1 class periods of 90 minutes each
Students will follow a set of directions that lead to the "oscillating clock" reaction. They will need to do careful molarity calculations and measurements for it to work correctly.
The Oscillating Clock demonstration, based on the Briggs-Rauscher (BR) reaction, provides a visually impressive reaction in which a solution oscillates in color between amber-orange and blue-black. The solution will oscillate approximately 10-15 times, with the time between color changes increasing as the reaction proceeds. After about 15 minutes, the oscillations will stop and the solution will remain blue-black in color.
The BR reaction was developed as a hybrid of two other oscillating reactions, the Bray-Liebhafsky (BL) reaction and the Belousov-Zhabotinsky (BZ) reaction. In the BL reaction, H2O2 was used as both an oxidizing and reducing agent to cause oscillations between I2 and IO3-. In the BZ oscillating reaction, organic compounds such as malonic acid were used along with electron transfer agents such as Mn(II) ions. The BR reaction utilized the H2O2 and IO3- from the BL reaction and the malonic acid and Mn(II) ions from the BZ reaction to develop a reaction that caused the oscillation of the evolution of oxygen and carbon dioxide gases and the concentration of iodine and iodide ions, which accounts for the observed color changes.
The mechanism for the BR reaction has been studied extensively, and the origin of the [I2] and [I-] oscillations has been explained. However, the proposed mechanism is unable to account for the production of CO2 gas and does not identify the final organic products of the reaction. This reaction, therefore, is a prime example of the ongoing exploration by chemists of phenomena that we can visibly observe, but not yet explain.
Lesson Design by Jordan School District Teachers and Staff.