Exp 7 The Determination of Equilibrium Constant

The Determination of an Equilibrium ConstantOBJECTIVES
• To prepare and test standard solutions of FeSCN2+ in equilibrium.
• To determine the molar concentrations of the ions present in an equilibrium system.
• To determine the molar concentration of Fe+3, SCN- and [FeSCN+2] using
spectrophotometric method.
• To determine the value of the equilibrium constant, Keq, for the reaction.
• To determine if the reaction is product or reactant-favored.
LECTURE TOPIC REFERENCES
Review the following before performing the experiment:
Chapter Title: Chemical Equilibrium
Section Title: The Concept of Dynamic Equilibrium; The Equilibrium Constant (K); Calculating
Equilibrium Constant from Measured Equilibrium Concentrations
Tro, N. (2017). Chemistry: A Molecular Approach. 4th Edition. Boston: Pearson. (Or latest edition)
CONCEPTS
The equilibrium state of a chemical reaction can be characterized by quantitatively defining its
equilibrium constant, Keq. In this experiment, you will determine the value of Keq for the
reaction between iron (III) ions and thiocyanate ions, SCN–.
Fe3+ (aq) + SCN– (aq) ↔ FeSCN2+ (aq)
When you mix amounts of Fe3+ and SCN–, a reaction occurs to produce FeSCN2+, but not all of
the reactants react. Thus, your beaker (or flask or cauldron) will contain some of each of these
three species, which is your equilibrium system. To learn more about the system, we need to
figure out a way to count the number of different ions in the reaction mixture. That is the major
objective of this experiment, and to achieve this objective you will take advantage of something
about FeSCN2+ – in aqueous solution it has a reddish color. The two reactants, Fe 3+ and SCN–,
are essentially colorless in solution, thus the red color you will see when you conduct the
reaction is produced by the FeSCN2+ ions.
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One of the more important numbers that help us understand an equilibrium system is called
the equilibrium constant, Keq. For the reaction between Fe3+ and SCN–, the Keq is defined by the
equation
To find the value of Keq at a given temperature, it is necessary to determine the molar
concentration of each of the three species in solution at equilibrium. You will determine the
concentrations by using a Vernier Colorimeter or Spectrometer to measure the amount of light
of a specific wavelength that passes through a sample of the equilibrium mixtures. The amount
of light absorbed by a colored solution is proportional to its concentration. The red FeSCN 2+
solution absorbs blue light, thus the Colorimeter users will be instructed to use the 470 nm
(blue) LED. Spectrometer users will determine an appropriate wavelength based on the
absorbance spectrum of the solution. The wavelength will be close to, but not exactly, 470 nm.
In order to successfully evaluate this equilibrium system, it is necessary to conduct two
separate tests. In Part I of the experiment, you will prepare a series of standard solutions of
FeSCN2+ from solutions of varying concentrations of SCN– and constant concentrations of H+
and Fe3+ that are in stoichiometric excess. The excess of H+ ions will ensure that Fe3+ engages in
no side reactions (to form FeOH2+, for example) which could interfere with your measurements.
In an excess of Fe3+ ions, the SCN– ions will be the limiting reagent, thus all of the SCN– will form
FeSCN2+ ions. The FeSCN2+ complex forms slowly, taking at least one minute for the color to
develop. It is best to take absorbance readings after a specific length of time has passed,
between two and four minutes after preparing the equilibrium mixture. Do not wait much
longer than five minutes to take readings, however, because the mixture is light sensitive and
the FeSCN2+ ions will slowly decompose.
In Part II of the experiment, you will prepare a new series of solutions that have varied
concentrations of the SCN– ions and constant concentrations of H+ ions and Fe3+ ions. You will
use the results of this test to accurately evaluate the equilibrium concentrations of each species
and calculate the Keq of the reaction.
