Introduction:
In this experiment we used colorimetric analysis to study the free energy and solubility of a cobalt salt. We did this by measuring the transmittance of four solution of the cobalt salt at different concentrations.
Alexander Krzyston Cobalt Alexander J Krzyston Cobalt| Alex James Krzyston
Alex Krzyston Cobalt| Alex J Krzyston Cobalt| Alexander James Krzyston
NORTHWESTERN UNIVERSITY Cobalt| EVANSTON Cobalt| BURR RIDGE
We then applied Beer’s Law to the data to show that A/r of the solution was equal. We then measured the absorbance of four other solutions of cobalt that were prepared at four different temperatures. A/r = kd we used this equation (value obtained from previous four solutions) and A = kCd to calculate the concentration of each of the solutions, A, B, C, and D. Because A, B, C, and D are saturated solution, the concentrations for them are also the solubilities, thus s = c and we can use the equation Ksp = (s)(2s)2 = 4s3 to find Ksp We can then make a graph of lnKsp vs. 1/T, which is a linear relationship. From the graph we can find the slope and intercept. From the slope we can calculate ΔHo and from the intercept we can calculate ΔSo.
Overview:
Sources of error in this experiment are that it was clear that some of the cobalt solution remained on the paper we used to weigh it out with, this result in a higher cobalt concentration than there actually was. In addition, not all of the cobalt may have been dissolved into the solution; this too would produce a solution with a lower concentration. Another source of error is that when we were using the 10mL pipettes to measure out the solutions it was difficult to get all of the solution out of the pipette, often times so would remain at the very tip. In addition, although the Solutions A, B, C, and D were prepared at different temperatures, they were all kept at room temperature so this may or may not have had an effect of their absorbance.
An interesting observation we found was that when we were measuring out 10ml of water to fill each of the 50mL beakers, we used the 10mL pipette and filled it to the line but when we out to distilled water in the beaker it measured more than 10mL. In this case, we went by the measurements of the pipette and not the beakers.
One way to improve this experiment would be to obtain data using both the UV-vis spectrophotometer and the Spec-20. This would allow students to compare data between the two machines and determine which one is more accurate. Also I think keeping the four solution that were saturated at different temperatures at the temperatures they were saturated at would improve the accuracy of the experiment.
*In calculating ΔHo and ΔSo from slope and intercept I had to use the average slope because my graph was not linear. To do this I calculated the slope from (0.0034, -12) to (0.0035, -13) which was -38000, the slope from (0.0035, -13) to (0.0036, -13) which was zero, and the slope from (0.0036, -13) to (0.0037, -12) which was 10000. I then found the average slope which was -12000. I then used the equation: m = -ΔHo / R to find the change in enthalpy. To find the change in entropy I used the average slope (-12000) that I had already calculated in the equation y = mx+b and applied each of the four data points to find an intercept. Using (0.0037, -12) I got an intercept of 360, using (0.0036, -13) I got an intercept of 350, using (0.0035, -13) I got an intercept of 340, and using (0.0034, -9.2) I got an intercept of 330. I then took the average of the four intercepts and got and average intercept of 340. I then used the equation b = ΔSo / R to find the change in entropy.
Experimental Procedure:
The procedure was the same as that given in the lab manual. We used the UV-vis spectrophotometer and the Spec-20, thus we did not need to calculate absorbance, we could directly read off the UV-vis spectrophotometer (but I did calculate transmittance in the lab report).
Data Analysis:
1. see attached sheet for calculation of cobalt concentration
2. see attached sheet for calculation of transmittance and mean absorbance
3. see attached sheet for calculation of molar absorptivity (k)
4. see attached sheet for Beer’s Law and calculation of A/r
Solution # Transmittance Absorbance A/r
0 0.13 0.072 0.072
1 0.13 0.027 0.079
2 0.13 0.036 0.072
3 0.13 0.048 0.073
5. see attached sheet for calculation of concentrations and Ksp
solution Saturated at (oC) Absorbance Concentration Ksp
A 0.0 0.185 0.015 1.5 x 10-5
B 6.0 0.098 0.0082 2.2 x 10-6
C 12.0 0.101 0.0084 2.4 x 10-6
D 18.0 0.356 0.030 1.0 x 10-4
6.
solution oC T (in K) 1/T (in K-1) Ksp ln Ksp
A 0.0 273 0.0037 1.5 x 10-5 -12
B 6.0 279 0.0036 2.2 x 10-6 -13
C 12.0 285 0.0035 2.4 x 10-6 -13
D 18.0 291 0.0034 1.0 x 10-4 -9.2
7.
8. see attached sheet for calculation of ΔHo and ΔSo from slope and intercept
*slope and intercept values are averages (see discussion of explanation)
Slope(m) = -12000
Intercept(b) = 350
ΔHo = 1.0 x 103kJ
ΔSo = 2910 J
In this experiment, we took a solid cobalt salt and dissolved it into water. The cobalt salt went from a solid to a solution; it was ionized which contributed to the entropy change, increasing it. In the previous experiment involving Sn and Fe, Sn and Fe were already in solution form.
Overview:
Sources of error in this experiment are that it was clear that some of the cobalt solution remained on the paper we used to weigh it out with, this result in a higher cobalt concentration than there actually was. In addition, not all of the cobalt may have been dissolved into the solution; this too would produce a solution with a lower concentration. Another source of error is that when we were using the 10mL pipettes to measure out the solutions it was difficult to get all of the solution out of the pipette, often times so would remain at the very tip. In addition, although the Solutions A, B, C, and D were prepared at different temperatures, they were all kept at room temperature so this may or may not have had an effect of their absorbance.
An interesting observation we found was that when we were measuring out 10ml of water to fill each of the 50mL beakers, we used the 10mL pipette and filled it to the line but when we out to distilled water in the beaker it measured more than 10mL. In this case, we went by the measurements of the pipette and not the beakers.
One way to improve this experiment would be to obtain data using both the UV-vis spectrophotometer and the Spec-20. This would allow students to compare data between the two machines and determine which one is more accurate. Also I think keeping the four solution that were saturated at different temperatures at the temperatures they were saturated at would improve the accuracy of the experiment.
*In calculating ΔHo and ΔSo from slope and intercept I had to use the average slope because my graph was not linear. To do this I calculated the slope from (0.0034, -12) to (0.0035, -13) which was -38000, the slope from (0.0035, -13) to (0.0036, -13) which was zero, and the slope from (0.0036, -13) to (0.0037, -12) which was 10000. I then found the average slope which was -12000. I then used the equation: m = -ΔHo / R to find the change in enthalpy. To find the change in entropy I used the average slope (-12000) that I had already calculated in the equation y = mx+b and applied each of the four data points to find an intercept. Using (0.0037, -12) I got an intercept of 360, using (0.0036, -13) I got an intercept of 350, using (0.0035, -13) I got an intercept of 340, and using (0.0034, -9.2) I got an intercept of 330. I then took the average of the four intercepts and got and average intercept of 340. I then used the equation b = ΔSo / R to find the change in entropy.
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