The Briggs-Rauscher Reaction An Oscillating Chemical Reaction

What is an Oscillating Chemical Reaction? An oscillating chemical reaction is a rare process that produces changes in color which repeatedly turn on and off over many cycles. These reactions are very complex and involve several chemical reactions occurring at the same time. Thomas Briggs and Warren Rauscher, two teachers at Galileo High School in San Francisco discovered one of the most dramatic examples of an oscillating chemical reaction in 1973 and it now bears their name.

Reactants for Briggs-Rauscher Reaction Potassium Iodate - KIO3 Hydrogen Peroxide - H2O2 Manganese Sulfate - MnSO4 Malonic Acid - HO2C-CH2-CO2H Sulfuric Acid - H2SO4 Starch Indicator + Water

How it works – Mechanism of Reaction Research into the B-R reaction by other scientists in 1982, led to the discovery that some 11 steps are involved! The simple explanation is as follows: The iodate ion is changed into iodine by hydrogen peroxide. The color changes to amber:

How it works – Mechanism of Reaction The free iodine reacts with malonic acid to produce iodide ions.

How it works – Mechanism of Reaction The free iodine combines with iodide very rapidly to form the negative ion I3 , which reacts with starch to form a dark blue complex:

The Blue Starch-Iodine/Iodide Complex

How it works – Mechanism of Reaction 4. After a period of time, the I3 ions are converted back into iodine and iodide ions, so the dark blue color disappears and the process repeats itself:

How it works – Mechanism of Reaction 5. Eventually the faster step 3 becomes dominant and the change of I3 back to iodine/iodide stops after about 15 cycles, so the solution remains dark blue. The overall chemical reaction is: