Consider two mechanisms below: 2I - (aq) + S 2 O 8 2- (aq) I 2 (aq) + 2SO 2 2- (aq)(a) Rate law for (a) is Rate = k[I - ] [S 2 O 8 2- ] and similar...

Consider two mechanisms below:

2I- (aq) + S2O8 2- (aq) → I2 (aq) + 2SO22-(aq) (a)

Rate law for (a) is Rate = k[I-] [S2O82-] and similar with the experiment's rate law.

2I- (aq) + S2O8 2- (aq) → I+ (aq) + 2SO42- (aq)  (b) slow

I-              + I+ (aq) → I2 (aq)      fast

Overall reaction: 2I- (aq) + S2O8 2- (aq) → I2 (aq) + 2SO42- (aq)

And Rate = k[I-]2

Activation energy of the experiment is 72.59 kJ/mol which means the reaction is exothermic.

which mechanism should be the best fitted mechanism? why?