of Kc and average Kc
Kc
Average Kc
7.19E+01
1.54E+02
2.15E+02

1.52E+02

1.80E+02

1.51E+02

All of the solution that was used during this experiment was transparent but when Fe3+ or Fe(NO3)3 was mixed with SCN- it turned into an orange color. The more of SCN- was added the dark the orange color became. When Fe(NO3)3 was diluted with HNO3- it had a transparent look to it. Although the chemical SCN- had a transparent coloring but when looked more closely it had a yellowish coloring.

Sample calculations:
Determining the concentration of SCN-
C1V1=C2V2
1. 0.0020 M x 3 x 10 -1 mL= 3mL X M
6 x 10-4 M x mL / 3 mL = 0.0002
Calculation of the molar extinction coefficient
Slope = molar extinction coefficient
2. y = 537. 24 x – 0. 0202
molar extinction coefficient = 537. 24
Calculation of the equilibrium concentration
Abs + b/ mx
3. 2.80 x 10-2 nm + 0.0202 / 537. 24
=8.97 x 10-5 M
Calculation for the equilibrium concentration of Fe3+
Initial Fe3+ M - equilibrium Fe(SCN-)2+ M = Equilibrium Fe3+M 4. 3.04 x 10 -4 M - 8.97 x 10-5 M = 3.31 x 10 -3 M
Conclusion:
An acid and a base were mixed together throughout the experiment, which resulted in a bright orange color. It was determined that using the colorimeter at 565nm the would give the optimum wavelength because it was the closest absorbance to 430nm. All of the cuvettes were filled to 3mL so there would not be another dependent variable. Whenever Fe3+ would come in contact with SCN- there would be a color change. Relatively all of the Kc were close to each other as they should be...
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