Determination of an Equilibrium Constant for a Chemical
Determination of an Equilibrium Constant for a Chemical Reaction Purpose: To find the equilibrium constant of the (Kc) for the reaction: Fe3+(aq) + SCN-(aq) ⇌ FeSCN2+ (aq) through several different concentrations of reactants, using spectrophotometric analysis. Procedure: Please refer to the CHEM 1A03/1E03/1AA3 Lab Manual 2013, pages 43-47, for a detailed procedure. Part A: Observations: Table 1: Values of Absorbance and Concentration for Different Volumes of Potassium Thiocyanate and 0.200 mol/L Ferric Nitrate Test Tube Number
Volume of KSCN (mL)
Absorbance (A)
1
1
0.182
Concentration of FeSCN2+ (mol/L) 4.00 x 10-6
2
2
0.368
8.00 x 10-6
3
3
0.669
1.20 x 10-5
Qualitative Observations: FeSCN2+(aq) and KSCN (aq) were both clear, colourless liquids initially. Solution turned to an orange and brown colour when the ferric nitrate was added to the potassium thiocyanate. Sample Calculations: Fe3+(aq) + SCN-(aq) ⇌ FeSCN2+ (aq) n(SCN-) = cv n(SCN-) = (.002 Mol/L) (.001(L)) n(SCN-) = .000002 Mol Due to 1:1 Mol ratio, n(FeSCN2+) = .000002Mol [FeSCN2+] = n/v [FeSCN2+] = (.000002(Mol)/.05(L)) [FeSCN2+] = .00004Mol/L
Graph 1: Absorbance vs. Concentration of FeSCN2+ Constant
The above graph shows the proportional relationship between the concentration and the absrbance of FeSCN2+. Using a line of best fit, a proportional constant of 5223 is determined to be the slope.
Part B: Sample Calculations: Initial [Fe3+(aq)] in Test Tube #1: C1V1 = C2V2 (0.00200 mol/L)(0.0050L) = (C2)(0.010L) C2= 0.00100 mol/L Initial [SCN-(aq)] in Test Tube #1: C1V1 = C2V2 (0.00200 mol/L)(0.0010L) = (C2)(0.010L) C2=0.000200 mol/L Equilibrium [FeSCN2+(aq)] in Test Tube #1: A = Absorbance = 0.243A y = proportionality constant = 5223 A/mol/L A = y[FeSCN2+] [FeSCN2+]= A/y 0.243A/5223 A/mol/L = 4.65x10-5 mol/L
Observations: ICE Table for Test Tube #1: Reaction: Fe3+(aq) + SCN-(aq) ⇌ FeSCN2+(aq) Initial 1.00x10^-3 mol/L 2.00x10^-3 mol/L ------Concentration (I) Change in -x -x +x Concentration (C) Equilibrium 1.00x10^-3 – x 2.00x10^-4 – x 4.65x10^-5 Concentration (E) x=4.65x10^-5 mol/L Equilibrium concentration for Fe^3+ (aq) = (1.00x10^-3 mol/L) - (4.65x10^-5 mol/L) = 9.54x10^-4 mol/L Equilibrium concentration for SCN^-1 (aq) = (2.00x10^-4 mol/L) – (4.65x10^-5 mol/L) = 1.54x10^4 mol/L
Equilibrium concentration for FeSCN^2+(aq) = 4.65x10^-5 mol/L Equilibrium Constant for Test Tube #1: Kc=Equilibrium Constant Kc=[Products]/[Reactants] Kc=[FeSCN^2+ (aq)]/[[Fe^3+ (aq)]*[SCN^-1 (aq)]] Kc=[4.65x10^-5]/[[1.54x10^-4]*[9.54x10^-4]] Kc=317
Observations: Table 3: Initial and Equilibrium Concentrations for Fe 3+(aq) and SCN-(aq), Equilibrium Concentrations for FeSCN2+(aq), and Values for Equilibrium Constant For Each Different Solution Test Tube Number
Initial [Fe3+] (mol/L)
1 2 3 4 5
1.00x10-3 1.00x10-3 1.00x10-3 1.00x10-3 1.00x10-3
Initial [SCN-(aq)] (mol/L) 2.00x10-4 4.00x10-4 6.00x10-4 8.00x10-4 1.00x10-3
Equilibrium [FeSCN2+(aq)] (mol/L) 4.65x10^-5 7.89x10-5 1.03x10-4 1.40x10-4 1.73x10-4
Equilibrium [Fe3+(aq] (mol/L) 9.54x10-4 9.21x10-4 8.97x10-4 8.60x10-4 8.27x10-4
Equilibrium [SCN-(aq)] (mol/L) 1.54x10-4 3.21x10-4 4.97x10-4 6.60x10-4 8.27x10-4
Kc Value 317 267 231 247 253
