Determining Equilibrium Constant for Thiocyanatoiron
Objective
Determine the equilibrium constant K_{c} for the chemical reaction:
Fe^{3+}(aq) + SCN^{–}(aq) ß à FeSCN^{2+}(aq)
Procedure
0.0020M Fe(NO_{3})_{3} (ml) | 0.0020M KSCN (ml) | H_{2}O (ml) |
5 | 2 | 3 |
5 | 3 | 2 |
5 | 4 | 1 |
5 | 5 | 0 |
18 (0.2M) | 2 | 0 |
(1) Calibrate the colorimeter.
(2) Prepare five test tubes with the designated solutions:
(3) Measure each solution using the colorimeter and computer software.
(4) Record an observation of the solution temperature.
(5) Calculate the average equilibrium constant for the reaction being studied, using the 5^{th} test as the standard solution.
Result Sheet
Trial: | 1 | 2 | 3 | 4 | 5 (standard) |
Absorption | 0.032 | 0.027 | 0.043 | 0.058 | 0.107 |
[Fe^{3+}]_{i} | 0.001 | 0.001 | 0.001 | 0.001 | 0.0018 |
[SCN^{–}]_{i} | 0.0004 | 0.0006 | 0.0008 | 0.001 | 0.0002 |
[FeSCN^{2+}]_{eq} | 5.98×10^{-5} | 5.05×10^{-5} | 8.04×10^{-5} | 1.08×10^{-4} | 0.0002 |
[Fe^{3+}]_{eq} | 9.4×10^{-4} | 9.4×10-4 | 9.4×10-4 | 9.4×10-4 | |
[SCN^{–}]_{eq} | 3.40×10^{-4} | 5.49×10^{-4} | 7.20×10^{-4} | 8.92×10^{-4} | |
K_{c }(M^{-1}) | 187.0 | 97.86 | 118.79 | 128.81 | |
Average K_{c} | 133.1 | ||||
Average Temperature: 23.5 C |
Sample Calculations
1. [Fe^{3+}]_{i }and [SCN^{–}]_{i }@ Trial 1
[Fe^{3+}]_{i}= 0.0020M Fe(NO_{3})_{3} x (5ml/10ml) = 0.001M, [SCN^{–}]_{i} = 0.0020M KSCN x (2ml/10ml) = 0.0004M
2. [FeSCN^{2+}]_{std}
[FeSCN^{2+}]_{std }= 0.0020M x (2ml/20ml) = 0.0002M
3. [FeSCN^{2+}]_{eq }@ Trial 1
[FeSCN^{2+}]_{eq}= (A_{eq}/A_{std}) x [FeSCN^{2+}]_{std} = [(0.032/0.107) x 0.0002M=5.98×10^{-5}M
4. [Fe^{3+}]_{eq }and [SCN^{–}]_{eq }@ Trial 1
[Fe^{3+}]_{eq}= 0.001 – (5.98×10^{-5}) = 9.4 x 10^{-4}M , [SCN^{–}]_{eq }= 0.0004 – (5.98×10^{-5}) = 3.4 X 1O^{-4}M
5. K_{C }@ Trial 1 (23.5 degrees C)
K_{c}= [FeSCN^{2+}]_{eq }/ [Fe^{3+}]_{eq }[SCN^{–}]_{eq }= (5.98×10^{-5})/(9.4 x 10^{-4})( 3.4 X 1O^{-4}) = 187.0
6. Average K_{c}
K_{c(avg)} = ∑(K_{c(trials 1->n)})/ (n) = (187+97.86+118.79+128.81) / 4 = 133.1
Analysis
In this experiment, the chemical reaction Fe^{3+}(aq) + SCN^{–}(aq) ß à FeSCN^{2+}(aq) was studied to determine the equilibrium constant, K_{c}. To determine this value, the absorptivity of several solutions were recorded using a colorimeter. Inclusion of a standard solution allowed for equilibrium calculations of the reactant and product concentrations. Experimentally, the average constant K_{c} was calculated at (133.1). Linear analysis of the K_{c} range has revealed flaws in the first trial. The source of this error is probably due to Ryan’s inaccurate calibration of the colorimeter or poor handling of the solution.