Finding the Constant - Kc
Overview
How can an equilibrium constant, Kc, be determined from absorbance data of a chemical species?
Objectives
- How can an equilibrium constant, Kc, be determined from absorbance data of a chemical species?
- Connect observations to AP Chemistry models and calculations.
Materials Needed
Procedure
Preparation of the 0.00100 M HSCN solution (done by teacher)
Pipette 5.00 mL of 0.10 M KSCN solution into a clean 500 mL volumetric flask. Dilute to the mark with 0.250 M HNO3. Mix well. Transfer the solution to a bottle. Label the bottle 0.00100 M HSCN. Preparation of the Beer’s Law Calibration Plot for the FeSCN+2 Solutions
Label four clean 10.00 mL volumetric flasks #1-4. Using pipettes, carefully prepare four standard solutions as described in Table I below.
Transfer 4.00 mL of the 0.100M Fe(NO3)3 solution to each of the four 10 mL volumetric flask. Use a 10-mL graduated cylinder for this process.
Add the appropriate volume of the 0.00100 M HSCN solution directly to the 10 mL flask as described in the table below. Use a graduated pipette for this process
Dilute each flask to the mark with 0.250 M HNO3. Mix the solutions thoroughly. TABLE I: Standard Solutions of FeSCN+2 Solution # 0.100M Fe(NO3)3 0.00100 M HSCN 0.250 M HNO3 1 4.00 mL 0.40 mL Dilute 2 4.00 mL 0.80 mL to 3 4.00 mL 1.00 mL the 4 4.00 mL 2.00 mL mark Using the Spectrophotometer to make the Beer’s Law Plot
Turn on the Spectrophotometer and let it warm up for 15 minutes.
Set the wavelength to 447 nm using the Wavelength Dial.
Set the Filter Position Lever to 340-599 nm.
Using the 0%T Dial, set the % Transmittance to read 0.0.
Fill a cuvette with 0.250 M HNO3. You will use HNO3 as your blank instead of distilled water. Clean the outside of the cuvette with a Kimwipe to remove oil from fingerprints left on the cuvette. Insert the cuvette correctly into the cuvette slot.
Use the 100%T / 0A Dial to set the Transmittance to read 100.0. Press the Mode Button on the Spectrophotometer to change it to read Absorbance. It should read .000. Record this number.
Remove the cuvette and insert another cuvette with one of your FeSCN+2 samples. Make sure to clean the outside of the cuvette with a Kimwipe first. Record the Absorbance of your solution.
Repeat this process with the rest of your solutions. Record the Absorbance of each solution. Determination of FeSCN+2 at Equilibrium
Clean and label four 10.00 mL volumetric flasks #1-4. Using pipets, carefully prepare four new solutions as described in Table II below.
Transfer 2.00 mL of the 0.0050 M Fe(NO3)3 solution to each of the four 10 mL volumetric flask. Use a graduated pipette for this process.
Add the appropriate volume of the 0.00100 M HSCN solution directly to the 10 mL flask as described in the table below. Use a10-mL graduated cylinder for this process.
Dilute each flask to the mark with 0.250 M HNO3. Mix the solutions thoroughly.
Find the absorbances of these four new solutions just as you did for Table I. Simply repeat steps 10 -13 above, except use your new solutions and record the absorbances). TABLE II: Equilibrium Solutions of FeSCN+2 Solution # 0.0050M Fe(NO3)3 0.00100 M HSCN 0.250 M HNO3 1 2.00 mL 2.00 mL Dilute 2 2.00 mL 4.00 mL to 3 2.00 mL 6.00 mL the 4 2.00 mL 7.00 mL mark