Lab 6
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Calculating an Equilibrium Constant
Pre-Lab Assignment
This written pre-lab is worth 3 points of your lab report grade and must be turned in to your lab instructor before
class begins.
1. Read the entire lab handout.
2. Make a table listing the names and chemical formulas of all chemicals used in this lab.
3. Make a table listing the suspected safety hazards in one column and the safety precautions that should be taken
to protect yourself from these hazards in the other column.
4. Write a brief summary of the experiments that will be performed today.
5. Answer the following questions:
a. Why is a solution that absorbs light at 447 nm a reddish solution?
b. What is a calibration curve? How will one be used in today’s lab?
Experimental Questions
What is the numerical value for the equilibrium constant for the reaction between iron (III) nitrate and
potassium thiocyanate?
Learning goals
Determine the concentration of reactants and products at equilibrium
Calculate an equilibrium constant.
Explain why the equilibrium constant should be the same regardless of initial conditions.
Background
Reversible reactions will achieve a state of dynamic equilibrium in which products and reactants will be present with
their concentrations appearing to remain constant. The ratio of reactants to products is called an equilibrium
constant, Keq, and can be calculated when the molar concentrations of all species at equilibrium are known.
The equilibrium system under investigation involves an aqueous reaction between iron (III) nitrate and potassium
thiocyanate. At equilibrium, these reactants will be in equilibrium with a single product: iron (III) thiocyanate ion,
which has a +2 charge. The net ionic equation for the reaction is shown below. Since the coefficient for each of the
species in the equation is “1”, the Keq will be calculated using the molar concentrations of each species at
equilibrium in the associated equation:
+3
-1
Fe (aq) + SCN (aq)
+2
FeSCN (aq)
Keq
[FeSCN 2 ]
[Fe 3 ][SCN 1 ]
+2
The product, FeSCN ion, absorbs light strongly in the region of 430 - 490 nm, causing the solution to take on a
reddish color after the reactants are mixed. The wavelength of maximum absorbance is 447 nm, which can be used
in conjunction with a calibration curve to determine the concentration of the product at equilibrium.
The initial concentrations of reactants will be known (concentrations initially added together). The
concentration of reactants at equilibrium will have to be calculated by subtracting the amount used in
forming the product. The amount of reddish product formed will be determined by measuring the absorbance
of the resulting mixture.
LeChatelier’s Principle is applied in two ways. First, in preparing the calibration curve, the equilibrium must be
forced to go all the way to products by increasing the concentration of one of the reactants by a factor of 100
compared to the other reactant. If this is not accomplished, a true calibration curve cannot be plotted since
reversible reactions do not go to completion and there would be no way to determine the concentration that
correlates to the absorbance measured.
Second, in preparing the equilibrium mixtures, differing amounts of one of the reactants will be used. This causes
different stresses on the second reactant, which results in a different amount of product being formed in each case. If
the equilibrium constant is indeed constant, in each case the resulting amount of product should reflect the same
ratio of products to reactants that defines the Keq to begin with.
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General Chemistry
Procedure
My lab partner is: ________________________________________
Part A: Prepare Solutions of Known Concentration for Calibration Curve.
Prepare the burets.
Use a beaker of deionized water to rinse the burets as follows.
Fill the buret approximately half full. (Make sure the valve is shut)
Open the valve over the sink or waste beaker, and allow 5-10 mL to drain before shutting
the valve. (If the flow seems slow, consult the instructor.)
Carefully tilt the buret sideways and allow the water to drain out the top while rolling the
buret in your fingers. Repeat the procedure twice more. (Total of 3 rinsings)
Fill the burets.
buret
Be sure the buret valve is shut. Pour approximately 10 mL of 0.00200 M SCN solution
into the first buret.
Open the valve over the sink or waste beaker and allow 2-3 mL to drain before shutting.
Carefully tilt the buret and allow the rest of the solution to drain out the top while rolling the buret.
Repeat once more to ensure the water from the rinse is fully removed.
Use a funnel to add enough liquid to the buret to make all four solutions (see the four solutions below).
