TF: __________________
Name: __________________________
EXPERIMENT 13
Thermodynamics of Complex-Ion Equilibria
Before You Come to Lab:
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Read the entire lab report, including the previous introduction and discussion, and the entire procedure.
Complete the Prelab, which is the last page of the lab report, and turn in the prelab to your TF as you enter the lab.
Safety in the Laboratory
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Safety glasses or safety goggles and lab coats must be worn at all times while in the laboratory.
Nitrile gloves must be worn at all times while performing this experiment.
The chemicals used in this experiment are toxic and irritant. Change gloves if you suspect that you have
spilled any chemical on your gloves.
Wash for 15 minutes at the eye wash station if your eyes are accidentally exposed.
Wash for 15 minutes at an emergency shower if you spill any chemicals on your person. Any contaminated
clothing must be removed.
You will be working with hot beakers and cuvettes in this lab. Use padded gloves to handle the hot beakers
and a test tube holder to handle the hot cuvettes.
Waste Disposal and Cleanup
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Empty the contents of all cuvettes into the “Used Chemicals” beaker at your lab bench. Use a squirt bottle to
rinse the cuvettes with water and pour the rinse into the “Used Chemicals” beaker.
Dispose of empty cuvettes and cuvette caps in the solid waste container.
Empty the “Used Chemicals” beaker into the waste collection bucket in the back of the lab.
Before You Leave the Lab
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Have your TF check your lab bench for cleanup.
Submit your data and lab report to your TF. This page and all subsequent pages must be stapled and turned in.
Wash your hands before leaving the lab.
Grading:
Prelab:
_____ / 10
Lab Report:
_____ / 20
Safety:
_____ / 3
Cleanup:
_____ / 2
Total:
_____ / 35
Experiment 13
I. Scientific Background and Introduction
Thermodynamic data for a reaction system provides researchers with information that is
important from both theoretical and practical points of view. There are several thermodynamic properties
that chemists pay close attention to when designing or carrying out experiments, such as thermodynamic
stability, the change in free energy of a reaction, and temperature dependence. By measuring and
tabulating ∆G° (Gibbs free energy, ΔG is positive, then the reaction is nonspontaneous and if it is
negative, then it is spontaneous), ∆H° (enthalpy, the measure of heat absorbed (+) or released (–) of a
system), and ∆S° (entropy, the measure of disorder) the values, chemists can learn more about familiar
reactions and make predictions about new reactions. For instance, a chemical company would not want to
waste time and money perfecting a new reaction if the reaction had a high positive ∆G. The
thermodynamic tables in the back of your textbook contain only a tiny fraction of the data collected by
chemists over the years.
In this experiment, you will learn how to determine these parameters from a controlled
experiment by using spectrometry to find concentration data at various temperatures. This will allow us
to determine the equilibrium constant at different temperatures, which can then be used to derive ΔG°,
ΔH°, and ΔS°. In the procedure section, you will find directions on how to setup the complexation of
aluminum ions with xylenol orange (reaction shown below). It will be up to you to decide how to collect
the necessary data to answer the questions in the lab report. You will design your procedure as part of the
prelab.
Complexation of aluminum ions with xylenol orange
The equilibrium we will study is the complexation of aluminum ions with xylenol orange,
symbolized as H4Q, which results in the formation of the complex ion AlQ–. Because both the reactants
and products strongly absorb light, but at different wavelengths, this reaction is ideal for
spectrophotometric study.
O
O
OH HO
N
HO
O
O
O
N
HO
OH
Al3+
HO
O
H3C
+
O
O
O
S
OH
OH
Xylenol Orange bound to Aluminum (AlQ-)
Red
Xylenol Orange (H4Q)
Yellow
H 4Q
yellow
O
H 3C
O
S
OH
OH
O
O O
N Al N
O O
3+
Al
colorless
→
←
AlQ–
red
+
+
4H
colorless
Recall that molecules are colored because they absorb certain wavelengths of visible light, while
allowing other wavelengths to pass through. Xylenol orange is a yellow-colored organic molecule
containing four acidic –COOH groups attached to a system of three benzene-like carbon rings, as shown
below. The xylenol orange acts as a ligand, which can bind to the aluminum ion through the six atoms
shown in boldface type. In the process, the four protons attached to the –COOH groups are released from
–
the molecule. The color of this complex ion AlQ is red.
