Question

For a reaction
\[\ce{2H2O2->[I^-][Alkaline medium]2H2O + O2}\]

The Proposed mechanism is given below: 
(1) H₂O₂+I^- → H₂O+IO^-(slow)
(2) H₂O₂+IO^-→H₂O+I^-+O₂(fast)

(i) Write the rate law for the reaction.
(ii) Write the overall order of a reaction.
(iii) Out of steps(1) and (2), which one is the rate-determining step? 

Answer 1

Let us consider the problem:  

\[\ce{2H2O2->[I^-][Alkaline medium]2H2O + O2}\]
In Some cases, the rate of reaction depends not only on the reactant but may also depend on the substance present as a catalyst. This is also seen in the above reaction. The rate of reaction depends on the slowest step in case of a complex reaction.

(1) \[\ce{H2O + I^- ->[slow]H2O + IO^- }\]

(2) \[\ce{H2O2 + IO^- ->[fast]H2O + I^- + O2}\]

(i) Rate law: 

Rate =`-1/2("d"["H"_2"O"_2])/"dt"="k"["H"_2"O"_2]["I"^-]`

(ii) The overall order of reaction: 

Rate = k[[H₂O₂] `["I"^-]`

Order = 1 + 1 = 2

(iii) Slowest step is the rate determining step. Hence, step 1 is rate determining step.