Determination of an Equilibrium Constant Essay

941 Words 4 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
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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

<li value="1"> Prepare a 0.00100 M solution of FeSCN2+(aq) by adding 0.100 moles of Fe(NO3)3 per liter of 0.5 M HNO3 and 0.00200 moles of KSCN per liter of acid.
<li value="2"> Determine the wavelength of maximum absorbance and record that absorbance. Note the concentration of Fe 3+ is much higher than the concentration of SCN-, leading to the assumption that all of the SCN-

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