The order of a reaction 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 molecularity of the following elementary reaction?

2CO + O₂ → 2CO₂ 

Trimolecular

The correct answer is 3. trimolecular.

The molecularity of a reaction refers to the number of molecules or ions that participate as reactants in an elementary reaction. An elementary reaction is a single step reaction in a reaction mechanism. The given reaction is:

\[ 2\text{CO} + \text{O}_2 \rightarrow 2\text{CO}_2 \]

Let's analyze the number of molecules involved in this reaction:

- \(2\text{CO}\): Two molecules of carbon monoxide are participating in the reaction.
- \(\text{O}_2\): One molecule of oxygen is participating in the reaction.

Now, adding the number of molecules involved:

\[ \text{Total Molecularity} = \text{Number of molecules of CO} + \text{Number of molecules of O}_2 \]

\[ \text{Total Molecularity} = 2 + 1 = 3 \]

Therefore, the molecularity of the given elementary reaction is 3, making it a trimolecular reaction. Trimolecular reactions involve the collision of three molecules or ions as reactants in a single step. In this case, the collision of two molecules of carbon monoxide and one molecule of oxygen leads to the formation of two molecules of carbon dioxide.