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Jan 29

Chemistry Lab Demonstrations: Check-In Day

by azmanam | Categories: fun | (13730 Views)

*For more cool stories, pictures, and videos of chemistry demonstrations, click here*

I’m TA’ing an undergrad organic lab this semester, and in order to keep myself and the students entertained, I’m trying to have some kind of demonstration at the beginning of each lab.  The goal is to have the demonstration have something to do with the theme of the lab.  The first lab of the semester was this week.  On the first day, all the students do is check in their glassware.  The lab doesn’t acutally have anything to do with organic chemistry.  So that means the demonstration doesn’t have to have anything to with organic chemistry, right?

This week, I did my favorite demonstration, the oscillating clock reaction.  Formally known as the Briggs-Rauscher reaction, mixing three colorless solutions together creates a solution which oscillates between blue, amber, and colorless for several minutes.

The explanation for what’s happening is actually really complex.  I’ll quote from the University of Leeds website:

In the Briggs-Rauscher reaction reaction the evolution of oxygen and carbon dioxide gases and the concentrations of iodine and iodide ions oscillate.  The somewhat simplified mechanism of this reaction can be represented by the following overall transformation:

IO3- + 2 H2O2 + CH2(COOH)2 + H+==> ICH(COOH)2 + 2 O2 + 3 H2O           (11.1)

This transformation is accomplished through two component reactions:

IO3- + 2 H2O2 + H+==> HIO + 2 O2 + 2 H2O                              (11.2)

HIO + CH2(COOH)2==> ICH(COOH)2 + H2O                            (11.3)

The first of these two reactions can occur via two different processes, a radical process and a nonradical process.  Which of these two processes dominates is determined by the concentration of iodide ions in the solution.  When [I-] is low, the radical process dominates; when [I-] is high, the nonradical process is the dominant one.  The second reaction (eq. (11.3)) couples the two processes.  The reaction consumes HIO more slowly than that species is produced by the radical process when that process is dominant, but it consumes HIO more rapidly than it is produced by the nonradical process.  Any HIO which does not react by eq. (11.3) is reduced to I- by hydrogen peroxide as one of the component steps of the nonradical process for reaction (11.2).  When HIO is produced rapidly by the radical process, the excess forms the iodide ions, which shut off that radical process and start the slower nonradical process.  Reaction (11.3) then consumes the HIO so rapidly that not enough is available to produce the iodide ion necessary to keep the nonradical process going, and the radical process starts again.  Each of the processes of reaction (11.2) produces conditions favourable to the other process, and, therefore, the reaction oscillates between these two processes.

You can read more about the reaction and learn how to do it yourself from about.com.

Here’s a video of the oscillating clock reaction.  No, this is not me from lab the other day.

6 comments

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  1. Noel

    I was a demo girl too! High five!

    The problem with Brigg-Rauscher (or as we call it, the iodine clock reaction) is that the reagents get old so quickly. We usually stock a few liters of it and it’ll go bad in 2-3 semesters. Otherwise, it’s pretty cool. It is especially cool at Cal because our school color is blue & gold. We do this during our rivalry week.

    My other favorite demos are the silver nitrate and copper wire reaction, throwing-alkali-metal-in-water reaction, and the obvious thermite (this one gets the freshman every time). The best thing about being the demo girl is that everyone thinks you’re awesome for blowing stuff up. Hoorah to glory of the past.

    Here’s the silver nitrate reaction:
    http://www.cchem.berkeley.edu/demolab/images/n1221772_30483517_9411.jpg

    1. azmanam

      I just made the solutions the night before, so I didn’t have to worry about them going bad.

      We did a magic show for middle-schoolers a while back. We did the metal-in-water and thermite demos. We also dunked things in liquid nitrogen, mentos/diet coke, and blew up a dry ice bomb.

      One of the problems I’m having with preparing the demos for the rest of the semester is trying to have them relate to the lab in some way. I think I’m ok for the next couple of weeks, but when we get to the SN2 lab, I don’t have a good idea for a demo that day. Maybe it just won’t have to relate. Unless anyone has a good idea…

    2. Enahs

      Silver nitrate and wire, hah.

      An old lab was getting rid of a bunch of chemicals. I got a 50 year old container that was 2kg of silver nitrate crystals. I also got a huge thing of copper powder!

      Made a saturated solution of silver nitrate, threw it in the sonicator and dumped in copper powder. Instantly raining silver metal. No seriously, I collected 1.2kg of silver powder this way!

  2. Fleaker

    You might consider looking into some of the demo ideas that Flinn Scientific puts out. As far as I know, most of them are tested for safety and hopefully for working out properly. It’s obvious that any demo one is planning on doing must be prepared and checked in advance so you don’t have a dud.

    Here’s the Flinn link:

    http://www.flinnsci.com/Sections/Chemistry/chemistry.asp

    There are far more in the catalog, most of them treating with subjects like kinetics, thermodynamics, organic chemistry, etc.

    Also, a tour through the Journal of Chem. Education would probably be helpful.

    azmanam, since you’re TA’ing an organic chemistry lab, make sure you show them Tollen’s reaction. I’ve heard that a copper mirror can be done as well. I’ve only done Pt, Pd, Rh, and Ag mirrors. For foolproofness/economy, the silver mirror test is the best.

  3. Jenny

    can i vote for a tollen’s reaction……?

    1. azmanam

      Thanks for the comments. I don’t have anything planned for the Wittig lab, yet. We use p-anisaldehyde, which should give a positive result to the Tollen’s reagent, so maybe we’ll do it that week. See you in lab.

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