Recently in Fall 2010 Category

Expt 16

Notes on Experiment 16:

- Go over the details of the well prep carefully. This includes not cutting to the bottom of the dish, not putting wells too close to each other, etc. Basically, make sure they know to take all precautions for keeping the solutions inside the wells and not all over the place.

- Mention that the red lead is, contrary to most of the electronics world, the ground. They should be using the black and green leads to take measurements.

- Use Raw Voltage 0-5 (not any of the other options) when you set up the voltmeter.

- Impress on students the importance of washing their hands if they don't want to be eating lead or cadmium with their lunch/dinner.

- I suggest to my students that they bend the metal strips so that one end is in the well, and the other is resting on the edge of the dish. This allows them to simply touch the leads to the metal strips, rather than painstakingly clipping and unclipping each one and/or removing and replacing strips from the wells.

- From Lindsay: having your students start the agar gel prep before prelab can also save time; this allows you to give your prelab lecture while the gels are cooling.

Expt 15

--They will have one colored (metal) solution next week. These are generally easy to identify, but we've had a problem in the past with KI being yellow. It is yellow again this week. If it is still yellow next week, the students should be able to identify the difference between KI and the chromium solution, which is also yellow.

--Remind the students that some of these solutions are quite strong (i.e., 6M HCl) and to keep their hands clean

--Students can cut down on their time substantially if they avoid solutions with common ions and make sure to follow their solubility rules (both on page 15-2).

Also, please make sure that the balance areas are kept clean. Although the balances themselves were clean when Stephen and I arrived this morning, the benchtops around them are crusted with acid and salt solutions that have dried out since last week. Thanks!

Expt 14

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Notes for this week:

1. Students should keep their NaOH and unknown acids for next week. Do not let them neutralize the leftover materials.

2. Impress on students the importance of not touching solid NaOH with their hands. If they do, they should wash their hands immediately.

3. Show the students what "bad" and "good" endpoint looks like when a student gets one.

4. Keep the balances clean of KHP and NaOH - these are particularly corrosive, and they will get dirty.

Expt 12

For this week's experiment, Stephen and I have only a few things to contribute:

1) The molar absorptivity value for this lab is 4296. Write this down for your students. (Also, b = 1 cm).

2) Make sure their spectrometers are set to 447 nm before they start taking data. It's easiest if you set it to Abs v. time, because they then can visually see the equilibrium.

3) for Part 2, tell them to only report the colors that they actually see. Also, somehow none of them ever seem to notice that the net ionic equations it asks for are on the preceding page.

That's about all we have for this week. Good luck!

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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