Measuring absorbance at equilibrium using Beer’s law
–
The complex AlQ absorbs very strongly at a wavelength of 550 nm. The unreacted molecule
H4Q, on the other hand, absorbs much more weakly at this wavelength. The total absorbance at 550 nm
can be determined by applying Beer’s law to both species and taking the sum:
–
A = ε1[H4Q]L + ε2[AlQ ]L
where the cell light path L = 1 cm, and ε is molar absorptivity
In our experiment, we will start with a certain initial concentration of H4Q and then heat the mixture to
–
–
form the complex AlQ . The complex AlQ does not form at room temperature, but will form as we heat
the mixture. At any moment, the sum of the concentrations of H4Q and AlQ– will be constant and equal
to the initial concentration of H4Q, which we can represent as [H4Q]i:
–
[H4Q]i = [H4Q] + [AlQ ]
By combining this equation with the expression for the total absorbance, we can express the absorbance
–
in terms of the initial concentration [H4Q]i and the equilibrium concentration [AlQ ]:
–
A = ε1[H4Q]i + (ε2–ε1)[AlQ ]
Note that the quantity (ε2–ε1) is positive because ε2 is much greater than ε1, and that we have incorporated
the knowledge that the path length is 1 cm. To further simplify this expression, we can define Ai as the
–
initial absorbance before the reaction starts, when there is none of the complex AlQ :
Ai = ε1[H4Q]i
Finally, by combining this with our absorbance equation and solving for the concentration of the
complex, we find that
[AlQ–] =
A − Ai
ε2 − ε1
4
1
1
The quantity (ε2–ε1) has been determined as 2.50 × 10 L mol– cm– . By measuring the initial absorbance
Ai and the absorbance at a specific time A, the concentration of the complex AlQ– can be determined.
€
Determining ΔG°, ΔH°, and ΔS° from equilibrium measurements
The equilibrium constant K for the complexation reaction is:
H4Q + Al
→
←
–
AlQ + 4H
+ 4
[AlQ ][H ]
K=
[H Q][ Al ]
−
3+
+
3+
4
€
In this experiment, we will need to determine the equilibrium concentration of AlQ– at several different
2
temperatures. The concentration of H+ will be controlled by an HSO4–/SO4 – buffer, which should
maintain a pH of 2.0. The equilibrium concentrations of the other species can be determined from the
their initial concentrations using simple stoichiometric relationships:
[H4Q] = [H4Q]i – [AlQ–]
3+
3+
[Al ] = [Al ]i – [AlQ–]
Therefore you will be able to calculate the equilibrium constant K at several different temperatures. From
this you can calculate ΔG° at each temperature from
ΔG° = –RT ln K
Finally, we can use the definition of Gibbs free energy to find ΔH° and ΔS° :
ΔG° = ΔH° – TΔS°
A plot of ΔG° as a function of temperature should give a straight line with a slope of –ΔS° and a yintercept of ΔH°. Be sure to use the correct units in all your calculations.
II. Procedure and Data Collection
In this lab, you will be writing a procedure to collect the necessary data to determine the thermodynamic
data, ∆G°, ∆H°, and ∆S°. Before beginning any part of the experiment, you should share your proposed
procedure with your lab partner. Pick which procedure you will follow and write a detailed procedure on
page 11. Check your experimental procedure with your TF before proceeding.
Using the Spectrometer with Logger Pro
1. If your computer is not already logged in, login to the computer by clicking on “Student” and entering
the password “g3n0m3”.
