Experiment 16

1.	Safety
2.	Objectives/Overview
3.	Procedures
4.	Observations
5.	Equipment
6.	View Techniques
7.	Waste Disposal/Clean-up
8.	Conclusions
9.	Calculations/Set-Ups
10.	Grading Scale
11.	Review Prelab Questions
12.	Review Postlab Questions

Experiment 16
Chemical Equilibrium: Finding a Constant, K_c

PROCEDURES

1.Obtain and wear goggles.

2. Pour about 30 mL of 0.00200 M Fe(NO₃ )₃ into a clean, dry 100-mL beaker. Label five 50-mL beakers 1-5. Pipette about 5 mL of the solution into each of the first four labeled beakers. Record the actual volume of solution used. The pipettes can be read to hundredths of a milliliter, so each volume should have 2 decimal places. Use a pipette bulb to pipette 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 0.00200 M KSCN into another clean, dry 100-mL beaker. Pipette 2, 3, 4 and 5 mL of this solution into Beakers 1-4, respectively. Record volumes to hundredths of a milliliter. Obtain about 25 mL of distilled water in a 100-mL beaker. Then pipette 3, 2, 1, and 0 mL of distilled water into Beakers 1-4, respectively, to bring the total volume of each beaker to 10.00 mL. Again, be sure to record to hundredths of a milliliter. Mix each solution thoroughly with a stirring rod. Be sure to clean and dry the stirring rod after each mixing. Volumes added to each test beaker are summarized below, but remember that your exact volumes may vary from those shown and should be recorded to hundredths of a mL:

Beaker Number	Fe(NO₃)₃ (mL)	KSCN (mL)	H2O (mL)
1	5	2	3
2	5	3	2
3	5	4	1
4	5	5	0
5*
*Standard solution see #3 on pages 13-5 for details

3. Obtain about 25 mL of 0.200 M Fe(NO3)3 in a 100-mL beaker. Prepare a standard solution of FeSCN2+ by pipetting 18.00 mL of 0.200 M Fe(NO₃)₃ into a 50-mL beaker labeled “5”. Pipette 2 mL (record volume to 0.01 mL) of 0.00200 M KSCN into the same beaker. Stir thoroughly.

4. Examine the layout of the computer screen for this experiment. There is just a “live” display and a data table. A graph is not necessary, because you are just trying to determine the absorbance values of each solution. They will be recorded in the data table versus Trial Number.

5. You are now ready to calibrate the colorimeter. 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 smooth sides facing toward the white reference mark at the right of the cuvette slot on the colorimeter.

Choose "Calibrate" from the "EXPERIMENT" menu and then click "Perform Now". 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 “Input 1” in the column labeled “Reading 1,” stabilizes, click the "KEEP" button. 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 “Input 1” in the column labeled “Reading 2” stabilizes, click the "KEEP" button. Click on the "OK" button. Leave the colorimeter set to the Blue LED position for the remainder of your measurements.

6. You are now ready to collect absorbance data for the four equilibrium systems and the standard solution. Keep your solutions identified by writing their numbers on a paper towel and placing the cuvette and beaker on the number. Fill each of five clean, dry cuvettes about 3/4 full with each solution from the test beakers. Wipe the outside of each cuvette with a tissue and then place the first cuvette (Beaker 1) in the colorimeter. Click on the "Collect" button. Close the lid and wait for the absorbance value to stabilize. Click on the "Keep" button and then type “1” in the edit box. Press the "ENTER” button. Your data will be recorded in the data table .

7. Repeat the Step 6 procedure for each of the remaining solutions.

8. Click the "Stop" button after completing the last solution.

9. Copy the absorbance values from the data table into your data table on Page 13-9.

10. Waste disposal: Keep a beaker at your lab station to collect waste. At the end of the experiment, pour the waste into the labeled container at the side bench designated for this waste. Your instructor must neutralize the acid before flushing the solution down the drain.

PROCESSING THE DATA

1. Write the K_c expression for the reaction in the Data and Calculation table. You will calculate experimental values of K_c by substituting concentration data from Page 13-9 into this expression. You will then have 4 experimental values of Kc, which you can average.

2. Calculate the initial concentration of Fe3+, based on the dilution that results from adding KSCN solution and water to the original 0.00200 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 =

Fe(NO₃)₃ mL

X (0.00200 M)

total mL

This should be the same for Beakers 1 - 4.

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

[SCN^-]_i =

KSCN mL

X (0.00200 M)

total mL

In Beaker 1, [SCN^-]_i = (2 mL / 10 mL)(.00200 M) = .000400 M. Calculate this for the other three beakers.

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

[FeSCN²⁺]eq =

A_eq

X [FeSCN²⁺]_std

A_std

where A_eq and A_std are the absorbance values for the equilibrium and standard beakers, respectively, and [FeSCN₂₊]_std = (1/10)(0.00200) = 0.000200 M. Calculate [FeSCN₂₊]_eq for each of the four trials.

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

[Fe³⁺]_eq = [Fe³⁺]_i – [FeSCN²⁺]_eq

7. [SCN^-]_eq: Calculate the concentration of SCN^- at equilibrium for Trials 1-4 using the equation:

[SCN^-]_eq = [SCN^-]_i – [FeSCN²⁺]_eq

8. 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.

9. Calculate the average and the range (range = largest - smallest) of your K_c values, as estimates of accuracy and precision respectively. Enter the results of your calculations in the "DATA AND CALCULATIONS" table.