Clock reactions
Clock reactions are named after the analogy between chemical changes in such
reactions and an alarm clock. You cannot tell whether the alarm clock is working or
not until it goes off; it is only then that you know it has been working the whole time.
In a clock reaction, chemical change becomes visible (“the alarm clock goes off”)
only after the reaction has reached a certain extent. There are three steps in each clock
reaction. The first is a slow formation of a chemical intermediate. The second is a fast
consumption of the intermediate by the limiting reagent. The third step is responsible
for the colour change. However, that step takes place only after the limiting reagent
has been consumed.