MATERIALS
Logger Pro
Computer
Vernier LabQuest*
Colorimeter
Plastic cuvette
Temperature probe
Automatic pipetters
2 small beakers (30 mL)
Several plastic Beral pipets
0.200 M iron (III), Fe3+, solution in
1.0 M HNO3
2
0.0020 M iron (III), Fe3+, solution in
1.0 M HNO3
0.0020 M thiocyanate, SCN–, solution
DI water
Lint free tissues or Kim® Wipes
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PRE-LAB EXERCISE
For the solutions that you will prepare in Step 6 of Part I (using Table 2 data) below, calculate
the [FeSCN2+]. Presume that all of the SCN– ions react. In Part I of the experiment, mol of SCN– =
mol of FeSCN2+. You will need the calculated [FeSCN2+] in Step 7. Record these values in the
following table:
Table 1 Concentration of [FeSCN2+] in the Test Standard Solutions
Mixture
[FeSCN2+]
1
0.00016
2
0.00012
3
0.00008
4
0.00004
PROCEDURE
Part I Prepare and Test Standard Solutions
1. Obtain and wear goggles and gloves.
2. Connect the Colorimeter to the computer interface. Open the file “10 Equilibrium” from the
Advanced Chemistry with Vernier folder of Logger Pro.
3. Prepare a blank by filling a cuvette 3/4 full with DI water. To correctly use cuvettes,
remember:
•
•
•
•
•
Wipe the outside of each cuvette with a lint-free tissue (Kimwipes).
Handle cuvettes only by the top edge of the ribbed sides.
Dislodge any bubbles by gently tapping the cuvette on a hard surface.
Cover the cuvette to avoid spillage inside the Colorimeter.
Always position the cuvette so the light passes through the clear sides.
4. Open the Colorimeter lid, insert the blank, and close the lid.
5. Calibrate the Colorimeter and prepare to test the standard solutions.
a. Press the < or > button on the Colorimeter to select the 470 nm wavelength.
b. Press the CAL button until the red LED begins to flash and then release the CAL button.
c. When the LED stops flashing, the calibration is complete.
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d. Empty the water from the blank cuvette. Using the solution in Beaker 1, rinse the
cuvette twice with ~1 mL amounts and then fill it 3/4 full. Wipe the outside with a
tissue, place it in the Colorimeter, and close the lid.
6. Label four small beakers 1–4. Using Table 2 below, prepare the four solutions by accurately
measuring the volumes of 0.200 M Fe(NO3)3, 0.0020 M SCN–, and DI. Use the automatic
pipette to measure the solutions (make sure you ask your instructor how to use this pipette
before using it). Mix each solution thoroughly. Record the temperature of one of the
solutions as the temperature for the equilibrium constant, Keq.
Note: Make sure the Colorimeter is ready before mixing the three reagents. Prepare the
test standard solution one at a time and immediately measure the absorbance.
DANGER: Iron (III) nitrate solution, Fe(NO3)3•9H2O: Causes severe skin burns and eye
damage. Do not breathe mist, vapors, or spray. WARNING: Potassium thiocyanate solution,
KSCN: Causes eye irritation and mild skin irritation.
Important: The mixtures you will prepare are light sensitive. You need to measure the
absorbance of these four mixtures within 2–5 minutes of preparing them.
Table 2 Volumes of Test Standard Solutions
Mixture
0.200 M Fe(NO3)3
(mL)
0.0020 M SCN–
(mL)
DI H2O
(mL)
Total Volume
(mL)
1
0.5
0.4
4.1
5
2
0.5
0.3
4.2
5
3
0.5
0.2
4.3
5
4
0.5
0.1
4.4
5
7. Collect absorbance-concentration data for the four standard equilibrium mixtures.
a. Clean (with a Kimwipe) and cover the cuvette containing the Mixture 1 and place it in
inside the colorimeter. Cover the colorimeter lid.
b. Click
. After the absorbance reading stabilizes, click
, type the
2+
concentration of FeSCN (from your pre-lab calculations shown in Table 1) in the edit
box, and click
.
c. Discard the cuvette contents as directed by the instructor. Rinse and fill the cuvette with
the Mixture 2 and place it inside the colorimeter. After the reading stabilizes, click
, type the concentration of FeSCN2+ in the edit box, and click
.
d. Repeat Part c of this step to measure the absorbance of Mixtures 3 and 4.
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e. Click
Examine,
after you have finished collecting data from the four mixtures. Click
, and write down the absorbance values in your data table.
8. Click Linear Fit, . A best-fit line (linear regression) equation will be plotted for your data.
Write down the equation in the Data and Calculations section.
IMPORTANT: Do not change anything in Logger Pro. You will use the best-fit line equation in
Part II. Best-fit line equation can also be obtained from the graph provided by the instructor.
