Chemical Kinetics

A reaction mechanism is the process of understanding the sequence of events that occur at the molecular level during a chemical reaction. It involves the study of how individual atoms, ions, or molecules interact with each other to form the products.To better understand this concept, think of it like cooking a complex dish with various ingredients. The reaction mechanism is like a step by step recipe that guides on how to combine the ingredients to create the final dish.

To understand the reaction mechanism, let us first understand elementary reactions. An elementary reaction involves a direct collision and transformation of individual molecules, atoms, or ions to produce specific products. An intermediate is not formed in elementary reaction. An example of an elementary reaction is the reaction between hydrogen gas and chlorine gas. In this elementary reaction, one molecule of hydrogen gas collides with one molecule of chlorine gas. This collision produces two molecules of hydrogen chloride.

Complex reactions are chemical reactions that involve multiple steps or elementary reactions. Elementary reactions are like the building-blocks of complex chemical reactions. Imagine a recipe for making a simple layer of a cake. Each step of adding ingredients and mixing them represents an elementary reaction. Now, a complex reaction is like a series of steps needed to create a fancy cake with many layers and decorations. Each elementary reaction in the series contributes to the overall process of making the fancy cake.

Let us take an example of the reaction that occurs in an internal-combustion-engine. In this reaction the isooctane reacts with the oxygen to produce carbon dioxide and water. At first glance, it might seem that this reaction happens in a single step. In this single step, the twenty five dioxygen molecules and the two isooctane molecules collide simultaneously to produce thirty four molecules of the product. However, this is highly unlikely to occur all at once.

It is more reasonable to believe that the reaction takes place through a series of individual steps. These individual steps are known as elementary reactions. Let us understand this with a simple example. Two molecules of Reactant A react with one molecule of Reactant B. This forms Product C. Now it might seem that this reaction is a single step reaction. However, it is not a single step reaction. This reaction actually takes place in two steps.

In the first step, two molecules of Reactant A react with each other to form X. In the second step, X acts as a reactant. X reacts with Reactant B to form Product C. Step one and step two reactions are called elementary reactions. These elementary reactions are the steps of the complex reaction.

Molecularity refers to the number of molecules or atoms that participate as reactants in an elementary reaction. In an elementary reaction, the reactants directly collide and convert into the products. The molecularity determines how many molecules or atoms are involved in this collision process. For example, hydrogen molecule reacts with bromine molecule to form hydrogen bromide. It is an elementary reaction. It involves two molecules. These molecules are hydrogen molecule and bromine molecule.

On basis of molecularity, we can classify the reactions into three types. These are the unimolecular reactions, bimolecular reactions and the termolecular reactions. In a unimolecular reaction, only one molecule undergoes transformation to form the products. These reactions often involve the decomposition or isomerization of a single molecule. For example, the conversion of Reactant A into product P is an example of the unimolecular reaction. Molecularity of the unimolecular reaction is one.

In a bimolecular reaction, two molecules collide and react to form the product. For example, the Reactant A collides with the Reactant B to form the product P. This reaction is a bimolecular reaction. It involes the collision of two reactant molecules. Molecularity of a bimolecular reaction is two.

Termolecular reactions involve the simultaneous collision of three molecules to generate the product. For example, the Reactant A, Reactant B and Reactant C collide simultaneously to form the product P. The molecularity of the termolecular reaction is three. Termolecular reactions are relatively rare. This is because the probability of three molecules colliding simultaneously is significantly lower.

The order of an elementary reaction can be determined from its molecularity. A unimolecular reaction is the first order reaction. A bimolecular reaction is the second order reaction. A termolecular reaction is the third order reaction.

We know that a complex reaction consists of multiple elementary reactions. In a complex reaction, the molecularity of one elementary reaction can be different from other. How can we determine the order of such complex reaction? The answer to this question lies in the rate determining step.

Different elementary reactions have different speed in a complex reaction. Some proceed at fast speed. Some proceed at slow speed. The slowest elementary reaction in a complex reaction determines the rate of reaction. This slowest elementary reaction is the called rate determining step.

Let us take an example of a complex reaction to understand the rate determining step. Substitution of tertiary alkyl halide by a nucleophile is an example of a complex reaction. This reaction consists of two steps. In the first step, the leaving group of the alkyl halide leaves. This results in the formation of a carbocation. In second step the nucleophile attacks the carbocation. This attack results in the formation of the product.

These two steps are also called elementary steps. In this reaction, first elementary step is slowest. First step determines the rate of reaction. The molecularity of first elementary reaction is one. It is a unimolecular reaction. This shows that it is a first order reaction.