Question

Write the chemical equations involved in this experiment and show that the rate of disappearance of [S₂O₈^2-] is proportional to the rate of appearance of the blue-black color of the starch-iodine complex.

chemical reactions: S₂O₈^2- (aq) + 2 I^- (aq) --------> I₂ (aq) + 2SO₄^2- (aq) &

I₂ (aq) +2S₂O₃^2- (aq) -------->2I^- (aq) + S₄O₆^2- (aq)

Answer 1

2 I^- (aq) + S₂O₈^2- (aq) -----> I₂(aq) + 2 SO4 ^2- (aq)

This reaction is referred to as a Clock Reaction because the progress of the above reaction, monitored by a secondary Clocking reaction which consumes the product I₂ as it is produced and triggers a color change when the Clocking Reagent is itself completely consumed. For this purpose, Thiosulfate (S₂O₃^2- ) as the Clocking Reagent to reduce the I₂:

2 S₂O₃^2- (aq) + I₂(aq) ------> S₄O₆^2- (aq) + 2 I^- (aq)

As soon as the Thiosulfate is used up, Iodine will begin to appear in the solution. The presence of Iodine is then dramatically detected by the formation of a blue Starch-Iodine molecular complex and the reaction is said to “Clock” at this point. Actually, the starch molecules complex with I^{3 -} which is formed by a reaction between I₂ and I ^- .