The web is loaded with videos explaining optical activity and demonstrating polarimeters of various types (see Sorrell, Fig. 4.3, p. 114). Here are two SHORT clever videos from David Whyte (and his cheeky daughter?):
- Home made polarimeter. Modern computer displays rely on liquid crystals and the light emitted by an LCD is polarized. David displays a yellow rectangle on his laptop screen so the yellow light emitted by the screen is a good approximation of polarized monochromatic (single wavelength) light. Next he loads some syrup (sucrose + water) into a transparent can on top of the screen. Finally, he films the screen+syrup setup through some polarized sunglasses and rotates the glasses back and forth. Things to notice:
- When he films the screen+syrup setup without the glasses in place, everything looks yellow and equally bright. You can’t see any effect of sucrose on the angle of polarization because your eyes are not sensitive to polarization angles.
- The first time he puts the sunglasses between the camera and the setup (0:31), the syrup goes black while the screen stays bright. Clearly, the syrup is rotating the plane of polarization, i.e., the light coming out of the syrup has a different angle from the rest of the screen. The syrup looks black because the light coming out of the syrup is rotated 90º from the sunglasses’ polarization.
- Three seconds later (0:34) he has rotated the sunglasses so that the screen goes black and the syrup is light. Again, this proves that the syrup is rotating the plane of the polarized light coming out of the screen. If the syrup were not optically active, the syrup and the screen would all go black simultaneously.
- Home made polarimeter, #2 by David Whyte. The previous video showed that syrup (sucrose) rotates plane-polarized light. This video shows that the angle is correlated with the path length of the solution. The same basic setup is used: computer screen (light source + polarizer #1) + sample + sunglasses (polarizer #2), but there are four samples: W (pure water), 1 (short layer of syrup), 2 (medium layer of syrup), 3 (tall layer of syrup). Also, in this video he holds the sunglasses steady and rotates the screen. Things to notice:
- Pure water, W, is achiral and optically inactive. Notice that the color and brightness of W matches that of the surrounding screen. Water has no effect on the angle of polarization.
- Syrup is optically active, but the amount of optical activity (the amount the sucrose sample rotates the plane of polarization) depends on the amount of sucrose in the light beam. The longest (tallest) sample, 3, deviates the most from pure water, i.e., it has the greatest effect on the plane of polarization. The shortest sample, 1, behaves almost identically to pure water. 2, as you might expect, falls in between.
- This video looks at a full 360º rotation of the screen polarization relative to the sunglasses. The screen should go dark twice during a full rotation: once when the light hitting the sunglasses is polarized 90º from the sunglasses’ polarization, and again when the light is polarized 270º from the sunglasses’ polarization. See if you can spot these two “go black” spots for each sample. (Hint: use the arrow that Whyte has drawn on the screen to read the angle; you should see a sample go black a second time when the arrow has reversed its direction from the first time the sample went black).
69 students took Quiz #1 in-class last week and 59 students turned in take-home quizzes. Here are some statistics on how the class performed:
- 40-50 points. 13 students scored in this range on the in-class quiz. After considering take-home results, this number rose to 23 students.
- 30-39.5 points. 33 students scored in this range on the in-class quiz. After considering the take-home results, this number fell to 31 students (more students rose out of this group via the take-home than entered from below).
- 20-29.5 points. 18 students scored in this range on the in-class quiz. After considering the take-home results, this number fell to just 12 students.
- 10-19.5 points. 5 students scored in this range on the in-class quiz. After considering the take-home results, this number fell to just 3 students.
As you can see, the take-home quiz appears to have given many students a boost (in fact, only 16 of 59 scored noticeably worse on the take-home quiz; see below). This data should strongly encourage anyone who wants to improve their understanding of the material (as well as their scores) to attempt the take-home quizzes.
I also took a closer look at how students performed on the take-home and how this affected student scores. Continue reading
As you prepare for our quiz later this week, keep two things in mind:
- There are lots of resources to help you: sample quiz (with answers) from 2013, practice problems online and in your book, online supplements that I have written, me, helpers in the DoJo.
- In order to benefit from these resources, you have to allow time for them to operate. Very few challenges can be turned around instantaneously. The “computer” that you call your brain is based on living, growing cells and the chemical reactions they undergo. You are not silicon-based.
For some quick inspiration, check out Advice from 2013’s Chem 201 class (part 1). Also, make sure to acquaint yourself with the guidelines for taking in-class and take-home quizzes on our Exams page.
I received this message from Kristin Bott in CUS. She’s been gathering data about your clickers and she NEEDS YOUR HELP. Read on,
Good afternoon, Chem 201!
A high-five to the 30 of you who have currently registered your clickers. For the remaining near-40 of you, please do so soon! I need to do some data manipulation with that data before Alan can use clickers in class successfully.
To register your clicker, fill out this form.
Your clicker number is a six-digit code on the clicker (see form for examples), a mixture of numbers and letters OR just numbers.
That form requires you to use your Reed credentials. If you are having problems accessing the form,
- open a NEW browser (if you’re working in Chrome, open Firefox, or Safari, or …)
- login at weblogin.reed.edu
- try to access the form again
If that still doesn’t work, you can send me an email with:
- Your full name (First Last)
- Your Reed email handle (everything before @reed.edu)
- Your lecture section
- Your clicker number (six-character code on back of clicker)
If you have any questions or significant confusions, let me know — email@example.com.
Welcome back to Reed.
The attached file contains information about the first week of Chem 201. Lab (p. 1) and lecture (p. 2). Please read this file right away.
