Wise Luck Deluxe (2nd Edition)
Reaction Mechanisms
The mechanism of a chemical reaction is the sequence of events that take place as reactant molecules are converted into products.
Introduction
The study of kinetics includes very complex and sophisticated reactions that cannot be analyzed without a proposed mechanism, a series of steps that a reaction takes before reaching the final products.
Reaction mechanisms are step-by-step descriptions of what occurs on a molecular level in chemical reactions. Each step of the reaction mechanism is known as an elementary process, a term used to describe a moment in the reaction when one or more molecules changes geometry or is perturbed by the addition or omission of another interacting molecule. Collectively, an overall reaction and a reaction mechanism consist of multiple elementary processes. These elementary steps are the basic building blocks of a complex reaction, and cannot be broken down any further.
A reaction mechanism is only a guess at how a reaction proceeds. Therefore, even if a mechanism agrees with the experimental results of a reaction, it cannot be proven to be correct.
Example 1
Consider the chlorination reaction of methane, CH4:
CH4(g)+2Cl2(g)⟶CH3Cl(g)+HCl(g)+Cl−(g)(overall reaction)
This reaction is proposed to occur via two successive elementary steps. Each step has its own characteristic reactants, product, rate law.
CH4(g)+Cl2(g)⟶CH3(g)+HCl(g)(step 1 (slow)) with an elementary rate law of, k1=[CH4][Cl2] CH3(g)+Cl2(g)⟶CH3Cl(g)+Cl−(g)(step 2 (fast)) with an elementary rate law of, k2=[CH4][Cl2] The steps combine to generate the final reaction equation,
CH4(g)+2Cl2(g)⟶CH3Cl(g)+HCl(g)+Cl−(g)(overall reaction)
Example 2a nsider the following reaction:
CO(g)+NO2(g)→CO2(g)+NO(g)(overall reaction)
This reaction equation suggests that the carbon monoxide directly reacts with nitrogen dioxide to form the products. However, its reaction mechanism shows this is not the case. Consider the two elementary processes below:
The mechanism of a chemical reaction is the sequence of events that take place as reactant molecules are converted into products.
Introduction
The study of kinetics includes very complex and sophisticated reactions that cannot be analyzed without a proposed mechanism, a series of steps that a reaction takes before reaching the final products.
Reaction mechanisms are step-by-step descriptions of what occurs on a molecular level in chemical reactions. Each step of the reaction mechanism is known as an elementary process, a term used to describe a moment in the reaction when one or more molecules changes geometry or is perturbed by the addition or omission of another interacting molecule. Collectively, an overall reaction and a reaction mechanism consist of multiple elementary processes. These elementary steps are the basic building blocks of a complex reaction, and cannot be broken down any further.
A reaction mechanism is only a guess at how a reaction proceeds. Therefore, even if a mechanism agrees with the experimental results of a reaction, it cannot be proven to be correct.
Example 1
Consider the chlorination reaction of methane, CH4:
CH4(g)+2Cl2(g)⟶CH3Cl(g)+HCl(g)+Cl−(g)(overall reaction)
This reaction is proposed to occur via two successive elementary steps. Each step has its own characteristic reactants, product, rate law.
CH4(g)+Cl2(g)⟶CH3(g)+HCl(g)(step 1 (slow)) with an elementary rate law of, k1=[CH4][Cl2] CH3(g)+Cl2(g)⟶CH3Cl(g)+Cl−(g)(step 2 (fast)) with an elementary rate law of, k2=[CH4][Cl2] The steps combine to generate the final reaction equation,
CH4(g)+2Cl2(g)⟶CH3Cl(g)+HCl(g)+Cl−(g)(overall reaction)
Example 2a nsider the following reaction:
CO(g)+NO2(g)→CO2(g)+NO(g)(overall reaction)
This reaction equation suggests that the carbon monoxide directly reacts with nitrogen dioxide to form the products. However, its reaction mechanism shows this is not the case. Consider the two elementary processes below: