Equilibrium Constant of an Iron(III) Thiocyanate Solution PDF

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Laboratory report on spectrophotometric analysis equilibrium constant...

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Equilibrium Constant of an Iron(III) Thiocyanate Solution April 22, 2020 General Chemistry II – CHEM 1412

Abstract In this experiment a spectrophotometer and linear regression were used to determine the equilibrium constants for an iron (III) thiocyanate reaction. The average of these equilibrium constants was 268.96. The equilibrium constants we evaluated for precision and generated a percent relative standard deviation of 1.58%. They were also graded on accuracy, using the expected constant value of 271.49. The average percent error was found to be 1.63%

1

Introduction Chemical equilibrium is the state reached by a reaction mixture when the rates of forward and reverse reactions have become equal [1]. This means the relative concentrations of said reactants and products are generally held constant until something interferes with the equilibrium. The chemical equilibrium of a given reaction is mathematically represented by the equilibrium constant, which can be found by dividing the concentration of the reaction’s products by the concentration of the reactants. The equilibrium constant for the reaction of ferric nitrate with sodium thiocyanate can be determined with spectral analysis via a spectrophotometer. A spectrophotometer works by measuring the spectral reflectance or transmittance curve of a specimen [2]. The science behind this analysis can be explained by the beer-lambert law which explains the relationship between the transmittance or absorbance of a light spectrum and the concentration of an absorbing species [5]. The Beer- Lambert law states that there is a linear relationship between the absorbance of light and the concentration of the species [5]. The equilibrium constant between iron (III) ion and thiocyanate ion to form a thiocyanatoiron (III) ion can be conveniently measured with visible spectrophotometry because the FeSCN 2+ solutions are deep blood-red [4]. This compound when analyzed in a spectrophotometer, produces a specific absorbance pattern that can be quantified and recorded. With said absorbance value, the concentration of the product can thus be determined using the previously stated concepts from the Beer-Lambert Law. Since a linear relationship between concentration and absorption exists, a line equation can be generated beforehand from calibration samples of the compound. With this line equation, the subsequently found absorbance values can be substituted into the equation to calculate that compounds specific concentration. That concentration is then used along with the concentrations of the reactants to accurately calculate the equilibrium constant for the reaction.

Procedure Gloves and safety glasses were used for the entirety of the lab. For part A, three reagents were used to prepare sample solutions: ferric nitrate 0.2 M, sodium thiocyanate 0.001M, and nitric acid 0.1M.

2 An Eppendorf pipette was used for the measurement and delivery of the ferric nitrate and sodium thiocyanate reagents. Clean pipette tips were placed on the Eppendorf for measurements and two different tips were labeled, one for the ferric nitrate and one for the sodium thiocyanate. The nitric acid was delivered using a standard transfer pipette. All samples in part A were prepped using six 25ml volumetric flasks, each with its own glass stopper. These flasks were cleaned with soap and water, rinsed with water, and finally rinsed with 0.1 M nitric acid (experiment solvent) and left to drain and dry completely on a paper towel before being used. The Vernier LabQuest 2 was plugged into the electrical outlet with the power adapter and was kept plugged in throughout the entire lab process. The LabQuest was turned on by pressing and releasing the power button on the upper left-hand side. The LabQuest’s successful startup was confirmed with the presence of the meter screen showing the mode, rate, and duration. The Vernier SpectroVis Plus spectrophotometer was then connected to the LabQuest. The source lamp of the spectrometer was verified as on before moving forward. On the meter screen, the red bar was tapped via fingertip to open the menu. The “change wavelength” option was selected from said menu. The “selected wavelength” field was then selected and changed to a wavelength of 447 nm. This wavelength was then accepted by clicking “OK”, followed by a return to the meter screen. On the meter screen, the “Mode” button was tapped via fingertip and the “events with entry” option was selected. The “name” field was selected and changed to “Concentration”. The “units” field was selected next and changed to M (mol/L) for molarity. The unit and name changes were accepted by clicking “OK”, followed again by a return to the meter screen. The meter screen was confirmed to have the correct wavelength (+/-2 nm) and mode (events with entry). The displayed absorbance was noted as not zero. The ferric nitrate tip was placed on the Eppendorf pipette. The pipette was then adjusted to 10ml. To condition the ferric nitrate tip the pipette was filled with ferric nitrate by placing the tip into the reagent and pushing the plunger to pull up sone of the reagent, filling the tip to the 10ml mark, and then pushed again to empty the reagent back into the beaker. Once conditioned the tip was again placed into the ferric nitrate and filled with reagent all the way to the 10ml mark. Following this the tip was placed

