Chemistry 12 (HL)
IB Topic 7.1
Determination of an Equilibrium Constant
Introduction
The purpose of this lab is to experimentally determine the equilibrium constant, Kc, at a given
temperature for the following chemical reaction:
Fe3+(aq) + SCN-(aq) ⇔ FeSCN2+(aq)
iron (III) ion + thiocyanate ion ⇔ iron (III) thiocyanate ion
very pale yellow
3+
colourless
blood red
-
2+
When the Fe and SCN ions react in solution, they react to form FeSCN and an equilibrium is
established between all three ions. This equilibrium is defined by the equilibrium constant, Kc:
Kc =
[FeSCN 2+ ]
[Fe3+ ] [SCN - ]
2+
The fact that FeSCN is a blood-red colour allows us to determine its equilibrium concentration by
measuring the absorbance of blue light with a wavelength of 470 nm. The equilibrium concentrations
of the other two ions may then be calculated, and the value of Kc determined.
In order to convert absorbance values to concentration values, a standard curve must be prepared.
In Part A of this experiment, you will prepare five “standard” solutions with a known concentration of
2+
3+
FeSCN . This is done by setting up reactions with a very large excess of Fe ions and a very
small amounts of SCN ions (a process sometimes known as “swamping”). Under such
conditions, the reaction essentially goes to completion instead of reaching an equilibrium state,
2+
and all of the SCN ions are converted into FeSCN ions. Beer’s Law states that the absorbance
2+
is directly proportional to its concentration. A plot of absorbance versus concentration of FeSCN is
called a standard curve, and it should show a linear relationship. This curve will then be used to
2+
determine the concentration of FeSCN in the equilibrium systems you will set up in Part B.
In Part B of this experiment, you will prepare four different equilibrium mixtures in order to determine
the value of the equilibrium constant for this reaction. As long as the temperature of the system is the
same, you should find the equilibrium constant is the same for all reactions. This shows that an
equilibrium is established regardless of the starting concentrations, and that concentration does not
have an effect on the value of the equilibrium constant.
Materials
MacBook with DataStudio
USB link
colorimeter
1 cuvette
thermometer
3 100-mL beakers
2 10.00 (or 5.00) mL graduated pipettes
pipette bulb
100.0 mL volumetric flask
5 15x120 mm test tubes (small)
4 22x150 mm test tubes (large)
glass stirring rod
0.200 M Fe(NO3)3 (prepared in 0.5 M nitric acid)
0.0020 M Fe(NO3)3 (prepared in 0.5 M nitric acid)
0.0020 M KSCN
CAUTION: Solutions
containing nitric acid
are corrosive.
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Chemistry 12 (HL)
IB Topic 7.1
Procedure
Goggles must be worn at all times.
Part A: Preparation of Standard Solutions and the Standard Curve
1.
Each group will be assigned one or two standard solutions to prepare using 0.200 M Fe(NO3)3
(provided in the burettes around the lab) and 0.0020 M KSCN. These solutions will then be
shared by the class in Part C.
2.
Obtain about 25 mL of 0.0020 M KSCN in a 100 mL beaker. You will use this for both Part A
and Part B.
3.
a)
b)
c)
d)
e)
f)
Label a 100.0 mL volumetric flask with your assigned standard solution and your
initials.
Add the required volume of 0.200 M Fe(NO3)3 from the dispensing burettes set up
around the room. See the table below.
Add the required volume of 0.0020 M KSCN, using a 10.00 mL or 5.00 mL pipette.
(Keep this pipette for use in Part B.)
Carefully add deionized water to the mark on the neck of the volumetric flask.
Thoroughly mix your solutions.
Place your standard solution in the assigned location so that all groups may use it in
the next step.
standard
solution
1
2
3
4
5
volume of
0.200 M Fe(NO3)3
(mL)
10.00
10.00
10.00
10.00
10.00
volume of
0.0020 M KSCN
(mL)
10.00
8.00
6.00
4.00
2.00
total volume of the
solution (after adding
water) (mL)
100.0
100.0
100.0
100.0
100.0
Part B: Preparation of Equilibrium Mixtures
Each group will prepare their own set of all four equilibrium mixtures.
1.
Obtain approximately 20 mL of 0.0020 M Fe(NO3)3 solution in a 100 mL beaker. Check that
you have the correct concentration!
2.
