CHM 001B CSU Le Chateliers Principle on Cobalt Chloride Pre Lab Question

Description

Please help with the Prelab assignment by answering the questions/requirements below: :
1) Descriptive Title of the Lab
2) Summarize the Purpose of the Lab
3) Hypothesis/Predictions (if applicable)
4) Any chemical equation, equations or formulas.
5) Answer the prelab questions.
6) Chemical Hazards
7) Summarize the lab ProcedureCHM 001B – General Chemistry II
Mission College, Faris
Exploring Cobalt Complex Equilibrium Through the Visualization of Color Change and UV-Vis
Spectroscopy
Adapted from: Ren, J.; Lin, T.; Sprague, L. W.; Peng, I.; Wang, L. Exploring Chemical Equilibrium for Alcohol-Based
Cobalt Complexation through Visualization of Color Change and UV-vis Spectroscopy. J. Chem. Educ. 2020, 97, 509516.1
Introduction
In this experiment we will explore cobalt(II) coordination complexes through UV-Vis
spectroscopy. But first, let us dive into the topic of coordination compounds. Coordination
compounds have a wide variety of applications as therapeutic drugs, chemical sensors, and
coloring agents. In your biology classes, you may have studied biological molecules that contain
transition metals, like hemoglobin, which bond in similar ways to coordination compounds.
Have you ever wondered why rubies are red and emeralds are green? Both gemstones contain
the same transition metal cation chromium(III), Cr3+. The difference is due to the crystal that
hosts the chromium(III) ion. Rubies are crystals of aluminum oxide in which about 1% of the
aluminum ions are replaced by the chromium(III) ions. Emeralds are crystals of beryllium
aluminum silicate, Be3Al2(SiO3)6, in which about 1% of the aluminum ions is replaced by the
chromium(III) ions. Why is the embedded chromium(III) ion in the ruby crystal red and in the
emerald crystal green? The color difference is due to the effect the host crystal has on the
energies of the atomic orbitals in the chromium(III) ions. Atoms in the crystal create a field around
the ion, called the crystal field, that splits the five degenerate d orbitals into two or more levels.
The color of the gemstone is caused by electron transitions between these levels. We will not go
into details regarding crystal field theory, the bonding model for transition metal complexes that
helps us understand color and magnetic properties of these compounds; however, whenever you
see different colors for the different coordination compounds in this experiment, know those
observations can be explained through crystal field theory. Let’s review over some basic
principles from general chemistry.
Transition metals have more uniform properties compared to main group elements. Most
transition metals have moderate to high densities, good electrical conductivity, high melting
points, and moderate to extreme hardness. The uniformity is due to their similar electron
configurations. Remember that electron configurations are a way for us to predict which atomic
orbitals each electron resides in which in turn helps us to predict an atom’s properties.
Atomic radii of transition metals stay relatively constant because as another proton is added, so
is an electron, however, the electron goes into the n-1 orbital. The number of outermost
electrons stays the same so the electrons experience nearly the same effective nuclear charge
(Zeff)as we move across, keeping the radii constant.
In a previous chapter, we learned that transition metals can form complex ions. A complex ion
contains a central metal ion bound to one or more ligands. A ligand is a Lewis base that forms a
bond with a metal. When a complex ion combines with a counterion or counterions to form a
Cobalt Complex Equilibrium
1
CHM 001B – General Chemistry II
Mission College, Faris
neutral compound, it is called a coordination compound. An example of a coordination
compound is shown in Figure 1.
Figure 1. Structure of [Co(NH3)6]Cl3.
The charge and size of the metal ion, as well as the type and number of coordinating ligands will
determine the overall structure of the coordination complex. These characteristics will influence
properties such as the observed color.
This lab will focus on cobalt(II), which can form complex ions in solution. When dissolved, the
cobalt(II) ion will be surrounded by either four or six ligands, yielding tetrahedral or octahedral
structures, respectively. Tetrahedral cobalt(II) complexes are blue/violet while octahedral
cobalt(II) complexes are pink. In this experiment you will interchange the ligands (methanol
versus propanol) and observe the color change.
