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Chapter 16: Problem 69

Distinguish between a complex reaction, a reaction mechanism, and an elementary step.

Short Answer

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A complex reaction is a chemical reaction occurring through a series of simple reactions or elementary steps, and its rate law cannot be determined from stoichiometry alone. A reaction mechanism is a detailed description of how a chemical reaction occurs at the molecular level and includes a sequence of elementary steps. An elementary step is a simple reaction that occurs in a single step without intermediates, and its rate law can be directly determined from its stoichiometry. Thus, a complex reaction consists of multiple elementary steps, and the reaction mechanism details the sequence of these elementary steps.

Step by step solution

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1. Definition of a Complex Reaction

A complex reaction is a chemical reaction that takes place through a series of simple reactions or elementary steps. These intermediate steps form the overall process, which can be described by the reaction mechanism. The rate law of a complex reaction cannot be determined from its stoichiometry alone, it requires experimental data or detailed knowledge of the reaction mechanism. For example, consider the reaction between nitrogen dioxide (NO2) and carbon monoxide (CO) to form nitrogen monoxide (NO) and carbon dioxide (CO2): \(2NO_{2} + CO \rightarrow 2NO + CO_{2}\) This reaction is a complex reaction, as it occurs through multiple elementary steps.
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2. Definition of a Reaction Mechanism

A reaction mechanism is a detailed description of how a chemical reaction occurs at the molecular level. It is a sequence of elementary steps that lead from the reactants to the products. Each elementary step in a reaction mechanism has its own rate law that can be determined from its stoichiometry. The overall rate law for a reaction is determined by combining the rate laws of the individual elementary steps in the mechanism. For example, the reaction between NO2 and CO mentioned earlier occurs through the following reaction mechanism: 1. \(NO_{2} + NO_{2} \rightarrow NO_{3} + NO\) (Elementary step 1) 2. \(NO_{3} + CO \rightarrow NO_{2} + CO_{2}\) (Elementary step 2) The overall reaction and its rate law can be determined by combining the rate laws of these two elementary steps.
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3. Definition of an Elementary Step

An elementary step is a simple reaction that occurs in a single step, without intermediates. It is the most basic component of a reaction mechanism, and its rate law can be directly determined from its stoichiometry. Elementary steps can be unimolecular, bimolecular, or termolecular depending on the number of molecules involved in the step (1, 2, or 3, respectively). For instance, in the reaction mechanism for the reaction between NO2 and CO, each of the steps is an elementary step: 1. \(NO_{2} + NO_{2} \rightarrow NO_{3} + NO\) - This is a bimolecular elementary step, which occurs between two NO2 molecules. 2. \(NO_{3} + CO \rightarrow NO_{2} + CO_{2}\) - This is also a bimolecular elementary step, occurring between NO3 and CO molecules. In summary, a complex reaction consists of multiple elementary steps, and the reaction mechanism details the sequence of these elementary steps. Understanding the differences between these concepts is essential for understanding and studying chemical reactions and their rates.

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Key Concepts

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

Complex Reaction
A complex reaction is not just a single, straightforward chemical change. Instead, it involves a sequence of several simpler reactions, known as elementary steps. These steps together make up the entire reaction.
  • Think of a complex reaction as a series of linked processes, where each individual reaction contributes to the formation of the final products.
  • The overall outcome of the reaction cannot be simply predicted by the initial reactants and final products alone because the interactions in between are essential to the process.
To fully understand a complex reaction, one must know all the elementary steps through which the reaction progresses.
This is why understanding the reaction mechanism (the detailed path the reaction takes) is crucial for determining how a complex reaction behaves.
The rate law for a complex reaction is determined experimentally, as it cannot be ascertained by just examining the balanced chemical equation.
Elementary Step
Elementary steps are the building blocks of any complex reaction. Each step involves a single chemical change and can be directly described using its stoichiometry, which means the proportion of reactants and products is clear.
There are a few characteristics of elementary steps:
  • An elementary step is a basic reaction that occurs in one go without forming any intermediates.
  • They can be categorized as unimolecular, bimolecular, or termolecular, depending on how many molecules are involved in the step (1, 2, or 3).
  • Because they're simple, the rate law of an elementary step can be directly deduced from its stoichiometry. For example, if an elementary step involves two reacting molecules, it is bimolecular, and the rate law reflects this as it depends on the concentration of both reacting species.
Understanding each elementary step in a reaction is fundamental to grasping the mechanism of the entire complex reaction.
This knowledge helps in predicting the behavior and the speed of the reaction.
Rate Law
The rate law is an essential concept in understanding how fast a chemical reaction proceeds. It tells us how the reaction rate is affected by the concentration of reactants.
Some important points about rate laws are:
  • The rate law of a chemical reaction shows how the rate depends on the concentration of reactants.
  • For elementary reactions, the rate law can be directly written from the stoichiometry of the reaction; however, for complex reactions, it requires either experimental data or a detailed understanding of the reaction mechanism.
  • The overall rate law of a complex reaction is determined by considering all the elementary steps that make up the reaction mechanism.
The rate law gives insight into which parts of the reaction mechanism are the slowest and therefore rate-limiting, impacting how we control and optimize reactions in industrial and laboratory settings.
By comprehending the rate law, chemists can predict, measure, and alter the reaction speed to efficiently produce desired compounds.

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Most popular questions from this chapter

Use the rate law in Example Problem 16.2 and the concentrations given in Practice Problems 31 and 32 to calculate the instantaneous rate for the reaction between \(\mathrm{NO}\) and \(\mathrm{H}_{2} .\) $$[\mathrm{NO}]=0.00500 \mathrm{M} \text { and } \left[\mathrm{H}_{2}\right]=0.00200 \mathrm{M}$$

Describe the effect on the rate of a reaction if one of the reactants is ground to a powder rather than used as a single chunk.

A two-step mechanism has been proposed for the decomposition of nitryl chloride \(\left(\mathrm{NO}_{2} \mathrm{CL}\right)\) Step \(1 : \mathrm{NO}_{2} \mathrm{Cl}(\mathrm{g}) \rightarrow \mathrm{NO}_{2}(\mathrm{g})+\mathrm{Cl}(\mathrm{g})\) Step \(2 : \mathrm{NO}_{2} \mathrm{Cl}(\mathrm{g})+\mathrm{Cl}(\mathrm{g}) \rightarrow \mathrm{NO}_{2}(\mathrm{g})+\mathrm{Cl}_{2}(\mathrm{g})\) What is the overall reaction? Identify any intermediates in the reaction sequence, and explain why they are called intermediates

The rate law is rate \(=k[\text { phenolphthalein }] .\) If the rate constant for the reaction is \(1.0 \times 10^{-2} \mathrm{s}^{-1}\) , what is the instantaneous rate of reaction when the concentration of phenolphthalein is 0.0025\(M\) ?

What role does the reactivity of the reactants play in determining the rate of a chemical reaction?
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