Chemical Equilibrium

Chemical Equilibrium: Finding a Constant, K_c

The purpose of this lab is to experimentally determine the equilibrium constant, K_c, for the following chemical reaction:

When Fe³⁺ and SCN^- are combined, equilibrium is established between these two ions and the FeSCN²⁺ ion. In order to calculate K_c for the reaction, it is necessary to know the concentrations of all ions at equilibrium: [FeSCN²⁺]_eq, [SCN^-]_eq, and [Fe³⁺]_eq. You will prepare four equilibrium systems containing different concentrations of these three ions. The equilibrium concentrations of the three ions will then be experimentally determined. These values will be substituted into the equilibrium constant expression to see if K_c is indeed constant.

In order to determine [FeSCN²⁺]_eq, you will use the colorimeter shown in Figure 1. The FeSCN²⁺ ion produces solutions with a red color. Because the red solutions absorb blue light very well, the blue LED setting on the colorimeter is used. The colorimeter measures the amount of blue light absorbed by the colored solutions (absorbance, A). By comparing the absorbance of each equilibrium system, A_eq, to the absorbance of a standard solution, A_std, you can determine [FeSCN²⁺]_eq. The standard solution has a known FeSCN²⁺ concentration.

To prepare the standard solution, a very large concentration of Fe³⁺ will be added to a small initial concentration of SCN^- (hereafter referred to as [SCN^-]_i. The [Fe³⁺] in the standard solution is 100 times larger than [Fe³⁺] in the equilibrium mixtures. According to LeChatelier's principle, this high concentration forces the reaction far to the right, using up nearly 100% of the SCN^- ions. According to the balanced equation, for every one mole of SCN^- reacted, one mole of FeSCN²⁺ is produced. Thus [FeSCN²⁺]_std is assumed to be equal to [SCN^-]_i.

Assuming [FeSCN²⁺] and absorbance are related directly (Beer's law), the concentration of FeSCN²⁺ for any of the equilibrium systems can be found by:

Knowing the [FeSCN²⁺]_eq allows you to determine the concentrations of the other two ions at equilibrium. For each mole of FeSCN²⁺ ions produced, one less mole of Fe³⁺ ions will be found in the solution (see the 1:1 ratio of coefficients in the equation on the previous page). The [Fe³⁺] can be determined by:

[Fe³⁺]_eq = [Fe³⁺]_i - [FeSCN²⁺]_eq

Because one mole of SCN^- is used up for each mole of FeSCN²⁺ ions produced, [SCN^-]_eq can be determined by:

[SCN^-]_eq = [SCN^-]_i - [FeSCN²⁺]_eq

Knowing the values of [Fe³⁺]_eq, [SCN^-]_eq, and [FeSCN²⁺]_eq, you can now calculate the value of K_c, the equilibrium constant.

MATERIALS

CBL System
TI-8X Graphing Calculator
Vernier Colorimeter
Vernier adapter cable
1 plastic cuvette
five 20 X 150 mm test tubes
thermometer tissues (preferably lint-free)
0.0020 M KSCN
0.0020 M Fe(NO₃)₃ (in 1.0 M HNO₃)
0.200 M Fe(NO₃)₃ (in 1.0 M HNO₃)
four pipets
pipet bulb or pipet pump
three 100-mL beakers

PROCEDURE

1. Obtain and wear goggles.

2. Label four 20 X 150 mm test tubes 1-4. Pour about 30 mL of 0.0020 M Fe(NO₃)₃ into a clean, dry 100-mL beaker. Pipet 5.0 mL of this solution into each of the four labeled test tubes. Use a pipet pump or bulb to pipet all solutions. Caution: Fe(NO₃)₃ solutions in this experiment are prepared in 1.0 M HNO₃ and should be handled with care. Pour about 25 mL of the 0.0020 M KSCN into another clean, dry 100-mL beaker. Pipet 2, 3, 4 and 5 mL of this solution into Test Tubes 1-4, respectively. Obtain about 25 mL of distilled water in a 100-mL beaker. Then pipet 3, 2, 1 and 0 mL of distilled water into Test Tubes 1-4, respectively, to bring the total volume of each test tube to 10 mL. Mix each solution thoroughly with a stirring rod. Be sure to clean and dry the stirring rod after each mixing. Measure and record the temperature of one of the above solutions to use as the temperature for the equilibrium constant, K_c. Volumes added to each test tube are summarized below:

3. Prepare a standard solution of FeSCN²⁺ by pipetting 18 mL of 0.200 M Fe(NO₃)₃ into a 20 x 150 mm test tube labeled "5". Pipet 2 mL of 0.0020 M KSCN into the same test tube. Stir thoroughly.