2. Open the Logger Pro software (Go → Applications → LoggerPro).
3. Look inside the spectrometer. You should see a light inside the spectrometer that has a violet color. If
the spectrometer light is not on, the spectrometer USB cable may be unplugged. Quit Logger Pro,
check the cable, and restart Logger Pro. If the light still does not come on, ask your TF for help.
Experimental Materials
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two nested beakers
distilled water
hot plate
cuvette holder
kimwipes
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plastic cuvettes
square cuvette caps
xylenol orange (H4Q)
solution
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aluminum ion (Al3+) solution
UV-Vis Spectrometer
padded gloves
temperature probe
Setting up the Reaction
1. At your bench, you will find two nested beakers. Add distilled water to the inner beaker to the 30-ml
mark. Then add water to the outer beaker so that its water level is below that of the inner beaker.
2. Set these beakers on the hot plate and heat the water. Keep an eye on the water; it should boil gently.
Add distilled water as needed if it boils too much.
3. Obtain clean plastic cuvettes and square cuvette caps from the center bench. The number of cuvettes
you will need will depend on your procedure written as part of your prelab. You should need at least
two cuvettes (one for DI water and one for the reaction). Note: Each cuvette has clear sides and
opaque sides. Handle the cuvettes only by the opaque sides. Never touch the clear sides; fingerprints
will interfere with your measurements. If the clear sides of the cuvette become marked or scratched in
any way, you must repeat the experiment with new cuvettes.
4. Fill one cuvette with distilled water from your wash bottle, and cap it with one of the square caps.
This is your “water cuvette”.
5. Add 1.75 mL of both xylenol orange (H4Q) and aluminum ions (Al3+) into the reaction cuvette(s).
Cap the cuvette(s) before bringing them back to your lab station.
Calibrating the Spectrometer and Setting Up for Data Collection
1. Gently insert the water cuvette into the spectrometer. Make sure it is inserted all the way into the
spectrometer. The opaque edges should be on the sides of the cuvette when it is inserted, and the light
should pass through the clear sides.
2. In Logger Pro, go to the “Experiment” menu. Select “Calibrate
Spectrometer.” Once the dialog
box appears, click “Finish Calibration.” You will have to wait a few seconds, and then you can click
“OK.”
3. Go to the “Experiment” menu. Select “Change Units Stainless Steel Temperature K.”
Measuring the Initial Absorbance (Ai)
1. Insert your reaction cuvette into the spectrometer. You should see the absorbance on the lower lefthand corner of the screen in Logger Pro.
2. Record this value in the data section of your lab report. It is the “Initial Absorbance (Ai)” that you
will need for your calculations.
Running the reaction
Below you will find a general procedure for heating the reaction mixture. Depending on the data
collection procedure you outline in your prelab, you may need to adapt the following steps. Include any
adaptations in the space provided below. Check with your TF before making any changes to the steps
outlined.
1. Place the reaction cuvette in the hot water bath (i.e. in the inner beaker). Be sure that the top of the
cuvettes is above the level of water in the inner beaker. You must make sure that no water gets inside
the cuvettes.
2. Set the hot plate between 260 oC and 270 oC.
3. Heat the water bath until it reaches 363K. Add distilled water from a wash bottle if needed to keep
the level of water up to cover most of the cuvette. Observe the cuvette and water bath during
heating and write any observations in your lab report.
Measuring the Equilibrium Constant of the Complexation Reaction
Now you will measure the equilibrium constant of the complexation reaction by monitoring the
absorbance of the reaction mixture as it cools. You will need to take temperature and absorbance
measurements in order to complete the lab report. Discuss with your lab partner your outlined procedure
from the prelab and pick which procedure you will follow. Write a detailed procedure below. Check your
experimental procedure with your TF before proceeding.
