A mechanism for a reaction is a collection of elementary processes (also called elementary steps or elementary reactions) that explains how the overall reaction proceeds.
A mechanism is a proposal from which you can work out a rate law that agrees with the observed rate laws. The fact that a mechanism explains the experimental results is not a proof that the mechanism is correct. A mechanism is our rationalization of a chemical reaction, and devising mechanism is an excelent academic exercise.
The animation here shows an elementary step of two molecules coliding with each other and exchange a hydrogen atom in the process. Since elementary processes are the language of mechanism, let us first define elementary processes or steps.
An elementary step is proposed to give the reaction rate expression. The rate of an elementary step is always written according to the proposed equation. This practice is very different from the derivation of rate laws for an overall reaction.
When a molecule or ion decomposes by itself, such an elementary step is called a unimolecular step (or process). A unimolecular step is always a first order reaction. The following examples are given to illustrate this point:
A bimolecular process involves two reacting molecules or ions. The rates for these steps are 2nd order, and some examples are given to illustrate how you should give the rate expression. The simulation illustrates a bimolecular process.
Elementary processes are written to show how a chemical reaction progresses leading to an overall reaction. Such a collection is called a reaction mechanism. In a mechanism, elementary steps proceed at various speeds. The slowest step is the rate-determining step. The order for that elementary process is the order of a reaction, but the concentrations of reactants in that step must be expressed in terms of the concentrations of the reactants.
ii. NO2 + F = NO2F (fast)
Since step i is the rate-determining step, the rate law is
1 d[NO2] - --- ------ = k [NO2] [F2] 2 dtAddition of i. and ii. gives the overall reaction.
This example illustrates that the overall reaction equation has nothing to do with the order of the reaction. The elementary process in the rate-determining step determines the order.
Other possible elementary steps in this reaction are:
To propose a mechanism requires the knowledge of chemistry to give plausible elementary processes. A freshman in chemistry will not be asked to propose mechanisms, but you will be asked to give the rate laws from a given mechanism.
|Molecularity||Elementary step||Rate law|
|1||A -> products||rate = k [A]|
|2||A + A -> products|
A + B -> products
|rate = k [A]2|
rate = k [A] [B]
|3||A + A + A -> products|
A + 2 B -> products
A + B + C -> products
|rate = k [A]3|
rate = k [A] [B]2
rate = k [A] [B] [C]
Recognize and name all three elementary steps.
Give the order of any elementary step.
Emission of light is first order, but use a number.