Note: lab will meet beginning tomorrow, Tu, Sept 2, but lecture will not.
201 is a complicated course that includes multiple sections, books, clickers, online homework, safety and chemical disposal training, and a whole lot more. This email just scratches the surface of what you need to know and look at regarding the lab. If you would like to learn more about Chem 201, visit our course and lab manual web pages:
http://blogs.reed.edu/chem201202/ (right here)
PS the course and lab manual web pages are still being modified (a lot) so bear with me for awhile
For years I have issued this advice to Chem 201 students before each exam, “Don’t stay up too late studying,” I say. “It doesn’t help. You need to get good sleep in order to make good long-term memories.” I have repeated this message over and over again not because I’m a “Mommy” who hates to see exhausted students working on o-chem exams (although I do hate to see this), but because my dim appreciation of sleep research has been that the science supports my advice.
So I was pretty excited when I saw an article in the 6 June 2014 issue of Science magazine that helps explain the relationship between sleep and memory. “Memories – getting wired during sleep” (p. 1087) reports how researchers have found a way (p. 1173) to observe changes in nerve fibers (in mice) that occur when the mice learn a new task. Specifically, the researchers examined the number and location of “dendritic spines.” These spines are structures that form in specific locations on a nerve fiber as a mouse improves its ability to perform particular tasks. Sleep promotes the formation of spines (and also improved task performance) while sleep deprivation prevented spines from forming (and also reduced task performance).
Neurophysiology is not my field, but these results are really exciting. Imagine being able to see structural changes in individual nerves that are associated with learning! And also discovering that sleep is essential for these changes to occur. Very, very cool.
And, of course, that advice of mine: if you really want a good score on the next exam, study. Then get to sleep. Good sleep.
An NMR spectrum provides important data on what’s in the NMR sample. Students are taught to look for certain contaminants – TMS, CHCl3, H2O – as a matter of routine. As many know, however, removing the last traces of chromatography solvent, unreacted starting material, and so on, can be a difficult chore so some Japanese science students have apparently hit on an elegant solution: using the ‘Delete Peak’ option in the JEOL NMR software to remove unwanted peaks. Needless to say, the editors of the journals that had published these spectra, and the students’ research mentors (once they were informed about the spectra) were not amused (“Cleaning up the record,” C&ENews, April 21, 2014, p. 32-3).
‘Silent’ alteration of data (by ‘silent’ I mean that the alteration was not reported to a referee or a reader), even if it involves removing ‘unimportant’ peaks, or peaks due to impurities, is nothing less than scientific fraud. If you have questions about how to process your data, or what data and data processing procedures to report, always bring your questions to your research supervisor, the journal editor, and colleagues in the field. Scientists, and more often, science students, tend to keep such questions to themselves because they think they will lose their colleagues’ respect by asking a “stupid question.” In fact, the sure-fire way to lose this respect is to engage in fraudulent practices.
Some of you may be planning a review of Chem 201 concepts before you take up the new challenges of Chem 202. Here are two short videos that review atomic structure and chemical bonding (note: this material is being taught by two very smart dogs).
Two weeks ago I offered my perspectives on exam scores (Some data …, Dec 1, 2013). Since then another exam has been completed and I can provide you with the latest results:
- Exam 1 – 44 (80-100%), 17 (60-80%), 3 (below 60%), median = 42
- Exam 2 – 40 (80-100%), 17 (60-80%), 7 (below 60%), median = 42
- Exam 3 – 9 (80-100%), 25 (60-80%), 30 (below 60%), median = 30
- Exam 4 – 14 (80-100%), 20 (60-80%), 30 (below 60%), median = 31
- Exam 5 – 32 (80-100%), 22 (60-80%), 11 (below 60%), median = 39.5
These figures may differ a bit from data provided earlier in the semester because the current figures are based on the 65 students who are still enrolled in the class (we started with 71 in lecture).
As you can see, exam scores have followed the historical pattern: a slight drop from #1 to #2, a large drop from #2 to #3, and increases from #3 to #4 to #5. Of course, these statements apply only to the class as a whole; individual scores may have followed a very different pattern. Nevertheless, this pattern supports two claims that I made at the start of the semester:
- organic chemistry “catches most students by surprise” at some point, and
- nearly everyone eventually figures out a successful strategy for learning with this material
Dec 31, 2013 Update. 20 students sent me their votes for the top Improvement and top Insight reflections.
Here are the results for the top Improvement:
- familiarize ourselves with the material before class – 5 votes
- don’t skip the more difficult problems and study together outside of class – 3 votes
- set up checkpoints and rotate responsibility for bringing model kits to class – 5 votes
- establish friendly and strong communication in the group – 3 votes
- study and review together before exams – 3 votes
- stay on task and don’t “be talking about nonsense” – 1 vote
Here are the results for the top Insight (many of these were prefaced by, “the greatest surprise to me is”:
- actively engaging with the material helps me understand, but a lecture can also clarify key connections – 1 vote
- learn better by explaining things to others – 8 votes
- group work can be difficult because sometimes I am too shy to speak up and I don’t want to slow the others down – 5 votes
- genuine commitment to group work makes it more effective, everyone contributes in some form or other – 2 votes
- the ability to memorize seems to make the difference on exam and we need to find ways (e.g. mnemonic devices) that help this – 3 votes
- differences between groups can be surprisingly large – 1 vote
To read the original ground rules and the full reflections that people voted on, just keep reading.