September 2010 Archives

Expt 11

Today's lab went very well. A few suggestions:

- Make sure to tell the students to use the "store latest run" option, otherwise it's very possible they'll end up deleting data and having to re-run trials.

- KI is used in both experiment 11 and experiment 13, students should be getting KI from the jugs with the pink labels.

- A common source of error in spectrometer measurements is droplets on the outside of the cuvette; impress on students the importance of keeping the spectrometers dry.

- As a matter of convenience, write the molar absorptivity (ε = 4284.8) on the board.

- If students are getting runs that read 0 absorbance, despite the rest of their solution turning yellow, try replacing the solution inside the cuvette with the yellow solution. Usually the trend will still be linear, and the run can be salvaged.

- Note for new TAs: the trials roughly trend upward in slope, from A to G (i.e. A has a small slope, and G is very high). This is a useful diagnostic for judging whether or not students need to redo a particular trial.

- Runs typically only need to go for 2-4 minutes before a sufficient linear section will appear. Also encourage students to prepare their next solutions while runs are in progress.

Part II
Today's lab is largely error-free. A few tips to keep things running smoothly:

- Remind students that they should be putting units on their calculations and answers, even for things like k and R.

(Units of k = (L/mol)^(order-1) / time)

- Make sure they record the data set number you give them.

- When students get to the Arrhenius equation, they are using the value of k they found (i.e., the slope of the correct order).
They'll get a linear graph out of this, the slope of which is -(Ea/R)
They can use that Ea value to plug back into the original equation to find A

- k and A have the same units.

Expt 10

Stephen and Alex's comments:

Lab went pretty smoothly this week. A couple things to watch out for:

- Balances will get extremely salty this week due to the freezing curve experiment, make sure to clean them carefully after lab.

Alex Adds: before you clean them, turn them off, and you can pull the balance plate off, then the cover. The cover tends to pick up salt and water you cannot see.

- Pay close attention during the freezing curve experiment and give students advice on how to improve their setup; otherwise some groups will end up doing the same thing over and over for 80 minutes.

Alex Adds: We usually don't let students run trials for more than three minutes. If they don't get it in three minutes, they *probably* won't get it. They should use a colder bath.

- Don't have students use the 600 mL beakers as baths...they can use something much smaller, such as the 250 mL.

Stephen Adds: Small test tubes work great too!

- For part IIB, the students should be using the low grade NaCl and not their previous salts. Also make sure that the salts are usually capped, as they tend to pick up water.

- Make sure the students are getting the connection between delta H and delta T, there's always quite a bit of confusion.

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