Problem 96 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following statements is correct? (1) order of a reaction can be known from experimental results and not from the stoichiometry of a reaction. (2) molecularity a reaction refers to (i) each of the elementary steps in (an overall mechanism of) a complex reaction or (ii) a single step reaction. (3) overall molecularity of a reaction may be determined in a manner similar to overall order of reaction. (4) overall order of a reaction \(\mathrm{A}^{\mathrm{m}}+\mathrm{B}^{\mathrm{n}} \longrightarrow \mathrm{AB}_{\mathrm{x}}\) is \(\mathrm{m}+\mathrm{n}\) Select the correct answer using the following codes: (a) 2 and 3 (b) 1,3 and 4 (c) 2,3 and 4 (d) 1,2 and 3

Step-by-Step Solution

Answer

No option is entirely correct; statements 1, 2, and 4 are accurate.

Step 1: Analyze Statement 1

Statement (1) claims that the order of a reaction can be known from experimental results and not from the stoichiometry of the reaction. This is correct because the reaction order is determined by how the concentration of reactants influences the reaction rate, which must be observed experimentally.

Step 2: Analyze Statement 2

Statement (2) states that molecularity refers to each elementary step of a complex reaction or a single-step reaction. This is correct, as molecularity applies to individual elementary reaction steps and indicates the number of molecules involved in the step.

Step 3: Analyze Statement 3

Statement (3) suggests that the overall molecularity of a reaction can be determined similarly to the overall order of reaction. This is incorrect because molecularity is strictly defined for elementary steps and does not apply to overall reactions, unlike the order which applies to the whole reaction.

Step 4: Analyze Statement 4

Statement (4) discusses the overall order of a reaction given as \(\mathrm{A}^{\mathrm{m}}+\mathrm{B}^{\mathrm{n}} \rightarrow \mathrm{AB}_{\mathrm{x}}\) being \(\mathrm{m} + \mathrm{n}\). This is usually true if the reaction mechanism is a single-step, implying that the reaction order coincides with the stoichiometric coefficients.

Step 5: Choose the Correct Answer

Now that the statements have been analyzed: Statement 1 is correct, Statement 2 is correct, Statement 3 is incorrect, and Statement 4 is correct. Among the options, option (b) suggests statements 1, 3, and 4 are correct. However, since only statements 1, 2, and 4 should be considered correct, there is no entirely correct option given the elimination of option (a), (b), (c), and (d).

Key Concepts

Molecularity

Molecularity in chemistry refers to the number of molecules or atoms that come together to participate in a particular reaction step. It is a fundamental aspect of the mechanism of a chemical reaction.
Molecularity is assessed in the context of individual elementary reactions rather than the overall process. Here's why:

• It tells us how many molecules are colliding in a single reaction step.
• It's always a positive integer: unimolecular (involving one molecule), bimolecular (involving two molecules), or termolecular (involving three molecules).
• Molecularity cannot be zero, negative, or a fractional number because it represents actual particles interacting.

While molecularity applies to individual steps in complex reactions, determining the overall molecularity is not applicable, as it pertains to these distinct elementary events.

Reaction Mechanism

A reaction mechanism illustrates the step-by-step sequence of elementary reactions by which an overall chemical change occurs. Understanding the mechanism is crucial for several reasons:

• It helps chemists predict how changes in conditions might affect the reaction rate.
• Informs the understanding of reaction intermediates, which are species that appear in the mechanism but not in the overall balanced equation.
• Mechanisms are often speculative and must be validated through experimental data.

Each elementary step typically has its own rate law, which shows how the concentration of reactants affects the rate of that step. Furthermore, mechanisms provide insight into the molecular details of how reactants transform into products, aiding in the development of more efficient chemical processes.

Stoichiometry

Stoichiometry is the calculation of reactants and products in chemical reactions and is based on the balanced chemical equation.
It entails using relationships between the amounts of substances involved in reactions. Key points include:

• It provides a straightforward way to calculate quantities of reactants needed or products formed.
• Stoichiometry involves mole ratios derived from the coefficients of a balanced equation.
• It helps predict yields and informs the efficiency of chemical processes.

Unlike reaction order, which is experimentally determined, stoichiometry is straightforward from the equation. However, it does not account for the reaction kinetic, so the stoichiometric coefficients may not always relate directly to the reaction rate.

Elementary Reactions

Elementary reactions are single reaction events that occur in one step and involve a specific change.
This is distinct from the overall reaction, which may involve numerous elementary steps:

• They typically involve a small number of molecules, leading to terms unimolecular, bimolecular, or termolecular.
• Each elementary reaction has its rate law directly derived from the reaction's stoichiometry.
• These steps can occur independently or as part of a larger mechanism, with intermediates formed and used.

Through elementary reactions, you clearly see the interactions and changes at the molecular level, laying the foundation for analyzing complex mechanisms.