When a smaller alcohol such as methanol is used as the solvent, cobalt(II) chloride dissolved in
methanol complexes six ligands to form a pink octahedral complex ion. In this experiment, we
start with a stock solution prepared by dissolving a very small amount of cobalt(II) chloride in
methanol, resulting in the pink octahedral complex ion, [CoCl(CH3OH)5]+. An equilibrium between
the octahedral and tetrahedral coordination complexes will be established when 2-propanol
(isopropanol) is added. A blue tetrahedral complex such [CoCl(CH3CH(OH)CH3)3]+ is produced
when the larger ligand, 2-propanol, is added to the pink octahedral complex solution to replace
the small methanol ligands. The tetrahedral complex is formed as a result of minimizing the steric
effect due to the larger 2-propanol ligands. Below is the reaction taking place. We will use ‘M’ for
methanol and ‘P’ for 2-propanol as abbreviations.
3

5
Equation 1
In this lab, you will explore the equilibrium constant through Le Chatelier’s Principle. Le
Chatelier’s Principle states that a change made to a chemical system at equilibrium will cause
the equilibrium position to shift in a way that will reduce the effect of the change. For example,
Cobalt Complex Equilibrium
2
CHM 001B – General Chemistry II
Mission College, Faris
if methanol is added to the equilibrium mixture, the equilibrium position will shift to the left. This
will increase the relative proportion of the octahedral complex ion, and thus shift the equilibrium
toward the octahedral (pink) side. Manipulating this property of equilibrium systems will allow
us to explore cobalt (II) complexation via color and analyzing these changes will require the use
of UV-vis absorption spectroscopy.
Below are the absorbance data you will observe as the equilibrium shifts with the addition of 2propanol.
Figure 2. Absorbance data for complex ion equilibria.1
Pre-lab Questions
1. What is the ground state electron configuration for cobalt(II)?
2. Predict the geometry and color of a solution of cobalt(II) chloride dissolved in methanol.
How does the geometry and color of the solution change if isopropanol is the solvent
instead of methanol? Write the chemical formula of the coordination complex in each
solution.
3. To determine the molar absorptivity coefficient of the octahedral complex CoBr2M4, a
student dissolves 0.26g of cobalt(II) bromide hexahydrate in 50.00mL of methanol to
make a stock solution. The student then creates solutions from the stock solution as
follows:
Solution
Volume of
stock (mL)
Volume of
isopropanol
(mL)
Absorbance at
530nm
1
0.500
9.500
0.303
2
1.000
9.000
0.577
3
1.500
8.500
0.855
4
2.000
8.000
1.149
Calculate the concentration of CoBr2M4 in each solution. Plot concentration (x-axis) versus
absorbance (y-axis) determine and report the molar absorptivity coefficient from a linear
regression analysis. Include the graph with your lab report.
Cobalt Complex Equilibrium
3
CHM 001B – General Chemistry II
Mission College, Faris
4. Explain in your own words, what the term “chemical equilibrium” means. At equilibrium
is there always an equal amount of products as reactants?
Cobalt Complex Equilibrium
4
CHM 001B – General Chemistry II
Mission College, Faris
Experimental
Safety hazards
Cobalt(II) chloride hexahydrate is an irritant and toxic to ingest, use with care. Methanol and
isopropanol are both volatile, flammable, and toxic solvents, prepare solutions in the fume hood
and avoid inhalation of any fumes.
Procedure
Part A. Determining the
of Co(oct) in methanol:
Obtain three small labeled beakers and collect 15mL of the stock solution (0.05M cobalt(II)
chloride dissolved in methanol), 10mL of 2-propanol, and 5mL of methanol, respectively. Place
parafilm over the beakers to reduce evaporation. Obtain a cuvette and fill it with methanol to
three-quarters full. Use a Kim Wipe to clean the sides of the cuvette. This cuvette will be used as
the “blank” for calibration. Before running your samples, be sure to run this “blank” beforehand
to calibrate the spectrophotometer. Using a 1.000mL micropipette, dispense 3.000mL of the
stock cobalt(II) chloride in methanol solution into another clean cuvette. Cap the cuvette and
place in the spectrophotometer. Collect absorbance data from 400-700nm for every 20nm. Also,
find the
where the solution has the maximum absorbance.
