Chemical Equilibrium

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Experiment 3: Chemical Equilibrium

Purpose
The purpose of this experiment was to determine the equilibrium constant for the formation of FeSCN2+.

Introduction
Chemical equilibrium is the point in a reversible reaction where the concentration of the reactants and that of the products remains constant. This point of equilibrium is referred to as the Kc value, which can be obtained using the formula: Kc = [product]

[reactant]
In this experiment, we used a spectrophometer to measure the concentration of FeSCN+3 ions. The spectrophometer measures the absorbency (A) of a solution and we could then calculate the concentration using the formula: A = K[FeSCN+2]

Where K is a constant characteristic of the light absorbing species (not to be confused with Kc value).

Procedure
Five test tubes were numbered (1 to 5). Approximately 2.5ml of 0.002M KSCN solution was added to each test tube. 2.5ml of 0.200M of FeCl3 solution was transferred into test tube #1 using a 5ml pipette. This test tube served as a standard.

Approximately 10.0ml of the 0.200M FeCl3 solution was transferred into a 25ml volumetric flask using a 10ml pipette. The volumetric flask was then filled to the 25 ml mark with distilled water. The volumetric flask was shaken and 2.5 ml of the solution was placed into test tube #2 using a clean pipette. The remaining contents of the volumetric flask were transferred to a beaker.

10 ml of the solution in the beaker was transferred to a new 25 ml volumetric flask. Distilled water was added to the flask to the 25 ml mark and the contents were shaken. 2.5 ml of this solution was transferred to test tube #3 using a clean pipette. The same dilution method was carried out two more times until all 5 test tubes were filled.

The absorbance for the solutions in each test tube was measured using a Novaspec II Spectrometer.

Observations and Results

Table 1: Concentrations, absorbance and equilibrium constant
Initial Concentrations (M)Absorbance (A)Equilibrium Concentrations (M)Equilibrium constant
[Fe+3]i[SCN-]i[FeSCN+2]eq[Fe+3]eq[SCN-]eq
Test tube #10.10.0010.648≈0.001≈0.09900
Test tube #20.040.0010.5520.00090.0390.0001230.8
Test tube #30.0160.0010.4260.00070.0150.0003155.6
Test tube #40.00640.0010.2980.00050.0060.0005166.7
Test tube #50.00260.0010.1510.00020.0020.0008125

Mean value of the equilibrium constant: 169.5

Calculations

Concentration of SCN- (for all test tubes):
Moles of SCN- = (Molality of SCN-)(L of solution)
= (0.002 M)(0.0025 L)
= 0.000005 moles

[SCN-]i = moles of solute
L of solution
= (0.000005 moles SCN-)
0.005 L
= 0.001 molal

Concentration of Fe (test tube #1):
Moles of Fe+3 = (Molality of Fe)(L of solution)
= (0.2 M)(0.0025 L)
= 0.0005 moles

[Fe+3]i = moles of solute
L of solution
= (0.0005 moles SCN)
0.005 L
= 0.1 molal

Concentration of Fe in flask #1 (10ml of 0.2M Fe+3):
Cf = CiVi
Vf
= (0.2M)(10 ml)
25 ml
= 0.08 M

Concentration of Fe (test tube #2):
Cf = CiVi
Vf
= (0.08M)(2.5 ml)
5 ml
= 0.04 M
Concentration of Fe in flask #2 (10ml of 0.08M Fe+3):
Cf = CiVi
Vf
= (0.08M)(10 ml)
25 ml
= 0.032 M

Concentration of Fe (test tube #3):
Cf = CiVi
Vf
= (0.032M)(2.5 ml)
5 ml
= 0.016 M

Concentration of Fe in flask #3 (10ml of 0.032M Fe+3):
Cf = CiVi
Vf
= (0.032M)(10 ml)
25 ml
= 0.0128 M

Concentration of Fe (test tube #4):
Cf = CiVi
Vf
= (0.0128M)(2.5 ml)
5 ml
= 0.0064 M

Concentration of Fe in flask #4 (10ml...
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