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Unimolecular reaction: Any reactioninvolving only one molecule of reactant changing into products is known as a Unimolecular reaction.

E.g. Dissociation of phosphorous pentachloride.

\({\bf{PC}}{{\bf{l}}_{\bf{5}}} \to {\bf{PC}}{{\bf{l}}_{\bf{3}}}{\bf{ + C}}{{\bf{l}}_{\bf{2}}}\)

It is unimolecular because only one molecule of \({\bf{PC}}{{\bf{l}}_{\bf{5}}}\) is dissociating in the reaction.

Bimolecular reaction:Any reactioninvolving two molecules of reactant changing into products is known as a Bimolecular reaction

E.g. Dissociation of HI

It is bimolecular because two molecules of reactant are dissociating in the reaction.

Elementary Reaction:Any chemical reaction proceeds througha series of intermediate steps before converting into products. These individual reactions are known as elementary steps or reactions.

E.g. Conversion of carbon monoxide to carbon dioxide

\({\bf{CO + }}{{\bf{O}}_{\bf{2}}} \to {\bf{2C}}{{\bf{O}}_{\bf{2}}}\)

It is an elementary reaction because the reactants are being converted to products in a single-step reaction.

Overall Reaction: The complete representation of a chemical reaction. The sum of the elementary steps/reactions are represented in a single reaction, known as the overall reaction.

Eg

\({\bf{CO + N}}{{\bf{O}}_{\bf{2}}} \to {\bf{NO}} + {\bf{C}}{{\bf{O}}_{\bf{2}}}\)

This reaction occurs in two steps, as shown below.

\(\frac{\begin{aligned}{l}{\bf{N}}{{\bf{O}}_{\bf{2}}}{\bf{ + N}}{{\bf{O}}_{\bf{2}}} \to {\bf{N}}{{\bf{O}}_3} + {\bf{NO}}\\{\bf{N}}{{\bf{O}}_3} + {\bf{CO}} \to {\bf{N}}{{\bf{O}}_2} + {\bf{C}}{{\bf{O}}_2}\end{aligned}}{{{\bf{CO + N}}{{\bf{O}}_{\bf{2}}} \to {\bf{NO}} + {\bf{C}}{{\bf{O}}_{\bf{2}}}}}\)