4. Plug the colorimeter into the adapter cable in Channel 1 of the CBL System. Connect the CBL System to the TI-8X calculator with the link cable using the port on the bottom edge of each unit. Firmly press in the cable ends.

5. Turn on the CBL unit and the TI-8X calculator. Press PRGM and select CHEMBIO. Press ENTER, then press ENTER again to go to the CHEM MAIN MENU.

6. Set up the calculator and CBL for the colorimeter.

• Select SET UP PROBES from the CHEM MAIN MENU.
• Enter "1" as the number of probes.
• Select COLORIMETER from the SELECT PROBE menu.
• Enter "1" as the channel number.
• PERFORM a NEW CALIBRATION.

7. You are now ready to calibrate the colorimeter. First prepare a blank by filling a cuvette 3/4 full with distilled water. To correctly use a colorimeter cuvette, remember:

• All cuvettes should be wiped clean and dry on the outside with a tissue.
• Handle cuvettes only by the top edge of the ribbed sides.
• All solutions should be free of bubbles.
• Always position the cuvette with its reference mark facing toward the white reference mark at the right of the cuvette slot on the colorimeter.

To calibrate the cuvette at 0% and 100% transmittance:

• Place the blank cuvette in the cuvette slot of the colorimeter and close the lid. Turn the wavelength knob of the colorimeter to the 0% T position. In this position, the light source is turned off, so no light is received by the photocell. When the voltage reading displayed on the CBL screen stabilizes, press TRIGGER on the CBL and enter "0" (the "reference") in the TI-8X calculator.
• Turn the wavelength knob of the colorimeter to the Blue LED position (470 nm). In this position, the colorimeter is calibrated to show 100% of the blue light being transmitted through the blank cuvette. When the displayed voltage stabilizes, press TRIGGER and enter "100" in the TI-83 calculator. Leave the wavelength knob of the colorimeter set to the Blue LED position for the remainder of the experiment.
• Press TRIGGER to return to the CHEM MAIN MENU.

8. Set up the calculator and CBL for data collection.

• Select COLLECT DATA from the CHEM MAIN MENU.
• Select TRIGGER from the DATA COLLECTION menu.
• Enter "5" as the number of samples.
• The percent transmittance reading is displayed on the screen of the CBL. When you get to Step 9, you will use TRIGGER on the CBL to store absorbance and percent transmittance data.

9. You are now ready to collect absorbance data for the four equilibrium systems and the standard solution. Empty the water from the cuvette and rinse it twice with ~1-mL portions of the Test Tube 1 solution. Then fill the cuvette 3/4 full with the solution from Test Tube 1. Wipe the outside of the cuvette with a tissue and then place the cuvette in the colorimeter. After closing the lid, wait for the percent transmittance value displayed on the CBL to stabilize. Once the reading has stabilized, press on the CBL to store the absorbance value.

10. 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 procedure in Step 9 to find and store the absorbance of this solution.

11. Repeat the Step 10 procedure to find the absorbance of the solutions in Test Tubes 3, 4, and 5 (the standard solution).

12. When you have finished collecting data, press ENTER to return to the CHEM MAIN MENU. Select VIEW DATA. Press STAT, then select Edit to view the absorbance data in L 2. Record the absorbance values (L 2) for Test Tubes 1-5 in your data table.

PROCESSING THE DATA

1. Write the K_c 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:

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:

In Test Tube 1, [SCN^-]_i = (2 mL / 10 mL)(.0020 M) = .00040 M. Calculate this for the other three test tubes.

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

where A_eq and A_std 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 K_c for Trials 1-4. Be sure to show the K_c expression and the values substituted in for each of these calculations.

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

DATA AND CALCULATIONS