Average Kc Value: (317+267+231+247+253)/5 = 263 Discussion: In this experiment I used different amounts and concentrations of ferric nitrate and potassium thiocyanate to determine the equilibrium constant for the reaction Fe2+ (aq) + SCN- (aq) ⇌ FeSCN2+ (aq). My first step was to add Ferric Nitrate to various amounts of KSCN. After this the absorbance was calculated using a spectrophotometer. After SCN- completely reacted with the excess Fe(NO3)3 ions, I was able to determine the concentration of FeSCN^2+, since the molar concentration between SCN- and FeSCN2+, was 1:1. After, obtaining these concentrations; I plotted them against the related absorbance values, thereby, showing a proportional relationship between the two variables. Ultimately, through this graph, I was able to determine the proportionality constant for the equation A=y[FeSCN2+], where y, the proportionality constant, was equal to 5223 A/mol/L. Essentially, this value aids in retrieving the concentration of FeSCN2+ ions at equilibrium. Furthermore, I found the initial concentration of the Fe3+ and SCN- ions using the dilution equation where C1V1=C2V2. Using these concentrations and ICE chart I was able to determine the change in concentration through equilibrium. This then allows me, which helped me in determining the equilibrium concentrations of all products and reactants. Using these new equilibrium concentration I was then able to determine the new equilibrium constant equation Kc = [Products] / [Reactants], for each test tube. Through this an average Kc value for all five test tubes are determined. The equilibrium constant (Kc), allows us to determine the state of the reaction, in which we are able to determine the amount of product in the solution compared to the amount of reactant. If the Kc
value is larger than one, the solution contains more product than the reactants. Ultimately, this means that the equilibrium shifts in the forward direction. Furthermore, this means that the SCNions completely react with the Fe3+ions, to form FeSCN2+. One source of error is that each test tube in the experiment had different values for their equilibrium constant. In theory these values should have been the same. Temperature and Concentration are the only sources that could have caused this difference. If the reaction is not completed then concentration can be affected, thus leading to an equilibrium which has not been fully reached. This in turn leads to and the wrong values were used to determine the equilibrium constant, thus giving us inaccurate proportionality constant, and inaccurate equilibrium concentrations. Additionally, water left in the test tubes after rinsing can have an impact on the concentration. The concentration is altered as the volume increases, causing the solution to be more dilute. Ultimately, this causes the onward calculations and results to be off. Conclusion: Through this experiment, I was able to find that the average Kc value for this reaction was around 263. This large Kc value indicates that the reaction proceeds strongly in the forward direction, thereby, suggesting more products are made then there are reactants.
Volume of KSCN (mL)
Absorbance (A)
1
1
0.182
Concentration of FeSCN2+ (mol/L) 4.00 x 10-6
2
2
0.368
8.00 x 10-6
3
3
0.669
1.20 x 10-5
Qualitative Observations: FeSCN2+(aq) and KSCN (aq) were both clear, colourless liquids initially. Solution turned to an orange and brown colour when the ferric nitrate was added to the potassium thiocyanate. Sample Calculations: Fe3+(aq) + SCN-(aq) ⇌ FeSCN2+ (aq) n(SCN-) = cv n(SCN-) = (.002 Mol/L) (.001(L)) n(SCN-) = .000002 Mol Due to 1:1 Mol ratio, n(FeSCN2+) = .000002Mol [FeSCN2+] = n/v [FeSCN2+] = (.000002(Mol)/.05(L)) [FeSCN2+] = .00004Mol/L
Graph 1: Absorbance vs. Concentration of FeSCN2+ Constant
The above graph shows the proportional relationship between the concentration and the absrbance of FeSCN2+. Using a line of best fit, a proportional constant of 5223 is determined to be the slope.
