Chemical Equilibrium: Finding a Constant, Kc

1. Click  to begin data collection.
2. Empty the water from the cuvette. Rinse it twice with ~1 mL portions of the Test Tube 1 solution.
3. Wipe the outside of the cuvette with a tissue and then place the cuvette in the Colorimeter. After closing the lid, wait for the absorbance value displayed in the meter to stabilize. Then click , type “1” (the trial number) in edit box, and press the ENTER key.
4. Discard the cuvette contents as directed by your teacher. Rinse the cuvette twice with the Test Tube 2 solution and fill the cuvette 3/4 full. Follow the Step-c procedure to find the absorbance of this solution. Type “2” in the edit box and press ENTER.
5. Repeat the Step d procedure to find the absorbance of the solutions in Test Tubes 3, 4, and 5 (the standard solution).
6. From the table, record the absorbance values for each of the five trials in your data table.
7. Dispose of all solutions as directed by your instructor.

PROCESSING THE DATA

1.   Write the Kc expression for the reaction in the Data and Calculation table.

   2.   Calculate the initial concentration of Fe³⁺, based on the dilution that results from adding KSCN solution and water to the original 0.0020 M Fe(NO₃)₃ solution. See Step 2 of the procedure for the volume of each substance used in Trials 1-4. Calculate [Fe3+]i using the equation:

[Fe³⁺]i =  Í (0.0020 M)

      This should be the same for all four test tubes.

   3.   Calculate the initial concentration of SCN^–, based on its dilution by Fe(NO₃)₃ and water:

 [SCN^–]i = Í (0.0020 M)

      In Test Tube 1, [SCN^–]i = (2 mL / 10 mL)(0.0020 M) = 0.00040 M. Calculate this for the other three test tubes.

   4.   [FeSCN²⁺]eq is calculated using the formula:

 [FeSCN²⁺]eq =  Í [FeSCN²⁺]std

      where Aeq and Astd are the absorbance values for the equilibrium and standard test tubes, respectively, and [FeSCN²⁺]std = (1/10)(0.0020) = 0.00020 M. Calculate [FeSCN²⁺]eq for each of the four trials.

   5.   [Fe³⁺]eq: Calculate the concentration of Fe³⁺ at equilibrium for Trials 1-4 using the equation:

 [Fe³⁺]eq = [Fe³⁺]i – [FeSCN²⁺]eq

   6.   [SCN^–]eq: Calculate the concentration of SCN- at equilibrium for Trials 1-4 using the equation:

 [SCN^–]eq = [SCN^–]i – [FeSCN²⁺]eq

7.   Calculate Kc for Trials 1-4. Be sure to show the Kc expression and the values substituted in for each of these calculations.

8.   Using your four calculated Kc values, determine an average value for Kc. How constant were your Kc values?