Question

Consider the decomposition of hydrogen peroxide (\( \mathrm{H_2O_2} \)) as per the equation given below :

\[ 2\mathrm{H_2O_2} \xrightarrow[\text{Alkaline medium}]{\mathrm{I^-}} 2\mathrm{H_2O} + \mathrm{O_2} \]

The mechanism for this reaction was found to be :

Step I : \( \mathrm{H_2O_2 + I^-} \rightarrow \mathrm{H_2O + IO^-} \) (slow)

Step II : \( \mathrm{H_2O_2 + IO^-} \rightarrow \mathrm{H_2O + I^- + O_2} \) (fast)

What is the molecularity of the reaction in Step II ?

Hint:

Molecularity is the number of species colliding in one elementary step. Step II has H2O2 and IO-, so it is 2.

Answer 1

Step 1: Recall the meaning of molecularity:
Molecularity is the number of reacting species (atoms, ions or molecules) that collide in one elementary step to give products.
It is a whole number and applies only to a single elementary step, not to the overall reaction.

Step 2: Count the reactants in Step II:
Step II is \(\mathrm{H_2O_2 + IO^-} \rightarrow \mathrm{H_2O + I^- + O_2}\).
The reacting species on the left side are one molecule of \(\mathrm{H_2O_2}\) and one ion of \(\mathrm{IO^-}\).
That makes two species colliding together.

Step 3: Name the molecularity:
A step with two reacting species is called bimolecular.
Step I has the same count, so it is bimolecular too.

Final Answer:
The molecularity of Step II is 2, so the step is bimolecular.
\[ \boxed{\text{Molecularity} = 2} \]