The
question can be answered if we go through the mechanism of reaction,
for example, chances to win the relay race competition by a team
depend upon the slowest person in the team Similarly, the overall rate
of the reaction is controlled by the slowest step in a reaction called the
rate determining step 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–