Question

Consider the following reaction in acidic aqueous medium:
(H2O2) +(2H+) +(I−) > (I2)+ (2H2O)
The following mechanism was suggested for the above reaction.
(H+) +(I−) > HI Fast
(HI) +(H2O2) > (H2O) +(HOI) Slow
(HOI) +(I−) > (I2) +(OH−) Fast
(OH−) + (H+) > (H2O) Fast
Show that the overall order is two.

Answer 1

The rate of the reaction is determined by the slowest step.

Therefore the rate law can be written for the slowest molecular reaction as:

Rate= K[HI][H₂O₂]

Therefore, the overall order is the sum of the orders with respect to the two rate determining species which are HI and H₂O₂

Overall order= 1+1

= 2