Clamp the buret vertically and tape a paper label to identify the contents.
Repeat the procedure for the buret that will contain the 0.0500 M HNO 3 solution. Be sure to label the burets.
Solutions Prep.
Label two of the 25mL beakers “1” and “2.”
Use the burets and the volumetric pipet to prepare the following two solution mixtures. You will use a clean,
dry 5.00 mL volumetric pipet to deliver the 0.200 M Fe 3+ solution.
Beaker
0.200 M Fe3+
0.00200 M SCN-1
0.0500 M HNO3
Total Volume
1
2
5.00 mL
5.00 mL
4.00 mL
2.00 mL
16.00 mL
18.00 mL
25.00 mL
25.00 mL
Stir each beaker with a glass stirring rod to ensure complete mixing. Rinse the stirring rod with DI water and
dry with a paper towel in between beakers to avoid cross-contamination of the solutions.
Fill a cuvette 3/4 full with each solution and label.
Discard each solution into the waste beaker.
Rinse each beaker with tap water and DI water, then dry with a paper towel.
Part B: Equilibrium Mixtures
Discard any leftover 0.200 M Fe3+ solution into the sink or waste beaker.
Rinse the empty container with tap water and DI water, then dry with a paper towel. Discard the
rinse.
Obtain 20 mL of 0.00200 M Fe3+ solution.
Rinse the 5.00 mL volumetric pipet with tap water and then DI water.
Draw 2-3 mL of the 0.00200 M Fe3+ solution into the volumetric pipet.
Remove the pipet bulb and invert the pipet over the sink over waste beaker, rolling the pipet
with your fingers while allowing the solution to drain.
Repeat the rinse with 2-3 mL of 0.00200 M Fe3+ to ensure that all rinse water is removed.
Re-label the two 25mL beakers “3” and “4.”Use the burets and the volumetric pipet to prepare
the following solution mixtures.
Beaker
0.00200 M Fe3+
0.00200 M SCN-1
0.0500 M HNO3
3
5.00 mL
2.00 mL
3.00 mL
4
5.00 mL
4.00 mL
1.00 mL
Again, use proper procedures to stir each mixture to ensure complete mixing.
volumetric
pipet
Total Volume
10.00 mL
10.00 mL
General Chemistry
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Fill two new cuvettes 3/4 full with its own solution and label.
Fill a “blank” cuvette with 0.0500 M HNO3 and label.
When your solutions are ready, transport the cuvettes in a beaker to an available spectrophotometer to read their
absorbances and %T. DO NOT CARRY TEST TUBES IN THE WOODEN TEST TUBE RACK. They will fall
out.
Set the wavelength to 447 nm using the wavelength selector knob.
Push the “Mode” button to select the %T function.
Open the sample chamber to be sure the chamber is empty.
Close the lid and use the left hand knob to set the readout to zero.
1. Open the chamber lid, insert the “blank” cuvette, and close.
2. Use the right hand knob to adjust the readout to 100%. This reading will drift a bit. Any setting from
96% to 105%will give satisfactory results.
3. Replace the “blank” cuvette with sample “1.”
4. Close the lid and read the %T.
5. Push the Mode button to select “ABS” and read the absorbance.
6. Push the Mode button to select “%T.”
7. Repeat steps 1-6 exactly until you have recorded data for all four samples.
Clean Up
All solutions may be rinsed down the sink. All glassware, including small borosilicate test tubes, should be rinsed with tap
water followed by a final rinse with DI water before returning to the cart or drawer. Fill the wash bottles with DI water from
the container near the large sink. Wipe the bench with a paper towel and return the key to the box. Obtain instructor’s initials
before leaving.
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General Chemistry
SAMPLE
%T
ABS
Instructor Initials ___________
Cal. 1
Cal. 2
Name_____________________
Equil. 3
Partner Name____________________
Equil. 4
Data Analysis
Answer the following questions ON THIS SHEET. Show all calculations.
