Determination of an Equilibrium Constant

Determination of an Equilibrium Constant

Objectives: To study the chemical reaction of Fe3+ and SCN- to produce Fe(SCN)2+ in aqueous solution. To measure concentrations of ions in a solution using a spectrophotometer. To determine the equilibrium constant of this reaction at a given temperature.

Procedure:

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Part 1: Calibration Curve Using Standard Solutions (Done with a group)

1. Obtain seven clean 150 mL beakers, one beaker per person.

2. Prepare solutions A-1 to A-7 in each beaker, each person preparing one solution.

3.Once all the solutions are prepared, set the wavelength of the spectrophotometer to 450nm.

4. Obtain a cuvette and fill it about halfway with solution A-1, the blank sample. Insert the cuvette carefully, as to not get any fingerprints on the sides, into the spectrophotometer, and zero the instrument. This will be the only time the spectrophotometer needs to be zeroed.

5. Pour out solution A-1, rinse the cuvette twice with A-2, and record the absorbance for A-2. Do this for the subsequent solutions.

6. Plot the data, absorbance versus concentration.

This will be the calibration curve used for Part 2.

Part 2: Solutions to Determine the Equilibrium Constant, K

1. Obtain 5 clean 50 mL beakers and prepare solutions B-1 to B-5.

2. Follow steps 3-5 from Part 1, using B-1 as the new blank.

3. Using the calibration curve from Part 1, determine the equilibrium concentration of [Fe(SCN) 2+] from the absorbance measured for B-1 to B-5.

At equilibrium, there will be very little product formed and mostly reactant present.

2. The purpose of a blank is to cancel out or zero the absorbance of all the other compounds in the sample except the component whose absorbance is to be measured. In this experiment the medium used was not water, so water would not be used as the blank, but instead the Fe(NO3) 3 and HNO3 solution is used. We do not want any KSCN to be present in the blank since we are finding the absorption of the product [Fe(SCN) 2+].

3. A =? bc = log (I0/It) = the thickness of the absorbing sample (units = centimeters) c = the concentration of the sample (units = moles/L) ? = the molar absorptivity (units = L mol-1cm-1) I0 = the intensity of the incident light It = the transmitted light

4. By using limited quantities of SCN- and excess quantities of Fe3+, known amounts of Fe(SCN) 2+ are produced. The amount of Fe(SCN) 2+ that is formed is essentially equal to the starting amount of the limiting reactant.

5. By using approximately equal concentrations of the reactants, this ensures that the reaction will not go to completion.

6. Absorptions that read off-scale can be a result of numerous factors. - Cuvette sides may be dirty - slot where cuvette is inserted may be contaminated - transmission decreases with path length, so if the path were too long this could cause a high absorbance and may fall below the sensitivity of the machine

7. x represents the equilibrium concentration of Fe(SCN) 2+. By first finding the absorbance of the solution, the calibration curve could be used to find the concentration of Fe(SCN) 2+.

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