Experimental Procedure and Data

Part A: Preparing the Standard Solutions

• Never pipet a solution directly out of the bottle, or it is likely to become contaminated. Pour some of the solution into a beaker before pipetting.
• Notice! There arhttps://courses.lumenlearning.com/chemistry2labs/wp-admin/upload.phpe two different concentrations of Fe(NO3)3 used in this experiment. Read the label of the bottle that you are using carefully to be sure it is the right one!
• Allow the spectrophotometer to remain on for at least 15 minutes prior to reading any absorbance value to ensure accuracy in measurements.

5. When filling the cuvets for measurements, allow the liquid level to ~ 1⁄2 fill the cuvet to ensure accurate absorbance readings. You must reread the blank anytime the instrument is opened to ensure proper calibration.
6. Record the absorbance for all standard solutions in the data section.

  

NEVER pipet a solution directly out of the bottle, or it is likely to become contaminated. Pour some of the solution into a beaker before pipetting.
Notice! There are two different concentrations of Fe(NO3)3 used in this experiment. Read the label of the bottle that you are using carefully to be sure it is the right one!

Allow the spectrophotometer to remain on for at least 15 minutes prior to reading any absorbance value to ensure accuracy in measurements.

Note that it is important to measure the absorbances of all the standard solutions and equilibrium mixtures on the same instrument for your results to be valid.
Prepare a standard solution from the table shown below. Since group 1 metals are always soluble, these compound completely dissociate and indicate whatever amount of NaSCN or KSCN is used is equivalent to dissociated SCN-. DispensetheamountofSCN-showninthetablefromthedispensingburetintoa25mL Erlenmeyer flask and dilute to the mark with 0.200 M Fe(NO3)3 in 0.500 M HNO3 solution (NOT the 0.002 M Fe(NO3)3)!. Note that this excess concentration of iron III nitrate ensures essentially all of the SCN- is complexed as FeSCN2+ for spectrophotometric analysis.

7. In Microsoft Excel, create a graph of Absorbance vs. Concentration using the absorbance values for your standard solutions. Plot a trendline to determine the relationship between the variables. Place your graph in the space provided.

Part B: Preparing the Equilibrium Solutions

1. Label three clean, dry 50 mL flasks as “2”, “3” and “5”.
2. Obtain about 35 mL of 0.00200 M Fe(NO3)3 in 0.500 M HNO3 solution (not 0.200 MFe(NO3)3 in 0.500 M HNO3 solution) in a clean, dry 250 mL beaker. Label the beaker.
3. Rinse a 5.00 mL volumetric pipet with small portions of this solution. Then use thispipet to add 5.00 mL of this solution to each beaker.
4. Intoeachofthelabeledflasksandusingthedispensingpipet,deliver0.00200MNaSCN in 0.500 M HNO3 solution. Deliver the exact number of milliliters (2, 3 or 5)indicated on the label of the flask and record the volume delivered.
5. Obtain about 15 mL of 0.500 M HNO3 solution in a clean, dry 150 mL beaker. Label thebeaker. Rinse a measuring pipet with this solution, and add enough 0.500 M HNO3 to each beaker to make the total volume equal to 10.0 mL (3, 2 or 0 mL, depending on the volume of 0.00200 M NaSCN in 0.500 M HNO3 solution which was added). Note that when using a measuring pipet, the liquid is not drained to the tip. The flow of liquid is stopped when the desired calibration mark is reached, and the delivered volume is taken as the final reading minus the initial reading.
6. Mix each solution thoroughly with a wooden stirrer. Be sure to use a unique and clean wooden stirrer for each solution. Immediately proceed with the next step as interfering side reactions may occur as the solutions age. The wooden stirrers may be discarded in the trash at the end of lab.
7. UsingthesamespectrophotometerasinPartA,re-calibratetheinstrumentusingthe 0.00200 M Fe(NO3)3 in 0.500 M HNO3 solution as a blank.
8. Measuretheabsorbanceofeachequilibriumsolution(remembertoproperlyrinseand dry all cuvets if they are being used from earlier in the experiment.
9. Record the absorbance values in the data section of your lab handout.

10.Use your standard curve from Part A to calculate the equilibrium concentration of

FeSCN2+. Use this concentration to determine the equilibrium concentration of the reactants. Calculate the equilibrium constant for the reaction.

