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?