Chem 2 experiment 34- Equilibrium Constant PDF

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February 12, 2020 Experiment 34: Equilibrium Constant

Hypothesis The equilibrium constant of the chemical equation can be determined using a spectrophotometer.

Discussion The objective of this experiment was to find the equilibrium constant (Kc) of the chemical equation. This was done by first preparing a variety of solutions based off of Tables 34.1 and 34.2 in the laboratory manual. Once this was complete, using the spectrophotometer the % Transmittance and Absorbance of each solution was recorded. Using this data, a graph was formed which represented Absorbance (A) of the first six solutions (blank-5) versus [FeNCS2+]. This established the calibration curve. The equation of the calibration curve allowed us to find the Kc value for each of the Test Solutions (6-10). The average equilibrium constant (Kc) of 8.1317E-05 which was found using the equation

FeNCS 2+¿ ¿ Fe 3+¿ ¿ SCN −¿ . The standard deviation is 6.94885E-05 and the %RSD is 117.0221368. The results were as ¿ ¿ ¿ ¿ ¿ ¿ expected, there was a decrease in %T and a decrease in Absorbance going from the blank solution to solution 10. Conclusion

The experiment was conducted following the manual and results were as expected. In this experiment we were determining the equilibrium constant of a chemical reaction between Fe3+ and SCN-. This was done using a variety of equations such as Beers Law as well as, spectrophotometer and measurements obtained from it. Kc, the equilibrium constant was successfully determined.

Table A. A Set of Standard Solutions to Establish a Standardization Curve 1. Molar Concentration of Fe(NO3)3 0.2 2. Molar Concentration of NaSCN 0.001 Standard Solutions A.1. Volume of NaSCN (mL) A.2. Moles of SCN- (mol) A.3. [SCN-] (25.0 mL) A.4. [FeNCS2+] (mol/L) A.5. Percent transmittance, %T A.6. Absorbance, A A.7. Plot Data of A versus [FeNCS2+]

Blank 0 0 0 0 100 0

1 1 0.000001 0.00004 0.00004 73.9 0.12

2 2 0.000002 0.00008 0.00008 55.4 0.256

3 3 0.000003 0.00012 0.00012 40.5 0.393

4 4 0.000004 0.00016 0.00016 29.3 0.53

5 5 0.000005 0.0002 0.0002 23.8 0.62

Results

Table B. Absorbance for the Set of Test Solutions 1. Molar concentration of Fe(NO3)3 2. Molar concentration of NaSCN Test Solutions B.1. Volume of Fe(NO3)3 (mL) B.2. Moles of Fe3+, initial (mol) B.3. Volume of NaSCN (mL) B.4. Moles of SCN-, initial (mol) B.5. Percent transmittance, %T B.6. Absorbance, A

0.002 0.002 6 5 0.00001 1 0.000002 62.4 0.205

7 5 0.00001 2 0.000004 52.4 0.281

8 5 0.00001 3 0.000006 40.7 0.391

9 5 0.00001 4 0.000008 31.1 0.506

10 5 0.00001 5 0.00001 26.6 0.575

Table C. Calculations of Kc C.1. [FeNCS2+], equilibrium, from

6 6.39985E-05

7 8 9 10 8.7818E-05 0.00012229 0.00015834 0.00017996

calibration curve (mol/L) C.2. Moles FeNCS2+ at equilibrium (10 mL)(mol)

6.39985E-07

8.7818E-07

1.2229E-06

1.5834E-06

1.7996E-06

[Fe3+], Equilibrium C.3. Moles Fe3+, reacted (mol) C.4. Moles Fe3+, equilibrium (mol) C.5. [Fe3+], equilibrium (unreacted). (10mL)(mol/L)

6.39985E-07 9.36002E-06 0.000936002

8.7818E-07 9.1218E-06 0.00091218

1.2229E-06 8.7771E-06 0.00087771

1.5834E-06 8.4166E-06 0.00084166

1.7996E-06 8.2004E-06 0.00082004

[SCN-], equilibrium C.6. Moles SCN-, reacted (mol) C.7. Moles SCN-, equilibrium (mol) C.8. [SCN-], equilibrium (unreacted). (10mL)(mol/L) C.9. Kc= [FeNCS2+]/[Fe3+][SCN-] C.10. Average Kc C.11. Standard Deviation of Kc C.12. Relative Standard Deviation of Kc (%RSD)

6.39985E-07 8.7818E-07 1.2229E-06 1.5834E-06 1.7996E-06 1.36002E-06 3.1218E-06 4.7771E-06 6.4166E-06 8.2004E-06 0.000136002 0.00031218 0.00047771 0.00064166 0.00082004 9.29901E-06 3.0054E-05 6.656E-05 0.00012071 0.00017996 8.1317E-05 6.94885E-05 117.0221368

Absorbance versus [FeNCS2+] 0.7 0.6

f(x) = 3190.71 x + 0 R² = 1

0.5 0.4 0.3 0.2 0.1 0

0

0

0

0

0

{FeNCS

Post Lab Questions 1. To ensure that 447 nm is the best setting for the spectrophotometer you should test a variety of different solutions with different concentrations, the absorbance level should increase as the concentration increases. 2. a. the equilibrium concentration of FeNCS2+ will be too high, this is proved using Beers Law.

0

b. the equilibrium concentration of Fe3+ will be e too low, because the calculated reacted number of moles will be to high. c. the calculated equilibrium constants will be too high because they are directly proportional to the concentration of FeNSC2+. 3.

a. the equilibrium concentration of FeNCS2+ in the Test Solutions will be too high because the calculated concentration will be too high. b. the calculated Kc for the equilibrium will be too high due to the erred data plot

4.

a. The absorbance reading for FeNSC2+ in the analysis would be too high because some of the light detected could be from fingerprint smudges. b. The equilibrium concentration for the FeNSC2+ would be too high because the higher the absorbance the higher equilibrium concentration value.

c. The equilibrium concentration of SCN would be too low because the number of moles within the creation would be too high. d. the Kc for the equilibrium will be too high.

5.

a. This technique error will cause the absorbance reading for FeNSC2+ to be lower because HNO3 is avoided there would be a lower concentration of Fe3+ ion.

b. Kc for the equilibrium will be too low because the FeNCS2+ is low which yields a lower Kc value.

6.

If the absorbance is very high, the solution must be diluted to a known concentration where the absorbance is within the acceptable range.

Reference Beran, J.A., (2008). Page 287-298 Laboratory Manual for Principles of General Chemistry...