Part B: Sample Calculations: Initial [Fe3+(aq)] in Test Tube #1: C1V1 = C2V2 (0.00200 mol/L)(0.0050L) = (C2)(0.010L) C2= 0.00100 mol/L Initial [SCN-(aq)] in Test Tube #1: C1V1 = C2V2 (0.00200 mol/L)(0.0010L) = (C2)(0.010L) C2=0.000200 mol/L Equilibrium [FeSCN2+(aq)] in Test Tube #1: A = Absorbance = 0.243A y = proportionality constant = 5223 A/mol/L A = y[FeSCN2+] [FeSCN2+]= A/y 0.243A/5223 A/mol/L = 4.65x10-5 mol/L
Observations: ICE Table for Test Tube #1: Reaction: Fe3+(aq) + SCN-(aq) ⇌ FeSCN2+(aq) Initial 1.00x10^-3 mol/L 2.00x10^-3 mol/L ------Concentration (I) Change in -x -x +x Concentration (C) Equilibrium 1.00x10^-3 – x 2.00x10^-4 – x 4.65x10^-5 Concentration (E) x=4.65x10^-5 mol/L Equilibrium concentration for Fe^3+ (aq) = (1.00x10^-3 mol/L) - (4.65x10^-5 mol/L) = 9.54x10^-4 mol/L Equilibrium concentration for SCN^-1 (aq) = (2.00x10^-4 mol/L) – (4.65x10^-5 mol/L) = 1.54x10^4 mol/L
Equilibrium concentration for FeSCN^2+(aq) = 4.65x10^-5 mol/L Equilibrium Constant for Test Tube #1: Kc=Equilibrium Constant Kc=[Products]/[Reactants] Kc=[FeSCN^2+ (aq)]/[[Fe^3+ (aq)]*[SCN^-1 (aq)]] Kc=[4.65x10^-5]/[[1.54x10^-4]*[9.54x10^-4]] Kc=317
Observations: Table 3: Initial and Equilibrium Concentrations for Fe 3+(aq) and SCN-(aq), Equilibrium Concentrations for FeSCN2+(aq), and Values for Equilibrium Constant For Each Different Solution Test Tube Number
Initial [Fe3+] (mol/L)
1 2 3 4 5
1.00x10-3 1.00x10-3 1.00x10-3 1.00x10-3 1.00x10-3
Initial [SCN-(aq)] (mol/L) 2.00x10-4 4.00x10-4 6.00x10-4 8.00x10-4 1.00x10-3
Equilibrium [FeSCN2+(aq)] (mol/L) 4.65x10^-5 7.89x10-5 1.03x10-4 1.40x10-4 1.73x10-4
Equilibrium [Fe3+(aq] (mol/L) 9.54x10-4 9.21x10-4 8.97x10-4 8.60x10-4 8.27x10-4
Equilibrium [SCN-(aq)] (mol/L) 1.54x10-4 3.21x10-4 4.97x10-4 6.60x10-4 8.27x10-4
Kc Value 317 267 231 247 253
Average Kc Value: (317+267+231+247+253)/5 = 263 Discussion: In this experiment I used different amounts and concentrations of ferric nitrate and potassium thiocyanate to determine the equilibrium constant for the reaction Fe2+ (aq) + SCN- (aq) ⇌ FeSCN2+ (aq). My first step was to add Ferric Nitrate to various amounts of KSCN. After this the absorbance was calculated using a spectrophotometer. After SCN- completely reacted with the excess Fe(NO3)3 ions, I was able to determine the concentration of FeSCN^2+, since the molar concentration between SCN- and FeSCN2+, was 1:1. After, obtaining these concentrations; I plotted them against the related absorbance values, thereby, showing a proportional relationship between the two variables. Ultimately, through this graph, I was able to determine the proportionality constant for the equation A=y[FeSCN2+], where y, the proportionality constant, was equal to 5223 A/mol/L. Essentially, this value aids in retrieving the concentration of FeSCN2+ ions at equilibrium. Furthermore, I found the initial concentration of the Fe3+ and SCN- ions using the dilution equation where C1V1=C2V2. Using these concentrations and ICE chart I was able to determine the change in concentration through equilibrium. This then allows me, which helped me in determining the equilibrium concentrations of all products and reactants. Using these new equilibrium concentration I was then able to determine the new equilibrium constant equation Kc = [Products] / [Reactants], for each test tube. Through this an average Kc value for all five test tubes are determined. The equilibrium constant (Kc), allows us to determine the state of the reaction, in which we are able to determine the amount of product in the solution compared to the amount of reactant. If the Kc
value is larger than one, the solution contains more product than the reactants. Ultimately, this means that the equilibrium shifts in the forward direction. Furthermore, this means that the SCNions completely react with the Fe3+ions, to form FeSCN2+. One source of error is that each test tube in the experiment had different values for their equilibrium constant. In theory these values should have been the same. Temperature and Concentration are the only sources that could have caused this difference. If the reaction is not completed then concentration can be affected, thus leading to an equilibrium which has not been fully reached. This in turn leads to and the wrong values were used to determine the equilibrium constant, thus giving us inaccurate proportionality constant, and inaccurate equilibrium concentrations. Additionally, water left in the test tubes after rinsing can have an impact on the concentration. The concentration is altered as the volume increases, causing the solution to be more dilute. Ultimately, this causes the onward calculations and results to be off. Conclusion: Through this experiment, I was able to find that the average Kc value for this reaction was around 263. This large Kc value indicates that the reaction proceeds strongly in the forward direction, thereby, suggesting more products are made then there are reactants.
