Consider the decomposition of hydrogen
peroxide which is catalysed by iodide ion in an alkaline medium 2H2O2 
-I
Alkaline medium
  2H2O + O2
The rate equation for this reaction is found to be



 
2
2
2
2
d H O
Rate
H O
I
d


 k
t
This reaction is first order with respect to both H2O2 and I– Evidences
suggest that this reaction takes place in two steps
(1) H2O2 + I– ® H2O + IO–
(2) H2O2 + IO– ® H2O + I– + O2
Both the steps are bimolecular elementary reactions Species IO- is
called as an intermediate since it is formed during the course of the
reaction but not in the overall balanced equation