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 involving two different reactants can never be a unimolecular reaction.

Step 1: Understand the terminology

In chemical kinetics, a unimolecular reaction involves only one reactant molecule, typically undergoing decomposition or rearrangement. A bimolecular reaction involves two reactant molecules coming together. The reaction order is determined by the sum of the exponents of the concentration terms in the rate law.

Step 2: Analyze unimolecular reaction

A unimolecular reaction only involves a single reactant molecule transforming into products. Therefore, it cannot involve two different reactants by definition.

Step 3: Evaluate first and second order reactions

A first-order reaction can have a rate law with a sum of exponents equal to one. It can involve two reactants if the concentration of only one is in the rate law. A second-order reaction can involve two reactants whose combined concentration terms add up to two.

Step 4: Assess bimolecular reaction

A bimolecular reaction involves two distinct reactant molecules. As such, it inherently requires the presence of two different reactants.

Step 5: Conclude based on definitions

Since a unimolecular reaction never involves two different reactants, it cannot be the correct description for a reaction involving two different reactants.

Key Concepts

Unimolecular Reaction

In the world of chemical kinetics, a unimolecular reaction is an intriguing concept. This type of reaction involves just a single reactant molecule. It typically undergoes a process like decomposition or a rearrangement to form products.
Unlike reactions requiring two different substances, unimolecular reactions are limited to one single entity beginning the transformation.
An example could be the isomerization of cyclopropane to propene. Here, a single molecule changes form, demonstrating the unimolecular nature.

• Involves only one molecular entity.
• Generally seen in decomposition or isomerization processes.
• Frequently affected by changes in molecular structure.

Understanding this fundamental idea helps clarify why a unimolecular reaction can't involve two different reactants, simply because its very nature restricts it to a single molecule.

Bimolecular Reaction

Bimolecular reactions are another category frequently encountered in chemical kinetics. In these reactions, two distinct molecules come together to react.
This could involve simple elements or more complex molecules interacting.
When the two molecules collide, they form products as a result of the chemical reaction. For instance, when hydrogen molecules react with iodine molecules to form hydrogen iodide, it exemplifies a bimolecular reaction.

• Utilizes two reactant molecules which could be the same or different.
• Forms new products upon interaction of the two reactants.
• Generally faster compared to unimolecular due to reactant interaction.

Because the definition insists on two reactants, bimolecular reactions are easily distinguished within chemical studies. It's clear why reactions needing two initial substances cannot be unimolecular under these guidelines.

Reaction Order

Understanding reaction order is key to grasping chemical kinetics. Reaction order is connected to the rate law of a chemical reaction, which is an equation displaying the relation between reaction rate and reactant concentrations.
It is determined by adding the exponents of the concentration terms in the rate law.
For instance, if the rate law is expressed as \( ext{Rate} = k[A][B] \), the reaction order would be two, due to the exponents on both \([A]\) and \([B]\) being one each.

• First-order reactions have a total order of one.
• Second-order reactions, like the example above, total to two.
• This order can vary, indicating the complexity of the reaction.

Through understanding these orders, it becomes clearer why a bimolecular reaction, involving two reactants, could align with first or second-order categories based on the concentration's role in the rate law.