Equilibrium Constant for FeSCN

Harrison Lee 1/13/15 Period 9-10 Ahsan Shawl The Determination of Keq for FeSCN2+ Objective: The purpose of this experiment is to calculate the equilibrium constant for the reaction of iron(III) ions with thiocyanate ions. Pre-Lab Questions: 1) Define equilibrium. Chemical equilibrium occurs when both forward and reverse reactions proceed at the same rate, and the concentrations of the products and reactions are the same. 2) The reaction for the formation of the diamminesilver ion is as follows: Ag+(aq) + 2NH3(aq)  Ag(NH3)2+(aq) a. Write the equilibrium constant expression for the reaction.

b. An experiment was carried out to determine the value of the equilibrium constant, Keq for the reaction Total moles of Ag+ present: 3.6 x 10-3 Total moles of NH3 present: 6.9 x 10-3 Measured concentration of Ag(NH3)2+ at equilibrium: 3.4 x 10-2M Total solution volume: 100mL Calculate the equilibrium concentration of Ag+ (uncomplexed).

Calculate the value of the equilibrium concentration of NH3 (uncomplexed).

Calculate the value of the equilibrium constant.

3) “The equilibrium concentration of FeSCN2+ ions in each reference solution listed below is essentially equal to the concentration of SCN- ions in solution before any reactions occurs.” Use LeChatelier’s Principle to explain why this statement is true. FeSCN2+ and SCN- have a 1 to 1 mole ratio in the reaction. Their concentrations would be the same no matter how the system is stressed, because LeChatelier’s Principle states that equilibrium will reestablish if there is ever an imbalance. 4) The five reference solutions in Part 1 are prepared by mixing 0.200 M Fe(NO3)3 solution and 0.00020 M KSCN solution in the amounts listed in the following table. Standard

Volume of 0.200 M Fe(NO3)3 Volume of the 0.00020 M Solution KSCN Solution Reference solution #1 8.0mL 2.0mL Reference solution #2 7.0mL 3.0mL Reference solution #3 6.0mL 4.0mL Reference solution #4 5.0mL 5.0mL Reference solution #5 4.0mL 6.0mL The concentration of Fe3+ ions in the first reference solution (M2) before any reaction occurs can be calculated using the so-called “dilution equation”. Use the dilution equation to calculate the concentration of SCN- ions in the five reference solutions before any reaction occurs. Enter these values in Data Table 1 as [FeSCN2+].

Data Tables: Data Table 1—Reference Solutions 21oC [FeSCN2+] 4 x 10-4M 6 x 10-4 M 8 x 10-4 M 1 x 10-3 M 1.2 x 10-3 M

Temperature Sample Reference Solution #1 Reference Solution #2 Reference Solution #3 Reference Solution #4 Reference Solution #5

Absorbance 0.170 0.255 0.362 0.430 0.512

Data Table 2—Test Solutions 21oC [Fe3+] 0.001M 0.001M 0.001M 0.001M 0.001M

Temperature Sample Test Solution #6 Test Solution #7 Test Solution #8 Test Solution #9 Test Solution #10

[SCN-] 2 x 10-4 M 4 x 10-4 M 6 x 10-4 M 8 x 10-4 M 1 x 10-3 M

Absorbance 0.128 0.218 0.322 0.428 0.538

Data Table 3—Results Sample Test Solution #6 Test Solution #7 Test Solution #8 Test Solution #9 Test Solution #10 Average value Average deviation

[FeSCN2+]eq 3.0 x 10-5 M 5.4 x 10-5 M 7.8 x 10-5 M 9.8 x 10-5 M 12.1 x 10-4

[Fe3+]eq 9.0 x 10-4 9.5 x 10-4 9.2 x 10-4 9.0 x 10-4 8.8 x 10-4

[SCN-]eq 0.00017 0.00035 0.00052 0.00070 0.00088

Keq 196.1 162.4 163.0 159.1 156.3

Post-Lab Calculations and Analysis: 1) (See graph paper) 2 and 3) (See Data Table 3) 4) Calculate the equilibrium concentration of Fe3+ ions in each test solution #6-10. (See Data Table 3)

5) Calculate the equilibrium concentration of Fe3+ ions in each test solution #6-10. (See Data Table 3) 6) Use Equation 4 in the background section to calculate the value of the equilibrium constant Keq for each test solution #6-10. (See Data Table 3)

7) Calculate the mean of the equilibrium constant for the five test solutions. (See Data Table 3)

8) Calculate the average deviation for Keq. (See Data Table 3)

9) The average deviation describes the precision of the results. Does the precision indicate that the equilibrium constant is indeed a “constant” for this reaction? Explain. The average deviation indicates that the equilibrium constant is indeed constant. 10) Describe the possible sources of error in this experiment and their likely effect on the results. Fingerprints on the cuvets could have affected the outcome, as well as any other contaminants that may have gotten onto them.

Post-Lab Questions: 1) Explain what is meant by an equilibrium constant. Was the value constant for all your experiments? Should it be constant? The equilibrium should be constant with all of the concentrations of the components involved in the reaction. The values were generally constant for all of the experiments, with some experimental error.

2) What does the calculated value of the equilibrium constant, Keq, indicate regarding the degree of completeness of the reaction? In other words, at equilibrium, are there mostly products, reactants, or relatively large amounts of both? The value of Keq determines whether a reaction is product or reactant favoring. In this experiment, the equilibrium constants were relatively large, at well over 1. As a result, there are mostly products. 3) Explain what a spectrophotometer is and what it measures. Describe how the “standard” solutions were obtained and used to determine concentrations of unknown solutions. A spectrophotometer measures the amount of light that can pass through a given substance. The standard, reference solutions were used to determined the concentrations of unknown solutions because we knew the concentration of FeSCN2+. We could thus use the absorbances of light to create a function that can be used to solve for the unknowns. 4) When you use a spectrophotometer, should you set the wavelength of light to be the same color as that of the solution, or would a different color be more appropriate? Explain. What was the color of light chosen for this experiment? What was the color of the FeSCN2+ ion? The spectrophotometer should not be set to a wavelength of light to the same color as the solution, because the light would be reflected back and not absorbed. This would not give accurate or favorable experimental results. Blue light was used for this experiment because the red color of the FeSCN2+ absorbs it best. 5) What degree of precision can you obtain with the spectrophotometer that was used? What is the major source of error in the experiment? The numbers attained from the spectrophotometer can be as precise to the thousandths place. The major source of error from the experiment would be substances accidentally coated on the cuvet that could block or absorb light. 6) Suggest other experiments which a spectrophotometer would be useful. A spectrophotometer could be useful in indicating slight changes in color to a solution that may be going through a chemical reaction.