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Chemical Kinetics - Molecularity of a Reaction

Grade 12CBSEChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Molecularity is defined as the number of reacting species (atoms, ions, or molecules) taking part in an elementary reaction, which must collide simultaneously in order to bring about a chemical reaction.

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It is a theoretical concept and is derived from the mechanism of the reaction, specifically for elementary steps.

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Molecularity can only be a positive integer (1,2,31, 2, 3). It cannot be zero, negative, or a fraction.

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Reactions are classified based on molecularity: Unimolecular (Molecularity =1= 1), Bimolecular (Molecularity =2= 2), and Trimolecular or Termolecular (Molecularity =3= 3).

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Molecularity greater than 33 is very rare because the probability of more than three molecules colliding simultaneously and in the correct orientation is extremely low.

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For complex reactions (reactions occurring in multiple steps), molecularity has no meaning for the overall reaction. Instead, we speak of the molecularity of each individual elementary step.

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Difference from Order: While Order of reaction is an experimental quantity and can be 00 or fractional, Molecularity is a theoretical value and must be a whole number.

📐Formulae

For an elementary reaction: aA+bB→Products\text{For an elementary reaction: } aA + bB \rightarrow \text{Products}

Molecularity=a+b\text{Molecularity} = a + b

Rate=k[A]a[B]b (Only for elementary reactions, where Order = Molecularity)\text{Rate} = k[A]^a[B]^b \text{ (Only for elementary reactions, where Order = Molecularity)}

💡Examples

Problem 1:

Identify the molecularity of the following elementary reaction: NH4NO2→N2+2H2ONH_4NO_2 \rightarrow N_2 + 2H_2O

Solution:

Molecularity =1= 1

Explanation:

This is a decomposition reaction involving only one reacting species (NH4NO2NH_4NO_2). Therefore, it is a unimolecular reaction.

Problem 2:

Consider the elementary reaction: 2HI→H2+I22HI \rightarrow H_2 + I_2. What is its molecularity?

Solution:

Molecularity =2= 2

Explanation:

In this reaction, two molecules of HIHI must collide to form the products. Hence, the molecularity is 22, making it a bimolecular reaction.

Problem 3:

A complex reaction occurs in three steps. Step 1 (Slow): A+B→CA + B \rightarrow C. Step 2 (Fast): C+B→DC + B \rightarrow D. What is the molecularity of the rate-determining step?

Solution:

Molecularity =2= 2

Explanation:

The slowest step is the rate-determining step. In Step 1, two species (AA and BB) react. Thus, the molecularity of the rate-determining step is 22.