3 into the blank solution flask and the plunger was pressed, releasing all the ferric nitrate into said flask and topped with its stopper. Next, the sample 1-5 flasks were each filled with 10ml of ferric nitrate and stoppered using the same method above with the Eppendorf pipette. The blank solution flask was then taken and diluted with nitric acid to the 25ml mark, making absolutely sure not to go past it. The dilution was done with the transfer pipette, filling with nitric acid and eye balling the volume each time until the 25ml mark was near. When close to final volume the flask was held at eye level and filled slowly drop by drop until the bottom of the meniscus touched the 25ml line. The flask was then stoppered and inverted 10 -15 times to thoroughly mix. Proper mixing was confirmed when the mixture reached a homogenous color. A plastic cuvet was cleaned with soap and water and then filled a third of the way and rinsed with 0.1M nitric acid by capping and shaking the cuvet. The nitric acid was then emptied into the waste beaker. The cuvet was then allowed to drain on a paper towel. The cuvet and its lid were then tapped on the paper to remove any remaining solution. On the meter screen, the red bar was tapped to open the menu and “calibrate” was selected. Before moving forward, the warmup was allowed to reach completion, taking between 10 to 15 minutes. The blank solution was then placed to the one-half to two-thirds mark on the previously cleaned and dried cuvet. The lid was then placed atop the filled cuvet. The sides of the cuvet were wiped down with a kimwipe. Making sure it was aligned properly, the cuvet was then placed in the sample compartment. “Finish Calibration” was then tapped via fingertip. The calibration was allowed to complete and “OK” was then tapped to return to the main meter screen. The absorbance value was confirmed to be 0.000 (+/- 0.002) as required. The cuvet was removed from the spectrometer and discarded into the waste beaker. The cuvet was then rinsed with 0.1M nitric acid and discarded into the waste beaker. Excess solution was removed by tapping the cuvet several times on the paper towel. The ferric nitrate tip was removed from the pipette and replaced by the sodium thiocyanate tip. The Eppendorf pipette was then adjusted to 1 ml by rotating the barrel at the top of the pipette by the plunger. Like the ferric nitrate tip, the sodium thiocyanate tip was then conditioned by filling it to the calibrated mark with the sodium thiocyanate and emptying it back into the reagent. The pipette was then

4 filled to the 1ml mark, placed into the sample 1 flask and emptied completely. Following the addition of the sodium thiocyanate a 5-minute timer was started to ensure consistency in sample reaction times. Visible confirmation was achieved by noting the color change from light yellow to a darker shade closer to orange. Sample 1 was then taken and diluted to the 25ml mark with nitric acid with the transfer pipette stoppered and mixed thoroughly as previously done with the blank solution. The exact same steps above were followed to prepare the 4 remaining samples with only a slight change in the volume of sodium thiocyanate for each sample as follows: For sample 2 -2ml of sodium thiocyanate was used, sample 33ml, sample 4 - 4ml and sample 5 - 5ml. All remaining samples were diluted to the same 25ml mark with nitric acid as done with sample 1. It was observed and noted that as sample proportion of sodium thiocyanate increased the color of said sample became darker shades of orange. After the first 5-minute timer went off a cuvet was then rinsed twice with sample 1 and emptied each time into the waste beaker. The cuvet was then filled between the halfway and the three-quarters mark with the first solution and capped with its lid. The sides of the cuvet were wiped with a kimwipe and the cuvet was tapped lightly on the counter to remove bubbles and it was placed in the sample compartment and confirmed to be aligned properly. The reading was allowed to stabilize for 15-30 seconds and the absorbance of said sample was then recorded. The sample was taken from the spectrophotometer and emptied into the waste beaker. Next the cuvet was pre rinsed and shaken with nitric acid emptied and tapped dry along with the lid. The process outlined in this paragraph, starting with the double rinsing of the cuvet and ending with the recording of the absorbance value was repeated for each of the 5 samples. In part B, the validation phase of the experiment, the reagent concentrations were adjusted as follows: ferric nitrate - 0.002 M, sodium thiocyanate - 0.002M. Nitric acid remained 0.1 M. Samples for this phase were prepared in 10ml volumetric flasks each with plastic snap tops. Just as the glassware in part A was prepped, the flasks for part B were washed with soap and water, rinsed with water, rinsed with nitric acid and allowed to drain and dry completely on a drying rack. In this phase a 5ml Eppendorf pipette was used to ensure greater accuracy. Two new tips were labeled, one for the ferric nitrate and the

