The iodine clock experiment, which consists of the following three reactions, is

Question

The iodine clock experiment, which consists of the following three reactions, is set up [S_2O_3^2-] [S_2O_8^2-]? Reaction 1: 3I^+(aq) + S_2O_8^2-(aq) rightarrow I_3^-(aq) + 2SO_4^2- (aq) slow Reaction 2: I_3^- (aq) + 2S_2O_3^2- (aq) rightarrow 3I^- (aq) + S_4O_6^2- (aq) fast Reaction 3: I_3^- (aq) + starch (aq) rightarrow 3I^- ---- starch (bluish black) fast A. No bluish block color would form because all the I formed in reaction 1 would be converted back into I_s: B. The bluish black color would be darker because more S_1O_2^2- is present. C. The bluish black color would be darker because more S_2O_2^2- is present. D. No bluish black color would form because all the I_2 formed in reaction 1 would be converted back into I^-.

Explanation / Answer

the answer is (D)

in this reaction I3- formed in first step reacts with the S2O32- to form I- .

if S2O32- is in lesser quantity than S2O82- , S2O32- will be consumed completely and the I3- then reacts with the starch to give bluish black color.

however if S2O32- is in greaer quantity than S2O82- , S2O82- will be consumed completely and  I3- formed in the reaction 1 would be converted back to I- .

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