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Chapter 10: Problem 96

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

Short Answer

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

Step by step solution

01

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.
02

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.
03

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.
04

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.
05

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.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

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.

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Most popular questions from this chapter

Two substance ' \(\mathrm{A}^{\prime}\) and ' \(\mathrm{B}^{\prime}\) are present such that \(\left[\mathrm{A}_{0}\right]=\) \(4\left[\mathrm{~B}_{0}\right]\), and half-life of ' \(\mathrm{A}^{\prime}\) is 5 minutes and that of ' \(\mathrm{B}^{*}\) is 15 minute. If they start decaying at the same time following first order, how much time later will the concentration of both of them would be same. (a) \(10 \mathrm{~min}\) (b) \(12 \mathrm{~min}\) (c) \(5 \mathrm{~min}\) (d) \(15 \mathrm{~min}\)

The hypothetical reaction, \(\mathrm{A}_{2}+\mathrm{B}_{2} \rightarrow 2 \mathrm{AB}\) follows the following mechanism: \(\mathrm{A}_{2}=\mathrm{A}+\mathrm{A}\) (fast) \(\mathrm{A}+\mathrm{B}_{2} \longrightarrow \mathrm{AB}+\mathrm{B} \quad\) (slow) \(\mathrm{A}+\mathrm{B} \longrightarrow \mathrm{AB} \quad\) (fast) The order of the overall reaction is: (a) \(\underline{1}\) (b) 2 (c) \(3 / 2\) (d) 0

For a reaction, \(\mathrm{A} \rightarrow \mathrm{B}+\mathrm{C}\), it was found that at the end of \(10 \mathrm{~min}\) from the start the total optical, rotation of the system was \(50^{\circ} \mathrm{C}\) and when the reaction is complete it was \(100^{\circ}\). Assuming that only \(\mathrm{B}\) and \(\mathrm{C}\) are optically active and dextro rotator, the rate constant of this first order reaction would be (a) \(6.9 \mathrm{~min}^{-1}\) (b) \(0.069 \mathrm{~min}^{-1}\) (c) \(6.9 \times 10^{-2} \mathrm{~min}^{-1}\) (d) \(0.69 \mathrm{~min}^{-1}\)

A graph plotted between concentration of reactant, consumed at any time \((\mathrm{x})\) and time ' \(\mathrm{t}\) ' is found to be a straight line passing through the origin. The reaction is of (a) first-order (b) zero-order (c) third-order (d) second-order

Which of the following statement(s) is/are correct? (a) An increase in temperature inereases the rate constant as well as the equilibrium constant of an exothermic reaction. (b) Densities of diamond and graphite are \(\mathrm{x}\) and \(\mathrm{y}(\mathrm{x}>\mathrm{y})\). Increase in pressure on the equilibrium favours backward reaction. \(\mathrm{C}(\) diamond) \(=\mathrm{C}\) (graphite) (c) Solubility of a solute (s) is dependent upon temperature as \(\mathrm{S}=\) A.e \(^{-\text {- } H R T}\) (Here \(\Delta H\) is the enthalpy of solution) (d) For the reaction \(\mathrm{N}_{2} \mathrm{O}_{4}(\mathrm{~g})=2 \mathrm{NO}_{2}(\mathrm{~g})\), if degrees of dissociation of \(\mathrm{N}_{2} \mathrm{O}_{4}\) are \(25 \%, 50 \%\) and \(75 \%\), then gradation of observed molar masses is \(\mathrm{M}_{1}>\) \(\mathrm{M}_{2}>\mathrm{M}_{3}\left(\mathrm{M}_{1}, \mathrm{M}_{2}\right.\) and \(\mathrm{M}_{3}\) are molar masses corre- sponding to vapour densities are \(25 \%, 50 \%\) and \(75 \%\) dissociation respectively)
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