Adam Bongarzone

Kinetics Lab Writeup

Section 5 Representin’

            Part 1

1. Titrated NaOH into a solution of chloropentammine cobalt (III) chloride with 12 drops of indicator. It turned from light purple to dark purple, indicating the equivalence point, and that it has a rich, fruity flavor with a flowery bouquet, and perhaps a hint of anise. Data as follows:

            Mass of cobalt complex: 0.2288 g

      Volume of NaOH titrated: 22.93 ml

      Molarity of NaOH: 0.1006 M

This gives:

      Moles of cobalt complex: 9.13(10^-4) mol

      Moles of NaOH: 0.0023

Calculations are included on the attached Microsoft ToiletPaper™ Spreadsheet

<Note: yes, I actually did write it out on a piece of toilet paper.>

Results

      Chloro: 4.39(10^-4) mol

      Aquo: 4.74(10^-4) mol

That’s about 52% chloro, 48% aquo.

2. Error in number 1. There was none. I’m just that good.

            Actually, I’m terrible at lab. Plenty of things could have gone wrong here: Incomplete dissolution of the cobalt complex is one likely culprit. We also may have gone past the titration point, since I wasn’t paying attention, and my lab partner is color-blind. Also, when we left during the break between sections, the resin dried out. I think there are some non-linear effects, but these were likely very small.

            As for significant figures, the error would lie in the fact that I don’t know how to use them correctly.

3. Check out the attached spectra. They are works of art. Bow before them.

4. a. We know visible light falls between 400 nm and 700 nm. 474 nm is near the violet range, also known as the “grape-flavored” range. Not coincidentally, the color of the cobaltammine is a purple/violet color. Hence, it will absorb at that wavelength AND have a refreshing grape flavor. So, we expect to see a color change to go along with the change in absorption spectra after aquation. Too bad I didn’t see any. And I didn’t taste it either.

b. The different absorption spectra could be from the different metal ligand bonds. The OH ligands have a different excitation frequencies than NH3 and Cl ligand bonds. The wavelength was shorter after aquation, which indicates higher energy, meaning that it’s stronger. I think this means that it has been eating its vegetables.

            Part 2

1. I have some absorbance data for both the .01N and .03N solutions. I thought it was good data. I nurtured it, sheltered it, loved it. Then Goalseek rejected it. It wouldn’t give me any reasonable answers for either solution. Luckily, the absorbance spectra machine isn’t as picky. It gave me some nice spectra, which are attached. I calculated A(Infinity) using the formula A(infinity)=ce, Where c is the initial concentration, and e is the extinction coefficient. There are some calculations and crappy graphs on the attached Excel sheets. To save you the trouble of writing, yes, I know I didn’t graph the linear fit or show y-intercepts or r squared. That’ll save you some red ink. Here’s a nice space for you to write however many points I lost from this, too:

2. The slope from the lines give rate constants of 203.053 L/min for the .01N and 126.945 L/min for the .03N. The data are approximately linear, so the rate law must be linear as well. That’s good, because if it weren’t, I’d be screwed.

3. We note that the concentration in acid does not cause a proportional (factor of 3) change in the rate constants. So it isn’t acid catalyzed. This leaves either reaction Sn1 or Sn2. I flipped a coin to decide that it was Sn1 (it was tails). The coin in question was a nickel, dated 1969, with something sticky under Jefferson’s nose, and normal aging on the back.

4. My sources of error are numerous and varied. Allow me to demonstrate: The absorbo-meter machine drifts like a boatful of drunk sailors. I’m also going to say that perhaps everything didn’t dissolve, since I’ve said that for everything this term and it seems to have worked. Also, the nickel that I used to decide between Sn1 and Sn2 had some junk on the heads side, so perhaps this extra weight caused it to be more likely to land on tails.

5.a. 550 nm is the normal wavelength for regular old visible light. I guess that means we were supposed to observe changes in the solution better because of this. Wish I had known that during the lab.

b. Because it’d be stupid to use HCl. We’ve got a cobalt chloride, so it would seriously mess with the equilibrium with twice the Cl ions. And if we’re lucky, we’d get some nice poisonous chlorine gas. Maybe that would’ve put me out of my pain anyway.

Conclusion: I’m glad it’s over.