5 other for the sodium thiocyanate. The pipette was adjusted to 5 ml volume and the ferric nitrate tip was conditioned with the new concentration by drawing the reagent in and draining it back out into the beaker. A new blank solution was prepped with the new 0.002 M ferric nitrate using the 5ml Eppendorf and diluted to the 10ml mark with the nitric acid from a transfer pipette. The blank solution flask was then capped and mixed as done in part A. A newly cleaned cuvet was rinsed with the new blank solution, tapped dry, filled halfway, capped, wiped with kimwipes, tapped lightly on the benchtop, placed in the spectrophotometer and calibrated at 447nm as done in part A. the cuvet was then emptied, rinsed and dried as done previously. As done in part A, all sample containers were filled and capped with the new ferric nitrate concentration using the Eppendorf pipette. The only difference was that a volume of 5ml was used as opposed to the 10ml volume in part A. All 5 samples were then filled with the sodium thiocyanate with the Eppendorf pipette with the properly labeled tip. The sodium thiocyanate reagent was added as follows: sample 6 – 1ml, sample 7 – 2ml, sample 8 – 3ml, sample 9 – 4ml, sample 10 – 5ml. These samples were then diluted with nitric acid to the 10ml marks, capped and mixed using the same method in part A. each sample was allowed to react for 5 minutes before being analyzed in the spectrophotometer and recorded using clean cuvets in the exact same manner as part A.

6

Experimental Data Trial 1 Sample ID

Blank

[FeSCN2+], M Absorbance, A.U. Table 1-A

0 0.002

1 4.00E05 0.151

2 8.00E05 0.284

3 1.20E04 0.435

4 1.60E04 0.596

5 2.00E04 0.759

AbsorbanceA.U.

Molar Concentration of FeSCN2+ & Light Absorbance - Trial 1 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

f(x) = 3765 x − 0.01 R² = 1

0

0

0

0

0

0

0

0

0

0

0

Concentration M

Graph 1-A Trial 2 Sample ID

Blank

[FeSCN2+], M Absorbance, A.U. Table 1-B

0 0.002

1 4.00E05 0.151

2 8.00E05 0.351

3 1.20E04 0.535

4 1.60E04 0.596

AbsorbanceA.U

Molar Concentration of FeSCN2+ & Light Absorbance - Trial 2 0.8 0.7 f(x) = 3788.57 x + 0.02 0.6 R² = 0.98 0.5 0.4 0.3 0.2 0.1 0 0 0 0 0 0

0

0

Concentration M

Graph 1-B

0

0

0

0

5 2.00E04 0.759

7 Trial 3 Sample ID

Blank

[FeSCN2+], M Absorbance, A.U. Table 1-C

0 0.002

1 4.00E05 0.171

2 8.00E05 0.351

3 1.20E04 0.535

4 1.60E04 0.712

5 2.00E04 0.886

Absorbance A.U

Molar Concentration of FeSCN2+ & Light Absorbance - Trial 3 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0

f(x) = 4447.86 x − 0 R² = 1

0

0

0

0

0

Concentration M Graph 1-C

Validation Trial 1 Sample Preparation Information Samples were prepared and analyzed at 19°C. Each sample was allowed to react for two (2) minutes after the thiocyanate reagent was added. Average Kc

193.57

Average Percent Error (%) Standard Deviation Percent Relative Std. Dev (%)

28.70% 18.02 9.31%

Trial 1 Sample ID Volume 0.002 M Fe3+ (mL) [Fe3+] initial (M) Volume 0.002 M SCN- (mL) [SCN-] initial (M) Absorbance (A.U.) [FeSCN2+]eq (M) 3+

[Fe ]eq (M) -

[SCN ]eq (M) Kc Percent Error (%)

6 5 0.001 1 0.0002 0.126

7 5 0.001 2 0.0004 0.25

8 5 0.001 3 0.0006 0.423

9 5 0.001 4 0.0008 0.545

10 5 0.001 5 0.001 0.655

2.88E-05

5.67E-05

9.56E-05

1.23E-04

1.48E-04

9.71E-04

9.43E-04

9.04E-04

8.77E-04

8.52E-04

1.71E-04 173.05 36.26%

3.43E-04 174.92 35.57%

5.04E-04 209.43 22.86%

6.77E-04 207.12 23.71%

8.52E-04 203.35 25.10%

8 Table 2-A Trial 2 Sample Preparation Information Samples were prepared and analyzed at 20°C. Each sample was allowed to react for five (5) minutes after the thiocyanate reagent was added. Average Kc

243.00

Average Percent Error (%) Standard Deviation Percent Relative Std. Dev (%)

10.82% 24.38 10.03%

Sample ID Volume 0.002 M Fe3+ (mL) [Fe3+] initial (M) Volume 0.002 M SCN- (mL) [SCN-] initial (M) Absorbance (A.U.) [FeSCN2+]eq (M) 3+

