Introduction: The equation of the reaction is Fe 3+ (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(NO 3 ) 3 II 2.0 x 10-3 M Fe(NO 3 ) 3 III 3.0 x 10-3 M KSCN IV 0.5 M HNO 3 Fig.2 [KSCN] KSCN (sol III) Fe(NO 3 ) 3 (sol I) HNO 3 (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 Beaker # 2.0 x 10-3 M Fe(NO 3 ) 3 (Solution II) 3.0 x 10-3 M KSCN (Solution III) 0.50 M HNO 3 (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 Data Tables and graphs : Solution 5 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 0.8 0.7 0.6 y = -0.0047x + 2.9771 R² = 0.8972 Absorbance vs wavelength Linear (Absorbance vs wavelength) 0.5 400 420 440 460 480 500 Solution # Absorbency I 0.108 Equilibrium Solutions II 0.205 III 0.378 IV 0.628 V 1.295
0.7 0.6 0.5 0.4 0.3 0.2 Concentration vs Absorbance y = 1217.7x + 0.0492 R² = 0.9951 Concentration vs Absorbance Linear (Concentration vs Absorbance) 0.1 0 0 0.0002 0.0004 0.0006 Absorbance Fe(NCS) 0.105 0.00005 0.213 0.00012 0.358 0.00027 0.641 0.00048 The Volume Used in Each Solution For Equilibrium Fe(NO 3 ) 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 HNO 3 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 6.025x10 2. 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.