Problem 210 - A Complete Resource Book in Chemistry for JEE Main

Question

A reaction involving two different reactants can never be? (a) Unimoleculur reaction (b) Ist order reaction (c) IInd order reaction (d) Bimoleculur reaction

Step-by-Step Solution

Answer

A reaction with two different reactants can never be a unimolecular reaction.

Step 1: Understanding the Problem Statement

The question requires identifying which type of reaction is impossible when there are two different reactants involved. We need to understand the definitions of the reaction types listed: unimolecular, first-order, second-order, and bimolecular.

Step 2: Defining the Reaction Types

A unimolecular reaction involves only one reactant molecule decomposing or changing. A first-order reaction depends on the concentration of a single reactant. A second-order reaction involves the concentration of either two molecules of the same reactant or one molecule each of two different reactants. A bimolecular reaction requires the collision of two reactant molecules.

Step 3: Identifying the Impossible Reaction Type

Since the reaction involves two different reactants, a unimolecular reaction is impossible because it involves only one reactant molecule. Both a bimolecular and a second-order reaction can naturally accommodate two different reactants.

Key Concepts

Unimolecular Reaction

In a unimolecular reaction, only one reactant molecule is involved. This type of reaction occurs when a single molecule undergoes a transformation by itself, such as breaking down into smaller molecules or rearranging its structure.
A good example of this is the decomposition of nitrogen dioxide \((NO_2)\), where \(NO_2\) breaks down into \(NO\) and \(O\), all from the same molecule.
Unimolecular reactions are characterized by:

• Involvement of a single reactant molecule.
• No requirement for collisions with other molecules to proceed.

In the context of the original exercise, a unimolecular reaction is impossible when two different reactants are involved, as it fundamentally requires only one reactant molecule.

Bimolecular Reaction

Bimolecular reactions involve the interaction between two reactant molecules. This can happen when these two molecules collide and undergo a reaction together.
These reactions are common in chemical kinetics and are essential in processes involving complex reactions.

• Two reactant molecules collide.
• The reaction typically depends on the concentration of both reactants.
• They are quite common in bio-chemical processes.

The concept of a bimolecular reaction fits perfectly when the original exercise mentions two different reactants. This is precisely the scenario where bimolecular reactions naturally occur.

Reaction Order

The reaction order is a fundamental concept in chemical kinetics that helps to describe how the rate of a chemical reaction depends on the concentration of the reactants.
It's essentially the power to which the concentration of a reactant is raised in the rate law equation.
The reaction order can be:

• Zero Order: Rate is independent of the concentration of the reactant.
• First Order: Rate depends linearly on the concentration of one reactant.
• Second Order: Rate depends on either one reactant squared or two reactants each raised to the power of 1.

Understanding reaction order helps in predicting how changes in concentration affect the speed of a reaction.

First Order Reaction

A first order reaction is a type of chemical reaction where the rate is directly proportional to the concentration of only one reactant.
This means that if you double the concentration of the reactant, the reaction rate will also double.
Key characteristics include:

• The rate constant \(k\) has units of \(s^{-1}\).
• It typically involves reversible reactions or single reactant processes.

In the original exercise context, a first order reaction can be possible with two different reactants, depending on the specific circumstances and pressure or catalyst, though commonly it involves one reactant.

Second Order Reaction

A second order reaction involves a rate that depends on the concentration of one reactant squared or the concentration of two different reactants.
This means that the reaction rate is proportional to the product of the concentrations of both reactants.
Characteristics include:

• The rate constant \(k\) has units of \(M^{-1} \, s^{-1}\).
• Involves either two of the same kind of molecule or completely different reactants reacting with each other.

The original exercise identifies second order reactions as possible when two different reactants are involved, aligning with typical second-order reaction definitions.