Elementary Reaction

Non-elementary reactions can be explained by numerous elementary reaction steps; elementary reactions build up to complicated reactions. A reaction mechanism is made up of a set of elementary reactions that predicts the elementary steps in a complex reaction. The reaction coordinates of two reactions are shown below. The first is an elementary reaction, whereas the second is a non-elementary reaction.

Different Types of Elementary Reactions

The number of reactant particles engaged in a reaction is referred to as its molecularity. The molecularity must have an integer value because there can only be discrete numbers of particles. There are three types of molecularity: unimolecular, bimolecular, and termolecular. There are no elementary reactions that involve four or more molecules that have been discovered.

Single Molecular Reaction

When a molecule rearranges itself to produce one or more products, it is called a unimolecular reaction. Radioactive decay is an example of this, in which particles are emitted from an atom. Cis-trans isomerization, heat breakdown, ring opening, and racemization are some further examples. The rate of decomposition of a material is proportional to its concentration. As Frederick Alexander Lindemann explained in his Lindemann mechanism, unimolecular reactions are frequently first-order reactions.

Bimolecular Reaction

A bimolecular reaction occurs when two particles collide. Organic reactions like nucleophilic substitution frequently involve bimolecular reactions. Bimolecular reactions are second-order reactions because their pace is determined by the product of the concentrations of each species involved.

Termolecular Reaction

The collision of three particles at the same time and location is required for a termolecular reaction. This type of reaction is extremely rare since it requires all three reactants to contact at the same time, with sufficient energy and the correct direction, in order to initiate a reaction. There are three types of termolecular reactions, all of which are third order.

Elementary Reaction Molecularity and Rate Law [1-4]

The number of reactants in an elementary reaction is known as the molecularity of the reaction. It is a deciding factor in how elementary reactions are classified. For example, the reaction given below  involves two HI molecules. As a result, its molecularity is two.

2HI-H2+I2 

The rate rule for an elementary reaction establishes a link between the reaction rate and the concentrations of the reactants. In the balanced equation, it can be represented in terms of the concentration and stoichiometric coefficients of the reactants. The number in front of the reactant is the stoichiometric coefficient. For example, rate = k[HI]2is the rate law for the above equation . The concentration of hydrogen iodide is represented by [HI].

The sequence of a chemical reaction can be determined by the number of reactant molecules. It’s calculated by adding all of the reactant’s stoichiometric coefficients together. The total reaction order is always equal to molecularity for an elementary reaction. Returning to the above  equation , the order is 2, because the reaction involves two molecules of HI. As a result, the breakdown of HI is a reaction of second order.

Elementary and non-Elementary reactions

Without any intermediates, reactants generate products directly in an elementary reaction. Process intermediates, on the other hand, form the ultimate products in a non-elementary or complex reaction. A non-elementary reaction is formed when two or more elementary reactions combine.

An example of a non-elementary reaction is as follows:

In two processes, nitrogen dioxide (NO2) combines with fluorine (F2) to form nitryl fluoride (NO2F).

Step 1: NO2 +F2 -[NO2F]*(Slow)

Step 2:  [NO2+F ](Fast)

The general consensus is:

2NO2F = 2NO2 + F2

The intermediate is [NO2+ F]* in this case. It’s denoted by the symbol []*, which means the intermediate is unstable and short-lived

Conclusion

The reaction mechanism is the series of distinct steps, or elementary reactions, by which reactants are changed into products during the course of a reaction. The rate of the slowest step, known as the rate-determining step, determines the total pace of a response. First-order rate laws govern unimolecular elementary reactions, while second-order rate laws govern bimolecular elementary reactions. The rate laws calculated from a response mechanism can be compared against experimentally observed rate laws to determine whether the mechanism is valid or credible.