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CHEM122 Temple University Thermodynamics of Cobalt Complexation Lab Report

I have attached the data sheet and the report sheet as well as a handout on terms related to thermodynamics. I have also attached the discussion (which I normally post on t he whiteboard) since I need you to read each step BEFORE you come to lab. The actual experiment, itself, will require you to prepare a solution (very precisely), cool it down, and then place it in a spectrophotometer and record the absorbance as a function of temperature. We will be in room W4-47 for both lab sections so that I might not be able to display this procedure on the board. However, I will go over all the calculations with you before you begin.

Data Sheet: Lab #8 – Thermodynamics of Cobalt Complexation
[Co(EtOH)4(Cl)2] [Co(EtOH)2(Cl)2] + 2 EtOH
Side of the reaction that appears more
“random”(greater entropy):
Weight of beaker + CoCl2.6H2O
Weight of empty baker
Color of sol’n after adding EtOH:
Weight of CoCl2.6H2O
Color of sol’n after cooling:
Formula weight of CoCl2.6H2O
Color on removal from spectroscope:
From the color changes, in which direction
does the reaction appear to be exothermic?
Moles of CoCl2.6H2O
Moles of Co ion
Initial Molarity of Co
plot ΔGo vs. Temp K
Actual Temp
X = conc. = abs/66.871
= initial -X
Temp K
ΔG° = -RT ln Keq
Perform your calculations. Even if you do use Excel formulas to calculate your values, show sample calculations from one complete trial on the back.
Plot a graph with Temperature on the x-axis and ΔG on the y-axis. Show the trendline.
Equation for your trendline =
ΔSo = – slope of line =
ΔHo = y-intercept =
Are the SIGNS of ΔH and ΔS from your graph consistent with your observations? Why or Why NOT?
Lab #8: Thermodynamics of Cobalt Complexation
1. Prepare an ice-salt bath: half fill a 250 ml beaker with ice, add salt (amount -tablespoon + ). Stir.
Add more ice and more salt while stirring to create a thick slush. (Dump out any water and add
more salt. Check the temperature using a thermometer from your drawer. The temperature
should be about -8 oC or lower once it stabilizes.
2. Turn on Spectrophotometer. Set the wavelength to 655 nm. Make sure filter at bottom of
spectrophotometer has been flipped.
3. Set up equipment as shown:
a. Fill a cuvette with distilled water. Place it in the spectrophotometer. Adjust for 100%
transmittance and zero absorbance with the lid of the sample holder close.
b. Open the lid of the sample holder and tape it so that is stays open when the light –
blocking cup is placed over it.
c. Place the light-blocking cup over the cuvette in the sample holder, insert the temperature
probe and adjust it so that it will be inside the solution in the cuvette but not so far down
that it interferes with the path of light through the cuvette. Check the absorbance to see if
it still reads zero. If not, make adjustments.
4. Weigh a clean, dry 50 ml beaker.
5. On a piece of tared weighing paper, weigh out between 0.25 and 0.28 grams of CoCl2.6H2O
6. Transfer the Cobalt compound completely to the clean beaker and re-weigh
7. Add about 20 ml of 95% ethanol ( density = 0.789 g/ml) to the beaker. Stir to dissolve. Add
additional ethanol in 1 – 2 ml increments until the substance is just dissolved.
8. Transfer the cobalt solution from the beaker to a 50 ml volumetric flask. Wash the beaker 2 -3
times with a ml or so of ethanol and add the washings each time to the volumetric, then fill the
volumetric to the mark with ethanol.
9. Fill one cuvette with ethanol. This will be your blank. Fill a second cuvette with the cobalt
solution from the volumetric flask.
10. Place the cuvette containing the cobalt solution into the ice bath and let it remain for about 15
minutes (the solution should change color completely, wait until after it has passed through the
intermediate purple stage)
11. Place the blank into the spectrophotometer and set the absorbance to zero. Remove the cuvette.
12. Record the final temperature of the ice bath. Then work quickly to
a. Remove the cuvette from the bath
b. Dry the outside of the cuvette
c. Place the cuvette into the sample holder of the spectrophotometer.
d. Insert the temperature probe as you did before and surround it with the light-blocking
cup. Attach the temperature probe to the LabQuest and turn it on.
e. Tape the cup down over the sample holder (NO LIGHT MAY BE ALLOWED TO ENTER)
13. Wait until the temperature stabilizes (the text suggests about -5oC but your temperature will
probably be higher) then record absorbance.
14. Record absorbance values for every 0.5 oC change in temperature. Continue recording
absorbance and temperatures until you have at least 10 to12 valid data points recorded. If your
solution warms rapidly, you may have to take readings every 0.2 oC or 0.3 oC – check with your
15. Remove the light blocking cup, remove and rinse and dry the temperature probe. Return the
probe and the LabQuest. Dump the cuvette as directed.
16. Determine the initial concentration of the Cobalt ion solution. Determine the equilibrium
concentrations of all species. Complete the calculations as directed on the data sheet.
17. Determine the equilibrium constant for the reaction at each temperature, Use the equilibrium
constant to calculate the value for ΔGo . Plot a graph of ΔGo vs. the temperature in Kelvin,
Calculate the values for ΔSo ( which will be the -slope of the line) and ΔHo (which will be the
y-intercept of the line).
Chemistry 122 – Lab Report (Plappert)
Your Name_______________________________________________________________ Experiment #__8_
Title: Thermodynamics of Cobalt Complexation
Name(s) of Partner(s) _______________________________________________________________________
Date Lab Performed_______________ Date Report Submitted________________
Purpose of Experiment:______________________________________________________________________
Procedure (SUMMARIZE succinctly in your own words: try for 75 words or less)
√ Attach the data sheet (spreadsheet) provided by the instructor .
√ Attach the graph . You will be plotting the absolute temperature on the
x-axis and ΔG (Gibbs free energy)
on the y-axis. The value for ΔG will be calculated from the equilibrium constant for the reaction. [I
STRONGLY suggest that you set up formulas in Excel and have Excel calculate the equilibrium constant as
well as the ΔG for each temperature! ]
The trendline MUST be included on the graph. From the trendline you will compute the value for ΔS (the
slope of the line) and ΔH the (y-intercept).
(You must also include a copy of this graph in your lab notebook).
√ Show a complete set of sample calculations for at least one complete trial on the back of this paper.
Make sure to show the equilibrium equation and calculations (ICE) used in the computation of the
equilibrium constant.
√ Attach your pre-lab and post-lab questions.
Please list below any relevant data or observations not shown on the data sheet.
CONCLUSION: State your obvious result(s) and briefly explain how these results did or did not achieve the
purpose of this lab.
ERROR ANALYSIS: Examine your data and results. What are the likely sources of experimental error? To
the degree possible, support this analysis with qualitative and quantitative evidence.


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