III. Lab Report
Useful information for calculating ΔG°, ΔH°, and ΔS° from measurements of the
equilibrium constant
A − Ai
ε2 − ε1
[AlQ–] The concentration of AlQ– (M)
[AlQ–] =
[H4Q] The concentration of H4Q (M)
[H4Q] =[H4Q]i – [AlQ–]
3+
[Al ] The concentration of Al
3+
(M)
€[Al3+] = [Al3+]i – [AlQ–]
−
K
The value of the equilibrium constant
+ 4
[AlQ ][H ]
K=
[H Q][ Al ]
3+
4
ΔG°
The calculated ΔG° (kJ/mol)
ΔG° = –RT ln K
€
You will need the value of Ai that you measured and the following parameters:
4
1
1
ε2–ε1 = 2.5 × 10 L mol– cm–
5
[H4Q]i = 2.0 × 10– M
3+
5
[Al ]i = 2.0 × 10– M
+
[H ]
= 0.01 M
3
1
1
R
= 8.31 × 10– kJ mol– K–
1. Fill in your initial absorbance and record any observations during the complexation reaction.
Initial Absorbance (Ai) =
Observations during reaction:
2. Take a screenshot of your Logger Pro data and send it to your TF. Please title it “Lab5_Your TF’s
name_Your lab group’s first names”. Example: “Lab5_ Tamara_Julie and Mike”.
3. Using excel, create a table with the following columns. Fill in the table with your data to calculate ΔG°.
Temp.&
Abs.&
[AlQ-]&
[H4Q]&
[Al3+]&
K"
ΔG°&
4. In excel, plot your ∆G° values as a function of Temperature in excel.
5. Fit a line to your data and determine the slope and y-intercept. Use those values to determine ΔH° and
ΔS° for this reaction.
Slope =
y-intercept =
ΔH° =
ΔS° =
6. Did you observe any changes in the reaction mixture as it cooled? How were those changes reflected in
your measurement of absorbance at 550 nm?
7. In your prelab (question 6), you predict whether the absorbance at 550 nm would increase or decrease as
the reaction mixture cooled. Was your prediction correct? If not, what was the error in your reasoning?
8. Does the calculated sign of ΔH° correspond to the shift in the equilibrium according to LeChatelier’s
Principle? Explain.
9. Does the calculated sign of ΔS° make sense in light of changes in entropy in the complexation reaction?
You may wish to write out the balanced reaction as part of your explanation.
10. Ask TF to give you literature results for ΔH° and ΔS°. How do your results compare? What assumptions
did you make when taking your measurements that may have affected your results?
When you are finished, please be sure to log off from your computer. If you do not log off, the
spectrometer light will remain on and the bulb will burn out.
Prelab
To be completed and handed in as you enter the lab.
1. For this experiment, you will be collecting absorbance data at a variety of temperatures. We can take
absorbance and temperature measurements while heating up the reaction mixture, or we can monitor the
absorbance as the reaction mixture cools, subsequent to heating. Which method do you think would be
easier and why?
2. Draw a detailed sketch of the reaction setup. Be specific in your sketch (ie: where would you put the
temperature probe?).
3. Draw a sketch of how your data collection setup will work. Again, be specific (ie: where would you put
the temperature probe?)
4. We have learned how to do several different types of measurements using Logger Pro (ie: collection of
individual data points as in the Titration Lab or continuous data collection over time as in the Kinetics
lab). Considering your data collection setup in #3, what data collection method will you use in Logger Pro
and why?
5. In this experiment, we will be heating the reaction mixture to ~85 oC and collecting data as the reaction
cools. Using the experimental materials listed in the “Procedure and Data Collection” portion of the
experiment, outline the steps you will follow to collect the necessary data to calculate thermodynamic
values ΔG°, ΔH°, and ΔS°. Your procedure should be detailed and should reference your sketch in #3 and
your answer to #4 (ie: if you plan to take individual absorbance/temperature measurements, at what
temperatures will you record your data?).
6. As the reaction mixture cools, would you expect the absorbance at 550 nm to increase or decrease?
Explain.
7. How do we know that [H+] in the reaction mixture is equal to 0.01 M?
8. In one experiment, the initial absorbance was 0.014. The absorbance at 86°C was 0.196. Using this
data, calculate ΔG° for this complexation reaction at 86°C.
(This Page Intentionally Left Blank)
Experiment 13