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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.