The order of a reaction with respect to a reactant with respect to a reactant is the power of concentration term of that reactant to which the rate of reaction is directly proportional. The overall order of a reaction is the sum of powers or exponents to which the concentration terms are raised in the rate law expression.

Molecularity is defined as the number of reactant species which collide simultaneously to form the products. It is always a positive integer like 1,2,3.. Higher molecularity (>3) are rare because chances of collisions of more than three molecules at a time are very less. Molecularity is defined only for elementary reactions. There is no meaning of molecularity for complex reactions. 

What is the order of the following chemical reaction?

2H₂O₂ → 2H₂O + O₂ 

The proposed mechanism is:

H₂O₂ → H₂O + [O]       (slow)

[O] + [O] → O₂             (fast)

First order

The correct answer is option 2. First order

The given chemical reaction is the decomposition of hydrogen peroxide (\(H_2O_2\)) into water (\(H_2O\)) and oxygen gas (\(O_2\)). The proposed mechanism consists of two elementary steps:

1. The decomposition of hydrogen peroxide into water and an oxygen atom (\([O]\)). This step is proposed to be the slow, rate-determining step.
\[ \text{Step 1: } H_2O_2 \rightarrow H_2O + [O] \]

2. The combination of two oxygen atoms (\([O]\)) to form a molecule of oxygen gas (\(O_2\)). This step is proposed to be a fast step.
\[ \text{Step 2: } [O] + [O] \rightarrow O_2 \]

The overall reaction is obtained by adding the two elementary steps together:

\[ 2H_2O_2 \rightarrow 2H_2O + O_2 \]

Now, let's analyze the proposed mechanism to determine the rate-determining step and the overall order of the reaction:

The rate-determining step is the slowest step in the reaction mechanism, which determines the overall rate of the reaction.

The rate law for a reaction is determined by the slow step, so the overall reaction order is equal to the order of the slow step.

In this case, the slow step is the first step:

\[ \text{Step 1: } H_2O_2 \rightarrow H_2O + [O] \]

Since the slow step involves the decomposition of one molecule of \(H_2O_2\) to form one molecule of \(H_2O\) and one oxygen atom \([O]\), the rate law for this step is first-order with respect to \(H_2O_2\).

Therefore, the overall order of the reaction is first-order with respect to hydrogen peroxide (\(H_2O_2\)).