In the iodine clock reaction system, how does the initial concentration of iodate ions affect the rate and/or the sequence of the first two reactions?

Answer 1

A summary of the iodine clock reactions might be written as follows:

(1) IO^3- + 5I^- + 6H⁺ ==> 3I₂ + 3H₂O

(2) I₂ + 2S₂O₃^2- ==> 2I^- + S₄O₆

The I₂ that is generated in reaction (1) is used in an oxidation / reduction reaction with thiosulfate (or some other agent) to produce I^- as shown in reaction (2). When the thiosulphate ions S₂O₃^2-) are used up the free iodine (I₂) then reacts with some starch indicator that is present from the start. This reaction produces blue/black color.

The initial concentration of iodate (IO₃^-) affects the rate of this reaction because as the concentration increases, the reaction rate gets faster. Rate = k[IO₃^-]ⁿ.