Part II Prepare and Test Equilibrium Systems
9. Prepare the solutions according to Table 3 below. Use 10.0 mL graduated cylinders to
measure the solutions. Mix each solution thoroughly.
Note: You are using 0.0020 M Fe(NO3)3 in this test. WARNING: Iron (III) nitrate solution,
Fe(NO3)3•9H2O: Causes skin and eye irritation. Do not breathe mist, vapors, or spray.
Table 3 Volumes of Test Equilibrium Systems
Mixture
0.0020 M Fe(NO3)3
(mL)
0.0020 M SCN–
(mL)
H2O
(mL)
Total Volume
(mL)
A
3.00
3.00
4.00
10
B
3.00
4.00
3.00
10
C
3.00
5.00
2.00
10
Calculating Equilibrium Concentrations
10. Collect absorbance-concentration data for the three equilibrium mixtures.
a. Using Mixture A, rinse the cuvette twice with ~1 mL amounts and then fill it 3/4 full.
Wipe the outside with a tissue and place the covered cuvette inside the Colorimeter.
b. Write down, in your data table, the absorbance of Mixture A.
c. Open the Analyze menu and choose Interpolate. Trace along the best-fit line equation to
find the FeSCN2+ concentration for Mixture A. Write down the concentration in your
data table. You can also calculate the FeSCN2+ concentration using Step 4 equation in
the Processing the Data section.
d. Discard the cuvette contents as directed by the instructor. Rinse and fill the cuvette with
Mixture B, cover it, and place it inside the Colorimeter. After the reading stabilizes, write
down the absorbance in your data table and use the Interpolate function to determine
the concentration of the sample. Repeat Step d for Mixture C.
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5
PROCESSING THE DATA
1. Write the Keq expression for the reaction in the Data and Calculations section.
2. Calculate the initial concentration of Fe3+, based on the dilution that results from adding
KSCN solution and water to the original 0.0020 M Fe(NO3)3 solution. See Step 9 of the
Procedure for the volume of each substance used in Mixtures A to C. Calculate [Fe3+]i using
the equation
3. Calculate the initial concentration of SCN–, based on its dilution by Fe(NO3)3 and water:
In Mixture A, [SCN–]i = (3 mL / 10 mL)(0.0020 M) = 0.00060 M. Calculate this [SCN–]i for the
other two mixtures.
4. [FeSCN2+]eq is calculated using the formula below. Use the best-fit line equation or
calibration curve from Part I: y = mx + b; where m is the slope and b is the y-intercept. If
the y-intercept is zero, for example, y = 3292 x, use the equation below to calculate the
concentration of [FeSCN2+]eq in Mixture A (for example, Absorbance = 0.205):
[FeSCN2+]eq =
𝐴𝑏𝑠𝑜𝑟𝑏𝑎𝑛𝑐𝑒 𝑜𝑓 𝑀𝑖𝑥𝑡𝑢𝑟𝑒 𝐴
𝑆𝑙𝑜𝑝𝑒 𝑜𝑓 𝐶𝑎𝑙𝑖𝑏𝑟𝑎𝑡𝑖𝑜𝑛 𝐶𝑢𝑟𝑣𝑒 𝑓𝑟𝑜𝑚 𝑃𝑎𝑟𝑡 𝐼
[FeSCN2+]eq =
0.205
3292
= 6. 23 x 10 -5 M
If the y-intercept is not zero, for example, y = 3292 x + 0.005950, rearrange the best-fit line
equation to calculate the equilibrium concentration of the reaction product, [FeSCN2+]eq as
follows:
[FeSCN2+]eq =
𝐴𝑏𝑠𝑜𝑟𝑏𝑎𝑛𝑐𝑒 𝑜𝑓 𝑀𝑖𝑥𝑡𝑢𝑟𝑒 𝐴−0.005950
FeSCN2+]eq =
3292
0.205−0.005950
3292
= 6.05 x 10-5 M
Calculate [FeSCN2+]eq for Mixtures B and C.
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5. [Fe3+]eq: Calculate the concentration of Fe3+ at equilibrium for Mixture A to C using the
equation:
6. [SCN–]eq: Calculate the concentration of SCN– at equilibrium for Beakers A to C using the
equation:
7. Calculate Keq for Mixtures A to C. Be sure to show the Keq expression and the values
substituted in for each of these calculations. Write all calculations and results in the
summary table in Data and Calculations section.
