Wise Luck Deluxe (2nd Edition)

Topics: Reaction rate, Chemical kinetics, Chemical reaction Pages: 8 (2286 words) Published: September 22, 2014
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: 2NO2(g)→NO3(g)+NO(g)(1)

CO(g)+NO3(g)→CO2(g)+NO2(g)(2)
CO(g)+NO2(g)→CO2(g)+NO(g)(overall reaction)
This reaction mechanism indicates that the compound NO3 performs a necessary role in allowing the reaction to take place, a concept not possible to confirm by looking at the overall equation. Example 2b

Take the reaction 2NO(g)+O2(g)→2NO2(g). A look at the overall reaction might suggest that two NO molecules must collide with an oxygen molecule to generate the product. However, termolecular processes are extremely rare; a further analysis of the reaction mechanism is required. The two elementary reactions below show: 2NO(g)→N2O2(g)(1)

2N2O2(g)+O2(g)→2NO2(g)(2)
2NO(g)+O2(g)→2NO2(g)(overall reaction)
The reaction mechanism shows that, in fact, three molecules combining simultaneously to form a product does not occur in this case. Instead, the overall reaction is a compilation of two individual elementary processes that take place. Description of a Reaction Mechanism

Because a reaction mechanism is used to describe what occurs at each step of a reaction, it also describes the transition state, or the state in which the maximum of potential energy is reached. A mechanism must show the order in which the bonds form or break and the rate of each elementary step. Also accounted for are the reaction intermediates, stable molecules that do not appear in the experimentally determined rate law because they are formed in one step and consumed in a subsequent step. Because a reaction cannot proceed faster than the rate of slowest elementary step, the slowest step in a mechanism establishes the rate of the overall reaction. This elementary step is known as the rate-determining step.  A mechanism must satisfy the following two requirements:

1. The elementary steps must add up to give the overall balanced equation for the reaction. 2. The rate law for the rate-determining step must agree with the experimentally determined rate law. Each of these events constitutes an elementary...
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