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Reaction mechanisms

Nearly all reactions do not occur in a single step but rather a series of steps termed a reaction mechanism. Several points can be made about these mechanisms.

The formula equation we write for a reaction is the sum of all the steps in a reaction.
The reaction mechanism can't be determined from the overall reaction.
Each step in the mechanism involves the collision of at most two particles. If the step involves only one reactant particle it is termed unimolecular. If the step involves two reactant particles it is termed bimolecular.
The mechanism is an educated guess based on experiments and theory.
The mechanism may involve the formation of certain molecules in one step that are later used up in the subsequent step. These molecules are termed intermediates, and do not appear in the overall reaction.
The slowest step in the reaction mechanism determines the rate, and therefore only reactants in the slowest step affect rate. This step is termed the rate determining step. Increasing the concentrations of these reactants will speeding up the slow step will increase the overall reaction rate. Increasing the concentration of reactant in fast steps will have little effect on rate. To understand how this works consider the following analogy.

What is a reaction mechanism?

In any chemical change, some bonds are broken and new ones are made. Quite often, these changes are too complicated to happen in one simple stage. Instead, the reaction may involve a series of small changes one after the other.
A reaction mechanism describes the one or more steps involved in the reaction in a way which makes it clear exactly how the various bonds are broken and made.

Elementary Processes or Steps

An elementary process is also called elementary step or elementary reactions. It expresses how actually molecules or ions react with each other. The equation in an elementary step represents the reaction at the molecular level, not the overall reaction. Based on numbers of molecules involved in the elementary step, there are three kinds of elementary steps: unimolecular step (or process), bimolecular process, and trimolecular process.

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.

The rate determining step

The overall rate of a reaction (the one which you would measure if you did some experiments) is controlled by the rate of the slowest step. In the example above, the hydroxide ion can't combine with the positive ion until that positive ion has been formed. The second step is in a sense waiting around for the first slow step to happen.
The slow step of a reaction is known as the rate determining step.
As long as there is a lot of difference between the rates of the various steps, when you measure the rate of a reaction, you are actually measuring the rate of the rate determining step.

Reaction mechanisms and orders of reaction

The examples we use at this level are the very simple ones where the orders of reaction with respect to the various substances taking part are 0, 1 or 2. These tend to have the slow step of the reaction happening before any fast step(s).

Molecularity of a reaction

This starts at the other end! If you know the mechanism for a reaction, you can write down equations for a series of steps which make it up. Each of those steps has a molecularity.
The molecularity of a step simply counts the number of species (molecules, ions, atoms or free radicals) taking part in that step.

For a reaction mechanism to be valid:

The molecularity of each elementary step should only be 1,2, or 3
The elementary steps must added up to the overall reaction
The rate determining step must agree with the rate law.

Summary

The number of particle involved in an elementary step is called the molecularity, and in general, we consider only the molecularity of 1, 2, and 3. Types of elementary steps are summarized below. In the table, A, B, and C represent reactants, intermediates, or products in the elementary process.

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]

Adapted From: http://www.chemguide.co.uk/physical/basicrates/ordermech.html