Determination Of An Equilibrium Constant Essay

1663 words - 7 pages


To determine the equilibrium constant for the following reaction:

Fe3+(aq) + SCN-(aq) -----> Fe(SCN)2+(aq)


In this experiment, the equilibrium constant, K, for the above reaction is given by the expression:

K = [FeSCN 2+]

[Fe 3+][SCN 1-]

where the concentrations of the substances are those at equilibrium. The equilibrium concentrations of these substances will be determined and used to determine K.

Since the reactants are essentially colorless, whereas the complex ion product is deeply colored, a spectrophotometer will be used to determine the maximum absorbance due to the FeSCN2+at its four different concentrations. Beer's Law states that the absorbance of a colored solution is directly proportional to the concentration of the absorbing species. In this experiment, the FeSCN2+ is the absorbing species. The formula for Beer's Law is A = kc where A is the absorbance of the solution, k is the Beer's Law constant and c represents the concentration of the absorbing species. To make easier to calculate the equilibrium concentration of FeSCN2+, the concentration vs. absorbance is plotted on the graph. Then the formula of Beer's Law is C = kA. To make the constant k more significant number, the concentration values will be used in mmol/L.

The experiment will be done in two parts. In the first part the value of the Beer's Law constant, k, will be determined and in the second part, the Beer's Law constant and the absorbance will be used to determine the equilibrium concentration of FeSCN2+. After the equilibrium concentration of Fe(SCN)2+ will be measured by way of the absorbance, the concentrations of the reactants Fe 3+ and SCN 1- can be calculated from their starting concentrations and the concentration of the complex ion.

Because the stoichiometry is 1:1:1 (reactant:reactant:product), the moles of each reactant used up in the reaction is equal to the moles of product formed. And since the initial concentrations of the reactants is known, it is a simple subtraction to find the moles of reactant remaining at equilibrium. The value for the equilibrium constant will be found from simple stoichiometric consideration, where x represent the concentration:

Fe3+(aq) + SCN-(aq) -----> Fe(SCN)2+(aq)

Initial concentration [Fe3+]0 [SCN-]0 0

Change -x -x x

Equilibrium [Fe3+]0 -x [SCN-]0 -x x

Finally, once the concentration of each substance in the reaction is determined, the equilibrium constant can be calculated


Part I -- Preparation of the Calibration Curve

Prepare a 0.00100 M solution...

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