Part B. Determining the molar absorptivity coefficient,
! , of Co(oct) in methanol:
Note that the same cuvette will be used to prepare all solutions described in Table 1. Also note
that solution A is already prepared from Part A.
After determining the
from Part A, proceed to measure the absorbance of solution A again,
. Remember to run a “blank” methanol cuvette prior to
but this time at the determined
taking the measurement. Additionally, take note of the color of the solution in your lab notebook.
After measuring the absorbance of solution A at
, discard the solution in the waste
container, rinse the cuvette with methanol three times, and prepare solution B (using a 1.000mL
micropipette) in the same cuvette. Since solution B is a mixture, carefully mix the two liquids in
the cuvette by drawing some of the solution into the micropipette and expelling it back into the
cuvette. Make sure to briefly mix all solutions that are not solely the stock solution. After taking
another blank, measuring solution B at
, and taking note of the color, prepare and measure
solution C in the same manner as well.
Solution
Volume of
stock (mL)
Volume of
Total volume
methanol (mL)
(mL)
A
3.000
0.000
3.000
0.050
B
2.000
1.000
3.000
0.033
C
1.000
2.000
3.000
0.017
[Co (oct)] (M)
Table 1. Solutions that will be used to determine the molar absorptivity coefficient of Co(oct) in
methanol.
Cobalt Complex Equilibrium
5
CHM 001B – General Chemistry II
Mission College, Faris
Part C. Equilibrium of Co2+ tetrahedral and octahedral coordination complexes:
To observe the equilibrium between the pink and blue cobalt complexes, the following solutions
in Table 2 will be measured. Again, only one cuvette is used for all solutions and the cleaning
procedure is the same as in Part B. Remember to run a “blank” methanol cuvette prior to each
sample measurement.
Solution
Volume of
stock (mL)
Volume of
isopropanol
(mL)
Total volume
(mL)
1
3.000
0.000
3.000
2
2.000
1.000
3.000
3
1.500
1.500
3.000
4
1.000
2.000
3.000
5
0.500
2.500
3.000
Table 2. Solutions used to observe the equilibrium between the Co2+ tetrahedral and octahedral
complexes.
Once you have prepared a solution in a cuvette, mix the solution, and record the color of the
mixture in your lab notebook. Take a “blank” methanol measurement, then measure the
absorbance at the
for both the pink (530nm) and blue (660nm) complexes. Repeat this
procedure for all five solutions.
After completing the experiment, dispose all used and unused solutions in the waste container.
Rinse pipette tips with deionized water and throw in the trash can.
Data and Results
Part A. Determining the
of Co(oct) in methanol:
Concentration (M) of cobalt(II) chloride in methanol stock solution: _______________________
Attach a computer-generated plot of wavelength (x-axis) versus absorbance (y-axis) for the stock
cobalt(II) chloride solution. Label axes and give the graph a title.
Based on your absorption spectrum for the cobalt(II) chloride stock solution, determine the value
of
.
(nm) of Co(oct) in methanol: ________________________
Cobalt Complex Equilibrium
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CHM 001B – General Chemistry II
Mission College, Faris
Part B. Determining the molar absorptivity coefficient,
! , of Co(oct) in methanol:
Complete the following Table 3, assuming negligible amount of tetrahedral complex present in
solution. Refer back to Table 1 for the [Co(oct)] (M).
Solution
Volume of
stock (mL)
Volume of
Solution color
methanol (mL)
A
3.000
0.000
B
2.000
1.000
C
1.000
2.000
[Co(oct)] (M)
Absorbance at
λmax
Table 3. Data collection to help determine the molar absorptivity of Co(oct) in methanol.
Using graphing software, construct a plot of Co(oct) concentration (x-axis) versus absorbance (yaxis). Graph the data as an XY scatter plot and find the best fit line for the data by using linear
regression analysis and set the y-intercept to zero. Include the equation of the line and the R2
value on the graph. Determine the slope of this best fit line which is equal to the molar
absorptivity, “#$% . Label the axes and the title and attach the graph to your lab report.
molar absorptivity, “#$% , of Co(oct): ______________________
Part C. Equilibrium of Co2+ tetrahedral and octahedral coordination complexes:
Complete the following Table 4 in your lab notebook.