Prepare the following equilibrium mixtures in large test tubes. Use appropriate sizes of
pipettes to prepare each mixture. Start with water, and then use the same pipette for one of
the other solutions.
equilibrium
mixture
volume of H2O
(mL)
1
2
3
4
5.00
4.00
3.00
2.00
volume of
0.0020 M Fe(NO3)3
(mL)
3.00
3.00
3.00
3.00
volume of
0.0020 M KSCN
(mL)
2.00
3.00
4.00
5.00
Stir each mixture with a glass stirring rod. Clean the stirring rod between mixtures.
p. 2
Chemistry 12 (HL)
IB Topic 7.1
Part C: Measuring the Absorbance of the Standard FeSCN
Mixtures
2+
Solutions and the Equilibrium
1.
Set up and calibrate a colorimeter:
a)
Connect the colorimeter to your MacBook before opening DataStudio.
b)
In the “set up” window, select blue absorbance.
c)
Open a “digits” window from the left side menu bar. Close the graph window.
3+
d)
Fill a clean cuvette with the “blank solution”. Since Fe ions have a pale yellow
3+
colour, we will use the diluted Fe solution (0.0020 M) to calibrate the colorimeter.
e)
Wipe off the cuvette with a tissue and place in the colorimeter. Close the cover.
f)
Do NOT start recording data!
g)
Press the green “calibrate” button on the colorimeter. The LED light should go on.
h)
When the LED light goes off, click “start” to ensure that the absorbance reading is
0.000.
i)
Remove the blank cuvette and dispose of the contents.
2.
Do not use
Measure the absorbance of each standard solution:
pipettes to fill
a)
Start with the most dilute solution. Pour some of the standard solution into the
the cuvette.
cuvette so that it is about ¾ full. It is not necessary to have an exact volume.
b)
Use the SAME cuvette for all solutions.
c)
Rinse the cuvette with distilled water. Gently tap the cuvette on a pile of paper towel
to quickly remove any water remaining in the cuvette.
3.
Measure the absorbance of each equilibrium mixture, following the same procedure.
4.
Measure and record the temperature of the reaction mixtures.
Processing the Data
Part A
1.
2+
Calculate the concentration of FeSCN in each standard solution. Remember that in the
standard solution, the reaction goes to completion. You may assume that all of the limiting
2+
KSCN reactant is converted into the product FeSCN .
1
–
2
3
4
5
moles of SCN used
moles of FeSCN
formed
2+
volume of solution
2+
[FeSCN ]
2.
2+
Prepare a standard curve by plotting absorbance versus concentration of FeSCN .
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Chemistry 12 (HL)
IB Topic 7.1
Part B
You may want to set up an ICE table for each trial to show the results of these calculations.
2+
1.
Calculate the equilibrium concentration of FeSCN in each equilibrium mixture, using the
recorded absorbance values in Part B and the standard curve.
2.
Calculate the equilibrium concentration of Fe
3.
4.
3+
in each equilibrium mixture:
3+
a)
Calculate the initial concentration of Fe in each mixture using the dilution formula
C1V1 = C2V2. This will be the same for all four mixtures.
b)
Some of the initial Fe is converted into FeSCN . Since the ratio of Fe to
2+
2+
FeSCN is 1:1, you may use the concentration of FeSCN present as the amount of
3+
3+
Fe that is used in the reaction. Calculate the equilibrium concentration of Fe as the
3+
concentration of Fe remaining in the mixture.
3+
2+
3+
-
Calculate the equilibrium concentration of SCN in each mixture.
-
a)
Calculate the initial concentration of SCN in each mixture using the dilution formula
C1V1 = C2V2. There will be a different concentration for each of the four mixtures.
b)
Some of the initial SCN is converted into FeSCN . Since the ratio of SCN to
2+
2+
FeSCN is 1:1, you may use the concentration of FeSCN present as the amount of
SCN that is used in the reaction. Calculate the equilibrium concentration of SCN as
the concentration of SCN remaining in the mixture.
-
2+
-
Calculate the value of Kc for each equilibrium mixture, and find the average value.
Conclusion and Discussion
1.
What is the value of Kc for this equilibrium system at the measured temperature?
2.
Comment on the consistency of the Kc values for the four equilibrium mixtures. Suggest
reasons for any variation between the Kc values.
3.
Why is necessary to measure the temperature?
4.
Explain why a 0.200 M Fe(NO3)3 solution is used to prepare the standard solutions in Part A,
but a 0.0020 M Fe(NO3)3 solution is used in preparing the equilibrium mixtures in Part B.
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