1. Calculate the Molarity of [FeSCN2+] in the two calibration solutions. (Hint: Since we used a large excess of Fe3+ for
solutions 1 and 2, we pushed the reaction all the way to the right; that means that the number of moles of FeSCN2+ will
equal the number of moles of SCN- added to the solution.)
2.
Use a graphing program (e.g. Excel) to prepare a calibration curve graph of absorbance vs. concentration using the data
from your solutions made in part A. Place concentration in molarity on the x-axis, absorbance (no units) on the y-axis. Be
sure to include a point for the origin (0.0 Abs, 0.0 M). Use a regression line for your printout. Do NOT connect the dots.
Re-write the equation for the line on your graph using “concentration” and “abs” instead of x and y.
3.
Determine [FeSCN+2] for each equilibrium sample. (Tubes 3 & 4). Use the equation for the line of your calibration curve
to calculate the concentration of each equilibrium sample (remember, you recorded an absorbance for each equilibrium
mixture). Record your results in the following table.
Results Table 1: Concentrations of Equilibrium Solutions
%T (%)
Solution 3
Solution 4
Absorbance
[FeSCN+2] (mol/L)
General Chemistry
4.
5.
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Calculate [Fe3+] for each equilibrium sample. (solutions 3 & 4 )
a. Calculate the initial number of moles of Fe3+ for each solution based on the amount of 0.00200 M Fe3+ added to each
equilibrium solution.
b.
Calculate the moles of Fe3+ used to form product in each tube based on the amount of FeSCN+2 product formed.
c.
Calculate the number of moles of Fe3+ at equilibrium by subtracting (b) from (a) for each tube.
d.
Calculate the [Fe3+] molarity at equilibrium by dividing (c) by the volume of the prepared mixture for each tube.
Calculate [SCN-1] for each equilibrium sample.
a. Calculate the initial number of moles of SCN-1 for each tube based on the amount of 0.00200M SCN-1 added to each
equilibrium solution.
b.
The moles of SCN-1 used to form product in each tube is equal to the moles of Fe 3+ used in 4b.
c.
Calculate the number of moles of SCN-1 at equilibrium by subtracting (b) from (a) for each tube.
d.
Calculate the [SCN-1] molarity at equilibrium by dividing (c) by the volume of the prepared mixture for each tube.
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General Chemistry
6.
Calculate Keq for each set of data using the equation on page 1. Place results in the table below. Be sure to include units
with concentrations.
Results Table 2: Concentrations of Equilibrium Solutions
Beaker
3
4
[FeSCN+2]
[SCN-1]
[Fe3+]
Keq
7.
Calculate the average value of Keq.
8.
The actual value for the equilibrium constant is 5.00 x 102. Calculate your percent error using the following formula:
actual value exp erimental value
%Error
x 100%
actual value
9.
In view of the level of procedural difficulty in this lab, a % error as great as 100% is considered acceptable. When dealing
with logarithmic functions such as Absorbance, it is fine for a student to come within the same order of magnitude (i.e.,
exponent value) when learning new techniques. Evaluate your results and speculate on sources of errors.
General Chemistry
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Questions
Answer the following questions on a separate sheet and attach to the lab report. These may be typed or neatly and legibly
handwritten.
1.
2.
3.
4.
a. Explain why the solution of Fe3+ in Part A was 100 times the concentration of the Fe 3+ solution in part B.
b. How does this relate to LeChatelier’s principle?
The wavelength used to read absorbance in this lab is 447 nm. What color is in this region of the electromagnetic
spectrum? Why does the solution appear reddish instead?
Explain why the calculated value for Keq should be the same regardless of the amounts of reagents used to prepare the
solutions.
Briefly describe one specific way in which an error might arise in each of the following areas AND explain its effect on
the value of the Keq (will Keq be higher or lower, and why).
a. the preparation of the solutions
b. graphing the data
c. reading the results on the Spec20
Lab Report
Staple in order:
This handout with original data in ink and answers to Data Analysis written neatly.
Calibration curve. Be sure it has your name, date of experiment, and lab number/name at the top.
Answers to Questions neatly handwritten or typed. Please put name and lab number at the top.