LAB SAFETY AND WASTE DISPOSAL
Waste Disposal:
Collect and dispose of wastes from Mixtures I to IV, and any unused reagents in the inorganic
waste bottle. All waste bottles are located in the Satellite Accumulation Area (SAA).
Lab Safety:
Wear the appropriate Personal Protected Equipment (PPE). Read all Safety Data Sheets (SDSs)
provided by instructor. Pay attention to the safety precautions mentioned in the procedure
and by the instructor.
References
Randall, J. and Volz, D. (2017). Advanced Chemistry with Vernier, 3rd edition. The
Determination of an Equilibrium Constant. Beaverton, OR: Vernier Software and Technology.
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DATA AND CALCULATIONS
Part I
Temperature: ________°C
Mixture
[FeSCN2+]
Absorbance
1
0.00016
0.540
2
0.00012
0.414
3
0.00008
0.286
4
0.00004
0.133
Linear regression equation
Y=3374x+0.005905
Slope
3374
Part II
Mixture
Absorbance
[FeSCN2+] at equilibrium
A
0.197
0.167
B
0.268
0.261
C
0.331
0.319
Calculations
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Equilibrium Constant Data and Calculations Summary (CODE:_______ )
Initial Concentrations
(moles/L or M)
Mixture
A
[Fe+3]i (A)
0.002 M Fe+3 x mL
Fe+3/mL solution
Equilibrium Concentrations
(moles/L or M)
[SCN-]i (B)
0.002 M x mL
SCN-/mL solution
[Fe+3]eq (C)
(A) – (E)
[SCN-]eq (D)
(B) – (E)
0.006M
0.0006M5.66×10−5=
5.434×10−4
5.434×10−4
0.0008M
0.006M07.77×10−5
=5.223×10−4
0.004M
0.0006M9.63×10−5
=5.037×10−4
0.0006M
B
Write the Equilibrium
Expression and Calculate Keq:
[FeSCN+2]eq (E)
Absorbance / slope from
Part I calibration
curve*****
0.197 − 0.005905
3374
=5.66×10−5
Keq=5.434×10−4 𝑥5.434×10−4 =
18876.167
0.0008M7.77×10−5 =
7.9223X10−3
0.268 − 0.005905
3374
=7.77×10−5
7.77𝑥10−5
=
5.223×10−4 𝑥7.9223×10−5
18.778
0.004M9.63×10−5 =
0.4399
0.331 − 0.005905
3374
=9.63×10−5
9.63𝑥10−5
=
5.037×10−4 𝑥0.4399×10−5
0.43461
Keq=
5.66𝑥10−5
0.006M
C
0.0006M
Note: ***** For correct calculation equation, see Step 4 of Processing the Data section. Show all calculations (May be submitted as a
scanned hand-written document with the completed Data and Calculation Section).
at ______22.3____________ oC
AVERAGE Keq = 6298.46
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1.
Calculations
(You can use Excel to perform/show your calculations)
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LABORATORY REPORT PREPARATION GUIDE
MASTER SQL
EXPERIMENT 7 Determination of an Equilibrium Constant
I. Introduction
1. What is chemical equilibrium and how is the state of equilibrium characterized?
2. What is equilibrium constant and what does it indicate?
3. How is equilibrium constant expressed?
4. What affects equilibrium constant?
5. What were the objectives of this experiment?
6. What parameters were needed in determining the equilibrium constant in this experiment?
7. What apparatus and concepts were used to determine these parameters? Briefly explain the requirements of this
apparatus.
8. What were the safety hazards and what safety precautions did you implement to minimize the risks of these hazards?
II. Results Analysis
A. Data and Calculations
Summarize important results and refer the reader to the attached Data and Calculations section.
B. Discussion
Part I Prepare the Test Standard Solutions (Provided by Instructor)
1. What chemical reaction was performed in determining the equilibrium constant in this experiment?
2. What was the limiting reactant and what was its role in determining the equilibrium constant of the reaction?
3. From the calibration curve of the standard solutions provided by the instructor, what happened to the absorbance as
the concentration of [FeSCN2+] is increased?
