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

The correct answer is (d) 1, 2, and 3.

Step 1: Analyzing Statement 1

Statement 1 claims that the order of a reaction is determined from experimental results rather than stoichiometry. This is correct because the reaction order is often determined by observing how changes in concentration affect the rate, something not inherent in stoichiometric coefficients.

Step 2: Evaluating Statement 2

Statement 2 says molecularity applies to each elementary step in a complex reaction or a single-step reaction. This is correct, as molecularity refers to the number of molecules involved in a single step and is relevant only to elementary reactions, not the overall reaction.

Step 3: Checking Statement 3

Statement 3 suggests that the overall molecularity can be determined similarly to reaction order. This is incorrect because molecularity is not a concept that can be applied to overall reactions; it's only applicable to individual elementary steps.

Step 4: Reviewing Statement 4

Statement 4 indicates the overall order of the reaction \(\mathrm{A}^{\mathrm{m}}+\mathrm{B}^{\mathrm{n}} \longrightarrow \mathrm{AB}_{\mathrm{x}}\) is \(\mathrm{m}+\mathrm{n}\). This is incorrect because stoichiometric coefficients do not determine reaction order unless verified experimentally.

Step 5: Selecting the Correct Options

Based on the analysis, Statement 1 and Statement 2 are correct. Thus, the correct option is (d), which includes 1, 2, and 3.

Key Concepts

Reaction Stoichiometry

When we talk about reaction stoichiometry, we are discussing the quantitative relationships between substances as they participate in chemical reactions. Stoichiometry uses the balanced chemical equation to help predict the amounts of each substance involved. For example, in the reaction \( A + B \rightarrow C \), the coefficients of the reactants and products in the balanced equation tell us how many moles of A react with how many moles of B to produce C.

Stoichiometry is essential in determining the amount of reactants needed and the expected amount of products formed in a chemical reaction. However, it does not tell us about the order of the reaction. Reaction order is derived from experiments, showing how concentration changes impact the rate.

### Key Points about Stoichiometry

• It describes quantities of reactants and products in a balanced chemical equation.
• Helps predict the amount of products formed from given reactants.
• Differs from reaction order, which needs experimental data.

Molecularity

Molecularity in chemistry refers to the number of species that must collide simultaneously to bring about a reaction in an elementary step. It's inherently related to the simplest steps in complex reactions. An elementary reaction occurs in a single step and involves direct contact between reactants.

### Understanding Molecularity

• Unimolecular: Involves a change in a single molecule leading to the product.
• Bimolecular: Involves a reaction between two molecules.
• Termolecular: Involves three molecules colliding simultaneously, which is relatively rare due to low probability.

Molecularity is an exact count of how many molecules participate in these steps. Unlike reaction order, it's not derived from experimental data but from the mechanism itself; thus, it's precise and inseparable from the basic reaction step.

Elementary Reactions

An elementary reaction is a single-step process in a chemical reaction where reactants convert directly into products without intermediate substances. In broader terms, complex reactions consist of multiple elementary reactions or steps.

Each step in a reaction mechanism is an elementary reaction, and the overall process can be understood by analyzing each elementary step.

### Characteristics of Elementary Reactions

• They occur in a single step and are usually very simple.
• The reaction mechanism often includes multiple elementary steps.
• They directly describe what happens in the reaction at the molecular level.

Understanding elementary reactions aids in grasping the sequence of events in a complete reaction mechanism. While stoichiometry describes the overall change in moles of reactants and products, elementary reactions illustrate the precise steps taken to form the final products.