Volumetrics

This past week we performed an experiment that investigated the kinetics of an SN1 reaction. I have been teaching with this experiment for years for the following reasons:

1) It fits in very well with the SN1/SN2 lecture material which is a key part of first semester organic chemistry,

2) it emphasizes some skills that are not always highlighted in organic chemistry such as the Excel graphing of an Arrhenius plot,

3) it reinforces some skills learned in previous chemistry courses such as end point determination and pipetting, and

4) I like the “class data” aspect of the experiment where students can compare their data to others and even use class data to determine the energy of activation.


The one big drawback is that it is a very sensitive reaction and students occasionally get widely varying results from the same procedure. Basically they need to measure three volumes of 0.15, 1.5, and 3.35 mL into 25 mL erlens. The smallest volume measure is very challenging with a 2/10 mL glass pipette. As a few students have pointed out, automatic pipettors would be better. Unfortunately, the biologists are not keen on loaning me their pipettors to pipette noxious chemicals. This year I thought about replacing the 2/10 mL glass pipettes with a precision Hamilton syringes. The most convenient size, a 250 microliter syringe costs around $50. I was able to buy one from EBay and two from LabX for somewhat less. That was an interesting experience and saved a few dollars.

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