4. What was the purpose of determining the calibration curve?
Part II Prepare and Test Equilibrium Systems
1. What were the procedural requirements in performing each chemical reaction and in determining the absorbance
using the colorimeter? Why was it necessary to read the absorbance at wavelength 470 nm and within 4 minutes of the
reaction?
2. What was the purpose of determining the absorbance of the equilibrium reaction systems?
3. What were the concentrations of the reactants and products at equilibrium?
4. What was the equilibrium constant of each reaction system? Show a sample calculation of one reaction system.
5. What was the average equilibrium constant? Write the equilibrium constant expression.
III. Conclusions
1. What can be concluded from the concentration of the reactants versus the calculated equilibrium constants? Is
equilibrium constant (Keq) affected by the change of initial concentration of the limiting reactant?
2. What can be concluded from the use of Beer’s law in determining the equilibrium constant of the reaction?
3. What can be concluded from the equilibrium constant of each reaction system? Is the equilibrium constant the same
at a given temperature?
4. What can be concluded from the average equilibrium constant? Is the reaction product-favored or reactant-favored?
Explain briefly your answer.
5. What is the practical application of the concepts you learned in this experiment? Give one example.
LABORATORY REPORT
Experiment 7 The Determination of an Equilibrium Constant
Name: ____________________________________________
Lab Partner’s Name: _________________________________
I. Introduction
II. Results Analysis
A. Data and Calculations
Date: ___________________
Score: _________________
B. Discussion
III. Conclusion
References (APA Style)
DATA AND CALCULATIONS
Part I Prepare and Test Standard Solutions
Temperature: ________°C
Table I.1 Standard Calibration
Mixture
[FeSCN2+]
Absorbance
1
2
3
4
Pre-lab Calculations as shown in Table 1 in Part I Procedure:
(Show all calculations below. You may use Excel to perform/show your calculations.)
DATA ANALYSIS (PART 1, USING LOGGERPRO)
Plot a graph of your data above, using [FeSCN ] on the x-axis, and Absorbance as the y-axis.
a. Open LoggerPro 3.16.1 (or newer version). Choose New from the File menu. An empty
graph and table will be created in Logger Pro (See graph image below).
b. Double click on the x-axis heading on the table, enter a name and unit (if applicable), then
enter the four values from your data table above. Do the same for the y-axis. Click Autoscale icon on the menu bar to see all of the points on the graph.
c. Select all points (Left-click on mouse, then drag the mouse so that you highlight gray all of
the points on the graph).
d. Click the Linear-fit icon on the menu bar. A best-fit line (linear regression) equation will be
shown on the graph. Write the linear regression equation and the slope in Table I.2.
2+
Note: You can also use Excel to create the graph and obtain the trend-line equation.
Attach your Linear Regression Graph below. You may submit it as a separate pdf file with
your lab report. Using the Linear Regression Graph, complete the table below:
Table I.2 Regression Line Data
Linear regression or trend-line
equation
Slope
Part II Prepare and Test Equilibrium Systems
Table II.1 Equilibrium Systems Data
Mixture
A
B
C
Temperature: ________°C
Calculations
Absorbance
Calculated [FeSCN2+] at
equilibrium
Equilibrium Constant Data and Calculations Summary
Initial Concentrations
(moles/L or M)
Mixture
[Fe+3]i (A)
0.002 M Fe+3 x mL
Fe+3/mL solution
[SCN-]i (B)
0.002 M x mL
SCN-/mL solution
Equilibrium Concentrations
(moles/L or M)
[Fe+3]eq (C)
(A) – (E)
[SCN-]eq (D)
(B) – (E)
[FeSCN+2]eq (E)
Absorbance / slope from
Part I calibration curve*
Write the Equilibrium
Expression and Calculate Keq:
Keq=
A
B
C
Note: * For correct calculation equation, see Step 4 of Processing the Data.
AVERAGE Keq =
at __________________ oC
1.
Calculations
(You can use Excel to perform/show your calculations)
DATA ANALYSIS (PART II)
1. How constant were your Keq values? Briefly explain the importance of the absorbance data in calculating the Keq.
2. Should the change in the initial concentration of SCN- affect the equilibrium constant of the reaction? Briefly explain.
3. Is the reaction product or reactant-favored? Briefly explain your answer.