Theory

This reaction is an example of a class of reactions called Belousov-Zhabotinsky (BZ) reactions. The overall reaction is usually given as:

3CH(COH)(aq) + 4BrO(aq) 4Br(aq) + 9CO(g)
+ 6HO(l)

Oscillation is brought about by two autocatalytic steps. Bromine is an intermediate in the reaction scheme - the red colour that is observed. An analogy with predator-prey relationships might be one way to give students some idea of what is going on. For example a population of rabbits (analogous to the bromine) will increase rapidly (exponentially) if there is plenty of food (reactants). However, the plentiful supply of rabbits will stimulate a rapid increase in the fox population (another intermediate that reacts with bromine) which will then deplete the rabbits. Lacking rabbits, the foxes will die, bringing us back to square one, ready for a rapid increase in rabbits and so on.

Extensions

The reaction can be investigated using a colorimeter with a chart recorder or interfaced to a computer.

Further details

The reaction will not work if tap water (Coventry) is used instead of deionised water. Chloride ions, via the addition of a pinch of potassium chloride or dilute hydrochloric acid will immediately stop the oscillations. Clean apparatus is therefore essential.


Safety

Wear eye protection.
The reaction mixture can be washed down the sink with plenty of water after the demonstration.