[Fe ]eq (M) -

[SCN ]eq (M) Kc Percent Error (%) Table 2-B

6 5 0.001 1 0.0002 0.148

7 5 0.001 2 0.0004 0.318

8 5 0.001 3 0.0006 0.464

9 5 0.001 4 0.0008 0.669

10 5 0.001 5 0.001 0.780

3.37E-05

7.19E-05

1.05E-04

1.51E-04

1.76E-04

9.66E-04

9.28E-04

8.95E-04

8.49E-04

8.24E-04

1.66E-04 209.90 22.69%

3.28E-04 236.30 12.96%

4.95E-04 236.31 12.96%

6.49E-04 273.68 0.81%

8.24E-04 258.82 4.67%

Trial 3 Sample Preparation Information Samples were prepared and analyzed at 20°C. Each sample was allowed to react for five (5) minutes after the thiocyanate reagent was added. Average Kc

248.08

Average Percent Error (%) Standard Deviation Percent Relative Std. Dev (%)

8.62% 16.73 6.74%

Sample ID Volume 0.002 M Fe3+ (mL) [Fe3+] initial (M) Volume 0.002 M SCN- (mL) [SCN-] initial (M) Absorbance (A.U.) [FeSCN2+]eq (M)

6 5 0.001 1 0.0002 0.179 4.07E-05

7 5 0.001 2 0.0004 0.348 7.87E-05

8 5 0.001 3 0.0006 0.471 1.06E-04

9 5 0.001 4 0.0008 0.600 1.35E-04

10 5 0.001 5 0.001 0.721 1.63E-04

[Fe3+]eq (M)

9.59E-04

9.21E-04

8.94E-04

8.65E-04

8.37E-04

1.59E-04 266.28 1.92%

3.21E-04 265.82 2.09%

4.94E-04 241.05 11.21%

6.65E-04 235.51 13.25%

8.37E-04 231.77 14.63%

-

[SCN ]eq (M) Kc Percent Error (%)

9 Table 2-C Trial 4 Sample Preparation Information Samples were prepared and analyzed at 19°C. Each sample was allowed to react for seven (7) minutes after the thiocyanate reagent was added. Average Kc

246.72

Average Percent Error (%) Standard Deviation Percent Relative Std. Dev (%)

9.12% 17.11 6.93%

Sample ID Volume 0.002 M Fe3+ (mL) [Fe3+] initial (M) Volume 0.002 M SCN-

6 5 0.001

7 5 0.001

8 5 0.001

9 5 0.001

10 5 0.001

[SCN-] initial (M) Absorbance (A.U.) [FeSCN2+]eq (M)

1 0.0002 0.175 3.98E-05

2 0.0004 0.300 6.79E-05

3 0.0006 0.501 1.13E-04

4 0.0008 0.612 1.38E-04

5 0.001 0.765 1.72E-04

[Fe3+]eq (M)

9.60E-04

9.32E-04

8.87E-04

8.62E-04

8.28E-04

1.60E-04 258.69 4.72%

3.32E-04 219.34 19.21%

4.87E-04 261.87 3.54%

6.62E-04 241.93 10.89%

8.28E-04 251.79 7.26%

(mL)

-

[SCN ]eq (M) Kc Percent Error (%) Table 2-D

Trial 5 Sample Preparation Information Samples were prepared and analyzed at 20°C. Each sample was allowed to react for seven (7) minutes after the thiocyanate reagent was added. The neck of each flask was wiped with a paper towel to remove excess solution before the sample was mixed. Average Kc 256.95 Average Percent Error (%) 5.36% Standard Deviation 3.73 Percent Relative Std. Dev (%) 1.45%

10 Sample ID Volume 0.002 M Fe3+ (mL)

6 5

7 5

8 5

9 5

10 5

[Fe3+] initial (M) Volume 0.002 M SCN- (mL) [SCN-] initial (M)

0.001 1 0.0002

0.001 2 0.0004

0.001 3 0.0006

0.001 4 0.0008

0.001 5 0.001

Absorbance (A.U.)

0.175 3.98E05 9.60E04 1.60E04 258.69 4.72%

0.339

0.501

0.637

0.765

7.67E-05

1.13E-04

1.44E-04

1.72E-04

9.23E-04

8.87E-04

8.56E-04

8.28E-04

3.23E-04 256.80 5.41%

4.87E-04 261.87 3.54%

6.56E-04 255.61 5.85%

8.28E-04 251.79 7.26%

[FeSCN2+]eq (M) [Fe3+]eq (M) [SCN-]eq (M) Kc Percent Error (%) Table 2-E Trial 6

Sample Preparation Information Samples were prepared and analyzed at 20°C. Each sample was allowed to react for five (5) minutes after the thiocyanate reagent was added. The neck of each flask was wiped with a paper towel to remove excess solution before the sample was mixed. Average Kc

268.96

Average Percent Error (%) Standard Deviation Percent Relative Std. Dev (%)

1.63% 4.25 1.58%

Sample ID Volume 0.002 M Fe3+ (mL) [Fe3+] initial (M) Volume 0.002 M SCN- (mL) [SCN-] initi...