Define these terms: (a) unimolecular reaction (b) bimolecular reaction (c) elementary reaction (d) overall reaction.

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

E.g. Dissociation of HI

\({\mathbf{2HI}} \rightleftharpoons {{\mathbf{H}}_{\mathbf{2}}}{\mathbf{ + }}{{\mathbf{I}}_{\mathbf{2}}}\)

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

Elementary Reaction: Any chemical reaction proceeds through a 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.

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