Question

The one which is a unimolecular reaction is (a) \(2 \mathrm{HI} \rightarrow \mathrm{H}_{2}+\mathrm{I}_{2}\) (b) \(\mathrm{N}_{2} \mathrm{O}_{5} \rightarrow \mathrm{N}_{2} \mathrm{O}_{5}+(1 / 2) \mathrm{O}_{2}\) (c) \(\mathrm{H}_{2}+\mathrm{Cl}_{2} \rightarrow 2 \mathrm{HCl}\) (d) \(\mathrm{PCl}_{3}+\mathrm{Cl}_{2} \rightarrow \mathrm{PCl}_{5}\)

Short answer

None of the options represents a unimolecular reaction.

Step-by-step solution

Analyze the given reactions

In this step, we analyze each of the given reactions by looking at the reactant side and count the number of molecules involved in the reaction. \n(a) \(2 HI \rightarrow H_{2}+I_{2}\): In this reaction, 2 molecules of Hydrogen Iodide are reacting. So, it's not unimolecular.\n(b) \(N_{2}O_{5} \rightarrow N_{2}O_{5}+(1 / 2)O_{2}\): This is incorrect. Reaction has the same molecule on both sides which is a mistake. \n(c) \(H_{2}+Cl_{2} \rightarrow 2 HCl\): Here, Hydrogen and Chlorine molecules are involved, so it's not unimolecular either.\n(d) \(PCl_{3}+Cl_{2} \rightarrow PCl_{5}\): In this reaction, two molecules, i.e., Phosphorus Trichloride and Chlorine are reacting. Hence, it's not unimolecular.

Final analysis and short answer

On analyzing all the options, it is seen that none of the reactions given in the options are unimolecular reactions. Therefore, none of the reactions mentioned in the question is unimolecular.

Key concepts

Understanding Reaction Mechanisms

Reaction mechanisms offer important insights into the steps and processes that occur in chemical reactions. They describe the specific molecular events and changes in bonds that take place, leading to the formation of products from reactants. These mechanisms are not just a one-step or a simple conversion but involve a sequence of detailed elementary reactions.
A comprehensive reaction mechanism maps out how reactants transition into products through an energy pathway and sometimes intermediates. The mechanism is often proposed based on experimental observations and may include several individual steps:

• Initiation Step: Where the reaction begins, often involving bond disruption or formation of radicals or ions.
• Propagation Steps: Where the active intermediate goes through various steps, each contributing to product formation.
• Termination Steps: Where the active species are consumed, leading to the final product formation.

Reaction mechanisms are vital in understanding the kinetics of a reaction, as they affect how fast a reaction proceeds. By knowing the mechanism, scientists can predict how changing conditions such as temperature or concentration can affect the reaction rate or direction.

The Basics of Chemical Reactions

Chemical reactions are processes where substances, known as reactants, are transformed into different substances, called products. These transformations involve breaking and forming of chemical bonds, resulting in changes in the arrangement of atoms.
Key points about chemical reactions include:

• Chemical reactions are usually accompanied by energy changes, typically in the form of heat, light, or work.
• They are governed by the laws of conservation of mass and energy, meaning the mass of the reactants equals the mass of the products.
• Reactions need a certain amount of energy to proceed, called the activation energy. This is the energy barrier that must be overcome for reactants to transform into products.

Chemical reactions are central to a wide range of scientific and practical applications, from industrial processes to the functioning of living organisms. Understanding them involves looking at different types of reactions, rates, and equilibrium. There are various classifications such as synthesis, decomposition, single replacement, and double replacement reactions, each with its own distinct patterns and energy changes.

Understanding Molecularity in Reactions

Molecularity refers to the number of molecules or atoms involved in an elementary step of a reaction mechanism. It is an important concept in classifying and understanding reaction steps. Molecularity is always a whole number and only applies to individual steps in a mechanism, not the overall reaction.
Key points about molecularity include:

• Unimolecular Reactions: These involve a single molecule undergoing a reaction, such as decomposition or isomerization. An example would be the breakdown of a sole molecule like \(_2O_5\) specified in the original document, if corrected, to proceed correctly.
• Bimolecular Reactions: These involve the collision and interaction of two molecules. Bimolecular steps are common and often form the basis of reaction rate theories.
• Termolecular Reactions: Involve three molecules coming together, which is rare due to the low probability of simultaneous collisions.

Understanding molecularity helps in deducing how reaction rates depend on the concentration of reactants. Unlike the overall reaction's stoichiometry, which refers to the ratio in which reactants are consumed and products formed, molecularity refers to individual reaction steps. This is essential for elucidating reaction mechanisms and making predictions about kinetic behavior.