Question

Which of the following statements are correct? (1) Order of a reaction can be known from experimental results and not from the stoichiometry of 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 \(\mathrm{A}^{\mathrm{m}}+\mathrm{B}^{\mathrm{n}} \rightarrow \mathrm{AB}_{\mathrm{x}}\) 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

Short answer

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

Step-by-step solution

Analyze Statement 1

Statement 1 proposes that the order of a reaction can be known from experimental results and not from the stoichiometric coefficients of the reactants in the balanced chemical equation. This statement is correct because reaction orders are determined experimentally by observing how changes in concentrations affect the rate, which may not necessarily match the stoichiometry.

Evaluate Statement 2

Statement 2 discusses molecularity, explaining it as (i) applying to each elementary step in a complex reaction or (ii) referring to a single-step reaction. This is correct; molecularity is defined for elementary steps, indicating how many molecules or atoms come together to form a reaction.

Consider Statement 3

Statement 3 suggests that overall molecularity can be determined in a similar manner to the overall order of a reaction. However, this is incorrect. Molecularity refers only to elementary steps and does not describe a complex reaction, whereas the overall order is established experimentally for an entire reaction.

Assess Statement 4

Statement 4 says the overall order of the reaction \[\text{A}^{m} + \text{B}^{n} \rightarrow \text{AB}_x \]is \(m+n\). This statement would be correct if the reaction is elementary, because for elementary reactions, the order can be directly related to the stoichiometric coefficients. However, the problem does not specify whether the reaction is elementary, so this conclusion cannot be drawn without additional experimental data.

Select the Correct Answer Based on Analysis

Summarizing the analysis: Statement 1 is correct, Statement 2 is correct, Statement 3 is incorrect, and Statement 4 cannot be confirmed without additional data. Therefore, the answer must exclude statements 3 and 4.

Key concepts

Molecularity of Reaction

Molecularity is a term that tells us how many reactant molecules are involved in an elementary reaction step. It is a simple concept but crucial in understanding reaction mechanisms. Here’s how it works:

• A reaction step that involves one molecule, such as the breakdown of a single compound, is termed unimolecular.
• If two molecules collide and react, it is bimolecular.
• Involving three molecules makes it termolecular, though these are less common due to the unlikely nature of three molecules colliding simultaneously.

Only makes sense when talking about elementary steps. In contrast, complex reactions consist of multiple elementary steps, each with its own molecularity. Molecularity is based on the physical act of molecules colliding, making it important for predicting rates and mechanisms. Its value is always a whole number, distinguishing it from reaction order, which is determined experimentally and can be fractional or zero.

Elementary Steps

An elementary step describes a singular event in a multi-step reaction pathway. Understanding elementary steps is key to linking observed kinetics with theoretical chemical mechanisms.

Each elementary step can be identified by observing the direct interaction and transformation of molecules. They represent the diagnostic steps that occur to complete a reaction mechanism. An elementary step gives immediate insight into molecularity, which we mentioned earlier, describing the precise number of molecules colliding.

Keep in mind that the rate law for an elementary step can be directly derived from its stoichiometry. For instance, for the elementary step:

\( \text{A} + \text{B} \rightarrow \text{C} \)

We can write the rate law as:

\( \text{Rate} = k[\text{A}][\text{B}] \)

Unlike complex reactions, where the overall rate law requires experimental determination, elementary steps' rate laws are straightforward due to their direct correlation with actual molecular events.

Stoichiometry vs. Experimental Data

In chemistry, stoichiometry is used to describe the quantitative relationship between reactants and products in a chemical reaction. It often provides a blueprint for what should happen when molecules react. However, stoichiometry alone can't always predict how fast a reaction will occur. This is where experimental data comes into play.

It's crucial to distinguish between stoichiometric coefficients and reaction orders. Reaction order is determined through experimental observations where the rate is measured, offering a more dynamic view of the process than stoichiometry can provide. For instance:

• The balanced equation \(\text{A}^{m} + \text{B}^{n} \rightarrow \text{AB}_x \) might suggest a reaction order \(m + n\).
• Yet, the true order might be different as combusted experimentally rather than predicted from the equation alone.

Moreover, reaction order can influence how temperature affects the rate or dictate reaction mechanism assumptions. Therefore, chemists rely on experiments to verify and refine the theoretical implications of stoichiometry.