Solution
Volume of
stock (mL)
Volume of
isopropanol
(mL)
1
3.000
0.000
2
2.000
1.000
3
1.500
1.500
4
1.000
2.000
5
0.500
2.500
Solution color
Absorbance at Absorbance at
λmax = 530nm λmax = 660nm
Table 4. Data collection in order to determine equilibrium of cobalt(II) tetrahedral and octahedral
coordination complexes.
Complete the following Table 5 in your lab notebook. Use the molar absorptivity, “#$% , derived
from Part B and Beer’s Law to calculate [Co(oct)]. Use the predetermined molar absorptivity “% %
of 338.51M-1 and Beer’s Law to calculate [Co(tet)]. Use the following literature values to calculate
the [P], concentration of isopropanol, and [M], concentration of methanol. To calculation [M],
assume the stock solution is made mostly of methanol.
‘()*+,-./#01#0 2#3 = 0.785
Cobalt Complex Equilibrium
8
8
; 9 ;< 9;**./#01#0#2 3 = 60.09 9 9 7 CHM 001B – General Chemistry II '()*+,- Mission College, Faris %? 2#3 = 0.791 8 ; 9 ;< 9;** 9 %? 2#3 = 32.04 8 9 After calculating the concentrations of all the species in each solution, calculate the equilibrium constant, Keq, for each solution using the equation from the introduction. Solution 1 [Co(oct)] (M) [Co(tet)] (M) [P] (M) [M] (M) Keq N/A* 2 3 4 5 Table 5. Calculating Keq for each solution using Beer-Lambert Law. *Keq value is not calculated for solution 1 since the [P] = 0M which would result in an equilibrium expression that requires division by 0. Post-lab Questions 1. Why is it important to place parafilm on top of the beakers when gathering the solutions for the experiment? How would not using the parafilm affect the results? Be specific in what pieces of data would be adversely impacted by this technical error. 2. Isopropanol was used as a ligand in this experiment to initiate the shift from octahedral to tetrahedral geometry. Do you think the same transition would occur if ethanol was used instead of isopropanol? Provide a rationale for your answer that addresses the structural differences of ethanol. 3. Using your knowledge from the previous cobalt complex lab (“Gibbs Energy Changes during Cobalt Complexation”), predict how an increase in temperature might affect the equilibrium of the reaction in this lab. Explain your prediction thoroughly. Broader Impacts You can choose one prompt to research and explore. Your response needs to be at least 350 words and include citations for any sources you utilize. 1. Complexes of the Co2+ ion have been the focus of two separate labs now but are not limited to the few complexes discussed. “Cobaloximes”, are a class of cobalt complexes that offer a promising platform for the green production of solar fuels (i.e. production of H2 from the splitting of water).3 Do some research into cobaloximes and discuss what some of the different ligands can do to alter the activity of a cobaloxime complex. 2. The idea of equilibrium can be a complex one, especially as reactions have more moving pieces. Take some time to explain why you think equilibrium is a necessary part of life by highlighting and discussing an important chemical equilibrium that exists in living things. Make sure to show the reaction taking place and discuss in detail what the downstream effects would be if the equilibrium was altered. Cobalt Complex Equilibrium 8 CHM 001B – General Chemistry II Mission College, Faris References 1. 2. 3. Ren, J.; Lin, T.; Sprague, L. W.; Peng, I.; Wang, L. Exploring Chemical Equilibrium for Alcohol-Based Cobalt Complexation through Visualization of Color Change and UV-vis Spectroscopy. J. Chem. Educ. 2020, 97, 509-516. Tro, N. J. (2018). Chemistry: Structure and Properties (2nd ed.). Pearson Education Inc. Dempsey, J. L.; Brunschwig, B. S.; Winkler, J. R.; Gray, H. B. Hydrogen Evolution Catalyzed by Cobaloximes. Acc. Chem. Res. 2009, 42, 1995-2004. Cobalt Complex Equilibrium 9 Purchase answer to see full attachment

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