AP Chem                                                                                 Unit 13

Chapter 13                                                                               Equilibrium

Lab1

 

Question

 

What method can we use to determine the natural balance of an equilibrium system, and what is the value or ratio of that balance for an equilibrium system.

 

Procedure:

 

In preparing the standard solution in the steps below of this experiment you will use a very low but known concentration of thiocyante ion, SCN-1(aq) and add a large excess of iron(III) ion ( Fe+3(aq)) in high (relative to SCN-1(aq)) concentration,).  You can assume that essentially all of the SCN-1 will be used in forming the deep red  thiocyanoiron(III) ion, FeSCN+2(aq)  (yes…it is a ligand), and that the equilibrium concentration of the FeSCN+2(aq)   will be essentially the same as the concentration of the SCN-1(aq)  with which you started.

 

Prepare 100 ml of 0.5M HNO3 (use MV=MV, HNO3 starts at 15.4M)

 

Prepare 50 ml of 0.002M KSCN and 50 ml of 0.2M Fe(NO3)3 both in the previously prepared 0.5M HNO3 solution (be as exact as you can)

 

Label and line up 8 clean 13x100 test tubes.  Add 4 ml of 0.0020 M potassium thiocyanate, KSCN, to each of these test tubes.  To the first test tube add 4ml of 0.2 M iron(III) nitrate, Fe(NO3)3.  This tube will be used as your standard.

 

When the 0.2M Fe(NO3)3 is added to the 0.002MKSCN they undergo an effect called mutual dilution. The effect on each is they will lower the concentration of the other. Since they are equal in volume they will cut the concentration in half , so the concentration of the 0.002 MKSCN is now .001M and the 0.2M Fe(NO3)3 is changed to 0.1M

 

Measure 10ml of 0.2 M Fe(NO3)3 in a 25-ml graduated cylinder and fill to the 25-ml mark with distilled water.  Pour the contents into a clean, dry beaker and mix thoroughly.  Measure 4ml of this diluted solution into the second test tube. It will wind up as a concentration of 0.04M after dilution.

 

Measure 10ml of the diluted Fe(NO3)3 solution prepared in Step B in a 25-ml graduated cylinder and fill to the 25-ml mark with distilled water.  Pour the contents into a dry beaker and mix thoroughly.  Measure 4ml of this diluted solution into the third test tube.

 

Measure 10ml of the diluted Fe(NO3)3 solution from Step C in a 25-ml graduated cylinder and fill with distilled water to the 25-ml mark.  Mix thoroughly.  Measure 4ml of this diluted solution into the fourth test tube.

 

Repeat the dilution process and use 4ml of the diluted solution for the remaining test tubes until there appears to be no detectable color change.

 

Be sure the solutions in the test tubes have been thoroughly mixed. Use a stirring rod or pipette to thoroughly mix.  Use the colorimeter to obtain a series of absorbance reading values. To use the spectrometer, turn on and allow the system to warm up for about 15 minutes before using. Set the wavelength to 465 nanometers. Place a wiped clean and dry “blank” cuvette containing distilled water in the receptacle with the notches aligned. Press the auto cal button. Clean and dry then place the reagent  samples of above (about 2/3 full) in the cuvette receptacle and read value. Clean and repeat for the other samples

 

Construct a on the basis of the assumption the initial concentration of FeSCN-2(aq)  is 0.001 M and the final concentration is 0 M

 

 

Calculations:

In your calculations assume that:

 

Iron(III) nitrate and the potassium thiocyanate exist in their respective solutions entirely as ions;

 

In the standard, essentially all the SCN-1 have reacted to form FeSCN+2.

The symbol [ ] is used to represent the concentration in moles per liter.  The formula within the brackets denotes the species.

 

Construct a graph of FeSCN-2(aq) using the 2 critical endpoint assumptions (initial concentration of FeSCN-2(aq) is 0.001M (…why 0.001M?…) and 0M

      

Determine via graph and/or calculation the concentration the FeSCN-2(aq)

 

 

Calculations

1.Show your calculations for determining the initial solution concentrations.

 

 

2. Determine the concentrations of all the ions (FeSCN-2(aq) , SCN-1(aq), and Fe+3(aq)) involved in each test case (show your work for the first cases only).

 

 

3. Determine the value of Keq given by the data derived (show your work for the first 2 setups only).

Questions:

 

1. Which of the combinations of concentrations gives the most constant numerical value?  This form is known as the equilibrium constant expression, Keq.

 

2. Restate the equilibrium constant expression in words using the terms reactants and products.

 

3. Give a possible explanation as to why such a relationship might exist?

 

4. What does the numeric value tell you about the degree of completeness of our experimental reaction

 

5. What 3 factors (according to Beers Law) affect the absorbance (transmittance) value you record

 

6. For most effective results, the color selected for measuring absorbance should closely match the color of the solution being tested. Why was 445nm used in this experiment.

 

7. Identify 3 parts of this procedure that can potentially produce major errors.

 

8. Discuss the implications of your procedure if your Keq is asymptotic in nature

 

9. How else might you use a photospectrometer?