Part C Demonstration of Le Chatelier’s Principle

1. Prepareawaterbathbyheatingabeakerofwaterto70-80°Conahotplate.Place1 drop of 1 M Fe(NO3)3 solution in a clean 18 x 150 mm test tube, and add 1 drop of1 M KSCN.
2. Add 12 mL of water and thoroughly mix the contents of the test tube. Record yourobservations. Divide the mixture into 2 mL portions in six 18 x 150 mm test tubes. One of the test tubes will remain untouched and serves as a “control” against which the other test tubes can be compared.
   1. To the first test tube:
      a. Add 3 drops of 0.1 M AgNO3 solution. Shake to mix. Record yourobservations.
      b. Next add 3 drops of 1 M Fe(NO3)3, shake to mix. Record yourobservations.
   2. To the second test tube
      1. Add 3 drops of 0.1 M AgNO3 solution. Shake to mix.
      2. Record your observations. Next add 3 drops of 1 M KSCN, shake to mix.Record your observations.
   3. To the third test tube

a. Add 3 drops of 0.5 M K3PO4 solution. Shake to mix.
b. Record your observations. Next add 3 drops of 1 M Fe(NO3)3, shake to mix. Record your observations.

4. To the fourth test tube
   1. Add 3 drops of 0.5 M K3PO4 solution. Shake to mix.
   2. Record your observations. Next add 3 drops of 1 M KSCN, shake to mix.Record your observations.
5. Place the fifth test tube in the 70-80° water bath for 1-2 minutes. Compare the warm solution to the solution in the unheated test tube (the “control”), and record your observations.

Pre-lab Assignment/Questions

N o t e – this pre-lab must be finished before you come to lab. (Please see syllabus for how to submit this assignment.)

1.

A. B. C. D. E. F. 2.

3.

Calculate the concentration of FeSCN2+ in each of the six standard solutions, A-F. Show your work.

4.

A 5.00 mL sample of 0.005 M NaSCN is diluted to 25.0 mL. How many mol of NaSCN are present?

If all of the SCN- is complexed as FeSCN2+, what is the molar concentration of the FeSCN2+?

When 50.0 mL of 0.10 M Fe3+ solution is complexed with 5.0 mL of a NaSCN solution, the molar concentration of the FeSCN2+ ion is found by spectroscopic analysis to be 1.75 x 10-4. If the K value is 1.20 x 102, calculate the equilibrium concentration of the SCN- ion.

In part C you will be looking at Le Chatlier’s principle. How would you expect the addition the following compounds to shift the equilibrium constant for the today’s reaction? How would you determine (by eye) whether this has happened.

a. Addition of AgNO3 b. Addition of KSCN

Experimental Data and Results

*Include all work, units and write answers in scientific notation (if applicable) using the correct number of significant figures for full credit. Obtain a signature prior to leaving lab.

Part A: Preparing the Standard Solutions

Attach the graph (including a trendline) of the Absorbance vs. Concentration of the Standard Solutions here.

Experimental Data and Results

*Include all work, units and write answers in scientific notation (if applicable) using the correct number of significant figures for full credit. Obtain a signature prior to leaving lab.

Part B: Preparing the Equilibrium Solutions

Attach a paper showing all calculations if necessary.

Experimental Data and Results

*Include all work, units and write answers clearly for full credit. Obtain a signature prior to leaving lab. Part C Demonstration of Le Chatelier’s Principle

Check this box if all materials in your tray have been cleaned, the trays and other materials have been returned to their proper position, and all items in your drawer are accounted for.

Instructor Signature_______________________________________________________________

Post Lab Questions

*See this syllabus for instructions on how to turn in this section of the lab handout.

1. Of the values that you calculated for K, do you consider one of them to be the “best” value?Explain why or why not.
2. Does your data indicate that K is independent of the initial concentrations of the reactants? If so, explain why this is expected. If not, explain what could have gone wrong.
3. ListimportantsourcesoferrorinthedeterminationofKandtheeffectthateachwouldhaveon your results.
4. For Part C, did the addition of AgNO3 cause an increase in the concentration of the product or the reactants? Explain what observation leads you to this conclusion.
5. For Part C, did the addition of K3PO4 cause an increase in the concentration of the product or the reactants? Explain what observation leads you to this conclusion.
6. For Part C, based on your observations of the solution in the test tube that was heated, do you think the forward reaction is endothermic or exothermic? Explain